Bally/Stern

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Click to go back to the Bally/Stern solid state repair guides index.

1 Introduction

Placeholder for Bally -17 Board Set
Bally -35 Board Set


Bally first started to experiment with solid state in the mid 70s with a couple conversions of EM games, Flicker and Bow & Arrow. Flicker's control system was designed by Jeff Frederiksen of Dave Nutting Associates, a Bally think-tank. This board utilized the Intel 4004 microprocessor. One prototype was built and was re-discovered in 1998. The Bow & Arrow system of which 17 were eventually built more closely resembles what actually ended up in the production games starting with Freedom. This boardset was designed by a Bally engineer named Doug MacDonald. There are at least two revisions of the prototype run's software in existence.

For an interesting read, the patent Bally filed for their first "computerized pinball machine" can be viewed here.

Placeholder for Stern M-100 Board Set
Stern M-200 Board Set



The early circuit boards were fabricated by a company called Universal Research Laboratories (URL). URL was purchased by Gary Stern as part of his strategy in acquiring the assets of the defunct Chicago Coin; it is this connection that enabled Stern to use essentially the same hardware for their solid state games as Bally. The software is remarkably similar as well, although Bally was more apt to use a "set" of operating system roms (the U6 chip) for several games, whereas Stern opted to custom-compile their roms for each game, or at most, pairs of games.

Bally Goldball special driver/voltage board


Some later -35 Bally games used a combination of the solenoid driver board and the lamp driver board.

The core system consists of:

  • A transformer to produce various AC voltages from line voltage
  • A rectifier board to convert those AC voltages to DC and provide fuse protection
  • A solenoid driver/voltage regulator board to regulate/filter the DC voltages and provide switching for solenoids
  • A lamp driver board to switch up to 60 lamps on/off


Optional components added to the system at later dates include:

  • Various sound boards, providing various type of sounds from simple tones, to background sound, to complex speech and sounds
  • Auxiliary lamp driver boards, to provide control for more than 60 lamps
  • Solenoid expander boards to allow more than 15 controllable momentary solenoids to be connected


2 Games

A list of solid state games by system and manufacturer (including those that aren't necessarily pinball). Source: http://www.ipdb.org

2.1 Bally

Game Title MPU Power Supply Lamp driver Sound Additional Boards
Freedom AS-2518-17 AS-2518-18 AS-2518-14 Chimes
Night Rider AS-2518-17 AS-2518-18 AS-2518-14 Chimes
Black Jack AS-2518-17 AS-2518-18 AS-2518-14 Chimes
Evel Knievel AS-2518-17 AS-2518-18 AS-2518-14 Chimes
Eight Ball AS-2518-17 AS-2518-18 AS-2518-14 Chimes
Power Play AS-2518-17 AS-2518-18 AS-2518-14 Chimes
Mata Hari AS-2518-17 AS-2518-18 AS-2518-23 Chimes
Strikes & Spares AS-2518-17 AS-2518-18 AS-2518-23 Chimes
Lost World AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-32
The Six Million Dollar Man AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-32
Playboy AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-32
Voltan AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-32
Supersonic AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-32
Star Trek AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-50
Game Title MPU Power Supply Lamp driver Sound Additional Boards
Paragon AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-50
Harlem Globetrotters AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-50
Dolly Parton AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-50
Kiss AS-2518-35 AS-2518-49 AS-2518-23 AS-2518-50 Auxiliary Lamp Driver AS-2518-43
Future Spa AS-2518-35 AS-2518-49 AS-2518-23 AS-2518-51 Auxiliary Lamp Driver AS-2518-43
Space Invaders AS-2518-35 AS-2518-49 AS-2518-23 AS-2518-51 Auxiliary Lamp Driver AS-2518-52
Nitro Groundshaker AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-51
Silverball Mania AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-51
Rolling Stones AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-51
Mystic AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-51
Hotdoggin AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-51
Viking AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-51
Skateball AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-51
Frontier AS-2518-35 AS-2518-18 AS-2518-23 AS-2518-51
Xenon AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-56 (Sounds Plus) Auxiliary Lamp Driver AS-2518-52, Vocalizer module AS-2518-57
Game Title MPU Power Supply Lamp driver Sound Additional Boards
Flash Gordon AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61 (Squawk & Talk) Auxiliary Lamp Driver AS-2518-52, early games used the AS-2518-56 sound board and Vocalizer module AS-2518-57
Eight Ball Deluxe AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61A (Squawk & Talk) Auxiliary Lamp Driver AS-2518-52, Solenoid Expander AS-2518-66
Fireball II AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61A (Squawk & Talk) Auxiliary Lamp Driver AS-2518-52, Auxiliary Driver - SCR Lamp Flasher AS-2518-67, Auxiliary Driver - GI Flasher AS-2518-68
Embryon AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61A (Squawk & Talk) Auxiliary Lamp Driver AS-2518-52
Fathom AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61A (Squawk & Talk) Auxiliary Lamp Driver AS-2518-52, Solenoid Expander AS-2518-66
Medusa AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61A (Squawk & Talk) Auxiliary Lamp Driver AS-2518-52
Centaur AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61B (Squawk & Talk) Say It Again AS-2518-81 (Reverb), Auxiliary Lamp Driver AS-2518-43, Solenoid Expander AS-2518-66, Auxiliary Driver - SCR Lamp Flasher AS-2518-67, Auxiliary Driver - GI Flasher AS-2518-68
Elektra AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61B (Squawk & Talk) Auxiliary Lamp Driver AS-2518-52, Solenoid Expander AS-2518-66, Auxiliary Driver - GI Flasher AS-2518-68
Vector AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61B (Squawk & Talk) Auxiliary Lamp Driver AS-2518-52, Solenoid Expander AS-2518-66, Auxiliary Driver - GI Flasher AS-2518-68, Auxiliary Driver - Triac Lamp Flasher AS-2518-82
Spectrum AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61B (Squawk & Talk) Uses a second AS-2518-23 for lamps (2 total in game), Auxiliary Driver - SCR Lamp Flasher AS-2518-67, Auxiliary Driver - GI Flasher AS-2518-68
Speakeasy AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-51 Auxiliary Lamp Driver AS-2518-52, Solenoid Expander AS-2518-66
Rapid Fire AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61B (Squawk & Talk) Ball Delivery Sensor & Motor Board AS-2518-102
Granny and the Gators AS-2518-133 AS-2518-132 AS-2518-107 (combo lamp and solenoid driver) Vidiot AS-2518-121
Mr. & Mrs. Pacman AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61B (Squawk & Talk) Auxiliary Lamp Driver AS-2518-52, Solenoid Expander AS-2518-66, Auxiliary Driver - GI Flasher AS-2518-68
Baby Pac-Man AS-2518-133 AS-2518-132 AS-2518-107 (combo lamp and solenoid driver) Vidiot AS-2518-121
Eight Ball Deluxe Limited Edition AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61 (Squawk & Talk) Auxiliary Lamp Driver AS-2518-52, Solenoid Expander AS-2518-66
Game Title MPU Power Supply Lamp driver Sound Additional Boards
BMX AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-51 Solenoid Expander AS-2518-66
Centaur II AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61B (Squawk & Talk) Say It Again AS-2518-81 (Reverb), Auxiliary Lamp Driver AS-2518-43, Solenoid Expander AS-2518-66, Auxiliary Driver - GI Flasher AS-2518-68
Goldball AS-2518-35 AS-2518-151 AS-2518-147 (combo lamp and solenoid driver) AS-2518-51 AS-2518-68 GI Flasher
Grand Slam AS-2518-133 AS-2518-151 AS-2518-147 (combo lamp driver/solenoid driver/regulator) AS-2518-51
X's & O's AS-2518-35 AS-2518-54 AS-2518-23 M-051-00114-B045 / A080-91603-B000 (Cheap Squeak)
Kings of Steel AS-2518-35 AS-2518-54 AS-2518-23 M-051-00114-B045 / A080-91603-B000 (Cheap Squeak)
Black Pyramid AS-2518-35 AS-2518-54 AS-2518-23 M-051-00114-B045 (Cheap Squeak) A084-91614-A000 Aux. Lamp Driver
Eight Ball Deluxe Classic AS-2518-35 AS-2518-54 AS-2518-23 AS-2518-61 (Squawk & Talk) Auxiliary Lamp Driver AS-2518-52
Spy Hunter AS-2518-35 AS-2518-54 AS-2518-23 M-051-00114-B045 (Cheap Squeak)
Fireball Classic AS-2518-35 AS-2518-54 AS-2518-23 M-051-00114-B045 (Cheap Squeak)
Cybernaut AS-2518-35 AS-2518-54 AS-2518-23 M-051-00114-B045 (Cheap Squeak)

2.2 Stern

Game Title MPU Power Supply Lamp driver Sound Notes
Pinball M-100 Chimes
Stingray M-100 Chimes
Stars M-100 Chimes
Memory Lane M-100 Chimes
Lectronamo M-100 SB-100
Wild Fyre M-100 SB-100
Nugent M-100 SB-100
Dracula M-100 SB-100
Trident M-100 SB-100
Hot Hand M-100 SB-100
Magic M-100 SB-100
Cosmic Princess M-100 SB-100
Meteor M-200 SB-300
Galaxy M-200 SB-300 Uses relay to turn on / off general illumination
Ali M-200 SB-300 Uses relay to turn on / off general illumination
Game Title MPU Power Supply Lamp driver Sound Notes
Big Game M-200 SB-300
Seawitch M-200 SB-300
Cheetah M-200 SB-300
Quicksilver M-200 SB-300
Stargazer M-200 SB-300
Nine Ball M-200 SB-300
Iron Maiden M-200 SB-300
Viper M-200 SB-300
Dragonfist M-200 SB-300
Cue M-200 SB-300
Flight 2000 M-200 SB-300 Uses voice module VS-100
Free Fall M-200 SB-300 Uses voice module VS-100
Lightning M-200 SB-300 Uses voice module VS-100, uses interface board for bonus display in playfield
Split Second M-200 SB-300 Uses voice module VS-100
Catacomb M-200 SB-300 Uses voice module VS-100
Orbitor 1 M-200 SB-300 Uses voice module VS-100

3 Technical Info

3.1 The Bally / Stern Board Set

this is a stub

3.2 Recommended Documentation

this is a stub

3.3 The Wiring Color Code

The Bally color coding system is a carry over from the EM era. Either one color or a two color code is used for every wire. The first number is the wire's insulation color, while the second number is the trace color, if one is used. The traces on the wire can either be dashes or stripes. Either type of marking has the same coding designation. An example of a wire with the code 15, would be a red wire with a white trace. A black wire with no trace would be code 80. If a wire color is used more than once, a "-" and the "nth" time the color is used is added as a suffix to the color code. An example of this is a red wire with white traces used a second time. The code for the wire would be 15-1.

Conversely, Stern never adopted a color coding system. Instead, the wire color was marked accordingly in the associated documentation, (ie. a white wire with a blue trace is referred to as W-BLU, red with yellow is R-Y, black is just B, etc.).

Below is the Bally color chart.

1 2 3 4 5 6 7 8 9 0 Jumper
Color
 Red 
 Blue 
 Yellow 
 Green 
 White 
 Brown 
 Orange 
 Black 
 Gray 
 No Trace 
 Jumper 

3.4 Connector Designations

All Bally / Stern machines have a common naming convention for all of the connectors in the game. A specific connection uses two parts - a prefix and a suffix. The prefix is the board number, and the suffix is the connection on the board. When referencing a specific connector pin within a housing, a dash follows the connection number. For example, the connector pin for the coin door self test switch signal on the CPU board is A4J3-1. A single connection, in this case the key on a display, for the player 2 display would be 2A1-14. The same connection on a player 3 display would be 3A1-14.

The following boards are assigned the same numbers throughout Bally / Stern games.

  • Lamp Driver Board - A5
  • MPU Board - A4
  • Solenoid Driver Board - A3
  • Transformer Module / Rectifier Board - A2
  • Displays - A1


3.5 Switch Matrix

Bally / Stern 5 x 8 Switch Matrix






The Bally / Stern switch matrix is a 5 x 8 matrix consisting of a maximum of 40 switches. There are a total of 5 switch strobes, (starting with 0, ending with 4), and 8 switch returns, (starting with 0, ending with 7). The numbering convention for the switches is to start at 01, increase by 1 in the same strobe, and continue consecutively to 40. To isolate the switches, a 1N4004 diode, or in some cases a 1N4148 diode, is placed across the switch itself. Not every switch in the matrix is used on every Bally / Stern game.

Although rare, Bally did add a 6th strobe (strobe 5) for some games, such as Centaur, Medusa and Spectrum.

Connections for the switch matrix originate from the two connectors located on the right side of the CPU board. Connector A4J3 is used for all the switches on the coin door, the ball roll tilt, and the pendulum tilt. While connector A4J2 is for all of the switches on the playfield.

Strobe 0

(A4J2-1 / A4J3-2)

 Strobe 1

(A4J2-2 / A4J3-3)

 Strobe 2

(A4J2-3)

 Strobe 3

(A4J2-4)

 Strobe 4

(A4J2-5)

Strobe 5 (When Used)

(A4J4-5)

Return 0 (A4J2-8 / A4J3-9)
 01
 09
 17
 25
 33
 41
Return 1 (A4J2-9 / A4J3-10)
 02
 10
 18
 26
 34
 42
Return 2 (A4J2-10 / A4J3-11)
 03
 11
 19
 27
 35
 43
Return 3 (A4J2-11 / A4J3-12) 04
 12
 20
 28
 36
 44
Return 4 (A4J2-12 / A4J3-13)
 05
 13
 21
 29
 37
 45
Return 5 (A4J2-13 / A4J3-14)
 06
 14
 22
 30
 38
 46
Return 6 (A4J2-14 / A4J3-15)
 07
 15
 23
 31
 39
 47
Return 7 (A4J2-15 / A4J3-16)
 08
 16
 24
 32
 40
 48

It is worth mentioning that the self test switch, which is located on the inside of the coin door and used to enter tests, audits, and bookkeeping, is not part of the switch matrix. A single signal is sent to the self test switch from the U10 PIA on the CPU board via connection A4J3-1, and returned to ground via a connection A3J2-7 on the solenoid driver board.

Also, the switch commonly referred to as "switch 33" on a Bally or Stern CPU board, and used to reset audits and bookkeeping, is not part of the switch matrix either. This switch receives a signal from the 6800 CPU chip, and when closed, sends the signal to ground. Starting with the first MPU-200 Stern game, Meteor, Stern added a second switch to the inside of the coin door to serve the purpose of remotely resetting audits and bookkeeping. The signal for this secondary switch is tied to the input line of switch 33, and leaves the CPU board via connection A4J3-5. Its return line is sent to ground via a daisy-chained lead to the self test switch.

3.6 Bally and Stern MPU Boards

Bally AS-2518-17 MPU Board
Bally AS-2518-35 MPU Board
Stern M-100 MPU Board
Stern M-200 MPU Board

All Bally and Stern MPU boards are built around the Motorola 6800 series CPU chip and 6820 or 6821 peripheral interface adapters (PIAs).

32 pins at J5 of a -17 MPU
33 pins at J5 of a -35 MPU

In most cases, if a Bally MPU board has taken a bad hit from alkali damage, the bottom right corner of the board typically takes a hit. Unfortunately, the bottom right corner of the board is also the location where the board number is stenciled onto the masking. If the masking has delaminated, it will take the stenciled board number information with it. At this point, it would be difficult to determine what board it actually is.

However, there is a quick and easy way to determine whether the board is either a -17 or -35. Simply look at the J5 header pin connection located at the top center of the board. If there are only 32 pins at the connection, the board is a -17. If there are 33 pins at this connection, the board is a -35.

3.7 Rectifier Boards

Bally AS-2518-18 Rectifier Board
Early Stern TA-100 Rectifier Board
Bally AS-2518-49 rectifier board
Later Stern TA-100 Rectifier Board
Later Stern TA-100 Rectifier Board (blank)
Bally AS-2518-54 Rectifier Board
Bally AS-2518-151 Rectifier Board (Note: 6A diodes used for rectifying 12v unreg are missing from board)

The rectifier board is the first part of the power train in any Bally or Stern machine. In addition to rectifying four discrete AC voltages to DC, the rectifier board consists of the bulk of fuses for the game, including the main fuse. After all associated voltages are fused, the power is dispersed to the backbox, playfield, and lower cabinet.

++++ Add more later, explain differences between -18 and -49 (Space Invaders / Kiss) rec. boards ++++

3.7.1 How To Hook Up a Bally AS-2518-18 Rectifier Board

Often folks buy a new or used rectifier board and then when they get it, they realize that they have to hook it back up to the old wiring harness. If this is you, and you forgot to take notes or pictures before you removed your old board, then here you go. The photo below shows an original AS-2518-18 rectifier board connected to a factory wiring harness. As far as I know, the wire colors are the same for ALL AS-2518-18 applications.

Bally AS-2518-18 Rectifier Board (Solder Side)
Board Connection Wire Color / Gauge Transformer Lug Circuit
E1 Red 18 AWG 5 Primary AC Hot
E2 Yellow 18 AWG 1 Primary Neutral
E3 Red 20 AWG 2 Solenoid Bus Hot
E4 White/Red 20 AWG 6 Solenoid Bus Neutral
E5 Green 20 AWG 8 Display High Voltage Hot
E6 White/Green 20 AWG 10 Display High Voltage Neutral
E7 Blue 18 AWG (2 wires) 17 GI Bus Hot
E8 Black 18 AWG (2 wires) 18 GI Bus Neutral
E9 Orange 18 AWG 13 Controlled Lamp Bus Hot
E10 Green 18 AWG 14 Controlled Lamp Bus Neutral
E11 White 20 AWG 15 12V Input for 5-Volt Regulator Hot
E12 White/Black 20 AWG 16 12V Input for 5-Volt Regulator Neutral


AWG = American Wire Gauge (18 = fat, 20 = skinny)

"E" solder pads are labeled on the top side of the circuit board

3.7.2 How To Hook Up a Stern TA-100 Rectifier Board

Stern TA-100 Rectifier Wire Color connections





As mentioned above when you replace or remove and reattach your rectifier board you need to know how to hook it back up. I have done this with a lot of Stern pins and made myself a chart for quick and easy reference when re-attaching the rectifier to the transformer. I figured it may be useful to others as well.

3.7.3 How To Properly Mount a Bally AS-2518-54 Rectifier Board

Bally AS-2518-54 Rectifier Board
Bally AS-2518-54 Rectifier Board (back)



The Bally AS-2518-54 rectifier board normally mounts on a large metal plate found in the bottom of the pinball machine’s cabinet. It uses the metal plate as a heat sink for the two large bridge rectifiers on the underside of the board, so mounting it properly is important. Carefully following the steps illustrated below will ensure the board is correctly mounted for long term reliability.

  • Mounting a Bally -54 rectifier board
  • Locate the two mounting holes for the bridge rectifiers, circled in red, and the seven mounting holes for the circuit board, circled in yellow.
  • Practice laying the board in the proper position once, so you’re sure you know where it goes. You want the two holes that go through the bridge rectifiers to line up with the holes in the plate. Also make sure the mounting screws around the edge of the board are lining up with their holes too.
  • Prep the surface of the mounting plate by cleaning the area, where the two bridges will mount, with some alcohol or acetone.
  • Make a circle of heat sink grease around each bridge mounting screw hole approximately the diameter of a dime. Heatsink grease is available from most all the popular vendors.
  • Carefully place the rectifier board in back on the mounting board. You want to get the board positioned as best as you can without moving it around a lot or you will smear the heat sink grease all over the place. What I like to do is peek through one of the two mount holes on the board and locate the corresponding screw hole on the plate. Then as you peek through this hole, guide the board into place while keeping the screw hole in sight.
  • Double check your work, being careful not to move the board around too much. To avoid smearing the heat sink grease, it’s best if you can place it directly into the proper position the first time, without having to scoot it around afterwards.
  • Next, start the two mounting screws that go through the bridge rectifiers, but do not tighten them all the way.
  • Now locate and install the remaining seven mounting screws along the perimeter of the board, but don’t tighten them all the way yet. Since all the screws are a little loose, you have a little bit of board movement available to help locate the mounting screw holes on the plate.
  • Once all the perimeter screws are in place, tighten the two bridge rectifier screws. You want them nice and tight so it spreads the heat sink grease into a nice, evenly spread thin layer. Just be careful that you don’t strip the threads out of the mounting holes. Then tighten up the perimeter screws after that.
    All done!
  • One word of warning: The mounting screws protrude way past the bottom of the board, so be careful where you place this assembly. The screws may scratch your workspace (a nice table, countertop, etc.).


3.8 Bally and Stern Solenoid Driver Board

Bally AS-2518-16 Solenoid Driver/Regulator
Bally AS-2518-22 Solenoid Driver/Regulator
Stern SDU-100 Solenoid Driver/Regulator
Another style of the Stern SDU-100 Solenoid Driver/Regulator

First of all, everything mentioned in the following sections specifically reference the Bally AS-2518-22 solenoid driver board, found in most Bally pins from 1977 through 1985. Since the Bally AS-2518-16 and Stern SDU-100 boards are very similar, and all are identical as far as the operation of the solenoid driver circuits, the information is applicable to these boards as well. The main differences between Bally and Stern solenoid driver boards used from the factory are the physical layout of parts, and whether or not the board is single-sided or double-sided. Since the physically connector locations are the same, Bally and Stern solenoid driver boards are similar enough to be interchangeable.

Secondly, if all this is Greek and you have no idea how decoder ICs work, or what a transistor is, take a look at the How Bally Coils are Driven for Dummies section to learn the basics of how things work.

Third, the solenoid driver board actually has three functions.

  1. Drive the solenoid and relay coils of a pinball machine.
  2. Regulate the logic voltage which provides a nice and steady +5VDC to the other boards for their various logic circuits.
  3. Regulate the high voltage (+190 VDC) for the display driver boards

The regulated voltage will not be discussed here, only the solenoid drive section.

Finally, don't forget that the Solenoid Driver board contains the high voltage circuitry for the displays. There is 190 volts DC here and if you're not careful, you'll get knocked on your butt. A shock from 190 volts DC will hurt. If you don't know what you're doing, stay away from it and have a professional fix it instead. In addition to high voltages, there are static sensitive parts on these boards. While working on boards, be sure to properly ground yourself before touching the board, and always work in a static-free workspace.

3.8.1 How Bally Coils are Driven for Dummies

Typical Bally Coil (Mata Hari)


There are three main characteristics regarding the coils and coil connections in Bally games.

  • Two big fat yellow wires going to one lug.
  • The small skinny wire going to the other lug.
  • The diode connected to the two lugs.

The majority of information in this section is applicable to Stern games too. Bally and Stern solenoid driver boards are similar enough in overall design for them to interchange between games / manufacturers. The only other primary difference between Bally and Stern is the wire color of the solenoid bus line in most cases.

One lug on every coil is connected via a parallel circuit by the fatter wires. This type of connection is also commonly referred to as a daisy-chained circuit. These fat wires supply each coil with positive 43 volts DC (+43VDC), so each coil is connected to the +43VDC bus. Most have two fat wires, but some may only have one. Flipper coils have these wires too, but they are connected a little differently, and are discussed elsewhere. For now, just assume regular solenoids are being discussed here.

Each coil has a small skinny wire on the other lug. This wire goes to the control circuits on the solenoid driver board. In order to energize the coil, there must be a path to ground for the +43VDC. In its at rest state, the coil does not have a path to ground. When the small skinny wire gets connected to ground, the path is complete and current will flow. This current flow turns the coil into an electro-magnet, which then pulls the plunger into the coil. When the wire is disconnected from ground, current flow stops, the electro-magnet is turned off, and the plunger returns to it's normal position with help from either a spring or gravity.

Finally, there is a diode on each coil. When the current is quickly turned off on an energized coil, the magnetic field around the coil collapses quickly. This action causes the coil to generate a huge voltage spike. The job of the diode is to prevent the majority of this spike from reaching the solenoid driver circuity. If the diode is bad (shorted) or installed backwards, the drive transistor will be destroyed the very first time the coil is energized, then released. The process is similar to the ignition system used in older cars. When the points open, the 12 volts is removed from the car's coil quickly, which causes another coil to generate a huge voltage spike to the spark plug.

Comparison of Electrical Sine Waves


The computer program that runs the machine also tries to limit this spike by turning off the coil near the zero crossing of the AC line. This helps because the DC that drives the coils is rectified, but not filtered, so it's not smooth DC, but "humpy", as shown in the adjacent picture.

By energizing the coils just after the zero crossing, the in-rush of current caused by a coil is limited. By turning the coils off just after the zero crossing, the voltage spike caused by the collapsing field is also kept to a minimum.

Depiction of Multiple Coils in a Bally Game with Manual Switches Inserted and tied to Ground



So, in the simplest form, the solenoid driver circuits in a Bally game look like what is shown in the accompanying pic. Look at it as a bunch of coils all connected to the +43VDC bus, and the other lugs going to switches which are connected to ground. If a switch were to close, it would connect the circuit from +43VDC to ground, and the coil would energize as long as the switch is closed.

One Switch Closed and Energizing the Coil



See how the circuit is complete due to the switch being closed, and the coil is energized. When the switch is opened again, the coil turns off, and is returned to the same state as the previous picture. If the diode were not there, there would be a big arc across the switch contacts upon the moment they opened up.

Depiction of Multiple Coils in a Bally Game with Transistors Shown

Now, take this one step further, and replace the manual switches with transistors. Transistors are normally used as amplifiers, but you can also use them as switches too. There are 3 leads on a transistor, the base, the emitter, and the collector. For NPN transistors like the ones on the Bally solenoid driver, the emitter and collector can be used like a switch. With no current supplied to the base, there is no current flow between the collector and emitter - the transistor switch is open, or OFF. If current is supplied to the base, the current will then flow between the collector and emitter, so now the switch is closed, or ON.

Without getting into too much detail - what happens is a current is applied to the base which is high enough to 'saturate' the transistor. This means the collector-to-emitter current will be amplified as high as it can, and the transistor will then conduct a large amount of current from COLLECTOR to EMITTER, in relation to the current flow from the BASE to the EMITTER. This is how it acts like a switch. The base goes high to turn it on, and low to turn it off. Since the collector is connected to the wire that goes to the coil (the small single wire), and the emitter is connected to ground, turning the transistor has the effect of connecting the collector to ground. This completes the circuit to the coil and it turns on (fires).

A coil can be tested by momentarily grounding the tab on the coil's drive transistor. For the TIP-102 or similar transistors used in the Bally solenoid driver, the metal tab is connected to the collector. Knowing this, and what was just discussed, grounding the tab is the same as grounding the collector, which will complete the circuit to ground and fire the coil. However, this test only tests the wiring from the solenoid driver to the coil. It DOES NOT test the transistor itself or any circuitry before the transistor.

The transistor and "control signal" in the simplified drawing above can be replaced with the actual circuit. More details regarding this circuit are found in the following section.

3.8.2 Overview

Two circuit boards will be discussed: the MPU board (AS-2517-17 or -35) and the Solenoid Driver board (AS-2518-22 or -16). The solenoid driver gets signals from the MPU board. These signals tell the solenoid driver which solenoid to fire. Up to 15 momentary and 4 continuous solenoids can be controlled by the solenoid driver. The flipper solenoids are enabled or disabled from the solenoid driver too, but are not controlled like the other solenoids.

3.8.3 How the Solenoid Driver Works

The solenoid driver is responsible for energizing the solenoid coils of the pinball machine. Four signals from the U11 PIA integrated circuit on the MPU board travel out from the J4 to the J4 connector on the Solenoid Driver board. These four signals tell the Solenoid Driver which solenoid to fire. This is accomplished by using a decoder chip that takes the binary pattern of the four signals (16 different patterns) and decodes (or demultiplexes) them into one of sixteen different outputs. The four signals are applied to the decoder then the decoder is strobed. Normally, all sixteen of the decoder output lines are held high (+5vdc). When strobed, the decoder lowers one of it's sixteen output lines, depending on the pattern of the four input signals. Detailed info about the 74LS154 decoder chip is availalbe here.

Typical Solenoid Driver Circuit

With no input supplied (strobe is high), the output lines of the decoder are high (+5 vdc). This puts a voltage at the base of Q1 (this transistor is one of seven in the CA3081 transistor array chip). This turns Q1 "on" and the voltage supplied to it's collector via resistor R1 passes through the transistor to ground. At this point, little or no voltage is present at the base of Q2, and Q2 is "off". With Q2 off, the 40vdc at the coil has no place to go, and the coil remains deenergized.

When the MPU board supplies the proper input signals (A-B-C-D) to the decoder, and the decoder is strobed (signal drops to low), the proper output signal will go low, which turns Q1 "off" (notice one of the two strobe lines goes to ground, so it's always low). This allows the +5vdc at Q1's collector to flow through the diode instead of Q1 on it's way to ground via resistor R3. This also puts a voltage at the base of Q2 and turns this transistor "on". When Q2 turns on, the 40vdc at the solenoid now has a path to ground through Q1 and current flows through the coil, thereby energizing it. Then the strobe to the decoder is released, the decoder output goes high again, Q1 turns on, Q2 turns off, and everything is back to normal.

Diode D1, resistor R3 and capacitor C1 work to slow the speed at which Q2 and the solenoid are able to turn off. This is important to prevent the "inductive kick" voltage that builds up when you try to turn off a solenoid quickly. A solenoid coil can build up hundreds of volts if it is switched off too quickly. For example, the spark in the spark plug of a car is generated from this inductive kick when the ignition coil is turned off quickly. In this case, D1 allows Q2 and the solenoid to turn ON quickly (which is OK) because the current that used to be flowing through Q1 can now flow forward through D1 and turn on Q2 quickly. However, when the decoder output goes back to high and Q1 turns back on, D1 prevents the charge from the base of Q2 from being sucked down Q1. The charge on C1 must drain off (slowly) through R3 and the base of Q2. This takes awhile and slows the turn-off of Q2 and the solenoid COIL, thus reducing the kick. Also, as the solenoid turns off and the voltage on the collector of Q2 starts to rise, this voltage is "fed back" by C1 to the base of Q2 and tends to keep Q2 on a little longer, slowing the turn-off of the solenoid even more. The OTHER diode (D2, across the solenoid) works to absorb the solenoid's turn-off kick by conducting when the voltage on the collector of Q2 is greater than about 40 volts.

3.9 Lamp Driver Board

Bally and Stern controlled lamps are driven discretely via SCRs located on the lamp driver board. Because of this, neither manufacturer utilizes a lamp matrix. Any variation of the lamp driver boards are capable of controlling a maximum of 60 discrete lamp circuits. Typically the 2N5060 SCRs can only control one lamp, while the MCR106 / SCR106 / C106 SCRs can control up to two lamps.

  • Bally Lamp Driver Boards
  • AS-2518-14
  • AS-2518-23



  • Stern Lamp Driver Boards
  • LDA-100
  • LDA-100 Rev. B
  • LDA-100 Rev. C


3.10 Combination Solenoid / Lamp Driver Board

File:Bally AS-2518-107 Partially stripped.JPG
Bally AS-2518-107 Combination Solenoid / Lamp Driver Board (Note that the board shown is missing the 5v regulator and heat sink, the 5v filtering capacitor, and several other discrete pieces)
Placeholder for Bally AS-2518-147 Combination Solenoid / Lamp Driver Board



3.11 Miscellaneous Boards

3.11.1 Strobe Board

The Bally strobe board was used on Flash Gordon. A small transformer in the cabinet bottom provides the voltage required for this board. That power is fused by an "in-line" fuse at the small transformer.

There isn't much to go wrong with this board as it's comprised of a few diodes and resistors. Other than replacing the actual strobe lamp and the "trigger" transformer, parts are easy to find. Some folks have been successful using a strobe lamp acquired from Radio Shack. A source for the trigger transformer is unknown.

The trigger transformer should measure about 32 ohms between the pins marked 1 and 2, and about .2 ohms between the pins marked 3 and 4.

If your strobe is producing a very weak strobe, or not working at all, test the two 22uf/250V electrolytic capacitors. Simply replacing out of spec capacitors may bring the strobe back to life.

Another area for concern, as with all single sided PCBs is the header pins. Replacing these pins or simply reflowing the solder on them is a good idea as they are subject to cracked solder joints.

If working properly, the strobe will fire 4 times at game power up. It also strobes along with the controlled lamps during "all lamp" test. It obviously will strobe during game play too.

  • Flash Gordon Strobe Board Components
  • Bally Strobe Lamp Board as found on Flash Gordon (front), with new 22uf capacitors.
  • Bally Strobe Lamp Board as found on Flash Gordon (rear).
  • The "HAL-9000 like eye" strobe lamp as seen from the front of the lamp insert panel.
  • Small transformer and in-line fuse in cabinet bottom.


3.12 Displays

Here are some different types of Bally displays

  • Bally 6 and 7 digit displays
  • AS-2518-21 6-Digit Display
  • AS-2518-15 6-Digit Display
  • AS-2518-58 7-Digit Display



Here are some different Stern displays. Note that the DA-300 circuit board is deeper than any of the other display types.

  • Stern 4, 6, and 7 digit displays
  • 4-Digit Display
  • DA-100 6-Digit Display (Double-Sided)
  • DA-100 6-Digit Display (Single-Sided)
  • DA-100 6-Digit Display (Single-Sided)
  • Different DA-100 6-Digit Display (Single-Sided)
  • DA-300 7-Digit Display


3.12.1 Bally 6-Digit Displays

A healthy Bally 6-Digit Display


The following information is focused on the AS-2518-21 display. Since the AS-2518-15 display is interchangeable with the "-21" display, everything mentioned here will apply to both displays. 7-digit displays behave the same too, so this information will be helpful for those too. The only real difference is an additional digit enable signal, and some more electronics on the board to drive the 7th digit.

If you could care less displays work, but are looking for information on how to repair them, then jump to the Repairing Bally Displays article below. We'll also show you some ideas on how to keep your displays working properly.

Example of a Bally AS-2518-15 6-digit display
Example of a Bally AS-2518-21 6-digit display


The two displays are a little different in the way the components are laid out. Regardless of the differences in appearance, they both perform the same function and are interchangeable with each other. The original display (at least the one mentioned in the Power Play Owner's Manual) is the "-21". The two shown here were upgraded with 1/2 watt 100K ohm resistors. A modification that should be done on all displays.

Example of a Stern 6-digit display
Example of a Stern 6-digit display


Likewise, there are different versions of Stern 6-digit displays. These displays are interchangeable with all Bally and Stern games.

3.12.1.1 How the displays work

The way the display circuitry works is really quite interesting. Although the human eye can not detect it, at any given moment, each display is only showing one digit at a time. The program that runs on the machine's computer is changing the digits so fast that it is not visible. If you were to film the displays and play the film back in slow motion, you'd see all the displays showing the same digit, and it cycles through all six, from left to right. It just cycles so fast that your brain thinks the whole display is lit all the time.

Each display has six digits, and upon closer inspection, each digit consists of seven segments. This is important stuff to know in order to understand how these displays work, and most important, how to fix them cheaply!

There are 4 main "parts" of a display assembly:

  • The glass display itself
  • The display driver consisting of the input decoder
  • The six digit driver circuits
  • The seven segment driver circuits

There is one digit driver circuit for each of the six digits, and one segment driver circuit for each segment of a digit. How the actual glass display does what it does in order to light various digits and segments is beyond the scope of this tutorial. The decoder takes a number from 0 - 9 as input and determines which segments need to be energized in order to represent this number. The digit drivers are responsible for applying the proper voltages to the proper pins of the display to tell it which digit to light. The segment drivers are responsible for applying the proper voltage to the proper pins of the display to tell it which segments of the digit to light. It is the MPU's job to supply the proper signals to the display driver to make it do all this stuff.

The decoder is a small integrated circuit (MC14543LE) called a "BCD To Seven Segment Decoder". This decoder happens to be a "latching" decoder, which means it latches onto its inputs and keeps them, even if they are no longer applied, until the decoder is told to release them (blanked). The decoder also has an input called a strobe. When strobed, the decoder will read it's four inputs and latch on to them. A strobe signal is usually a quick off/on/off pulse. There is also an input for the blanking signal.

Labeled Segments and Truth Table


BCD stands for Binary Coded Decimal and is a fancy term for storing a number from 0 - 9 in a half byte of storage (four bits). Using BDC encoding, each byte of memory can store two digits. The input of the decoder is a BCD number from 0 - 9, and the output of the decoder is seven signals. These seven signals are either on or off, and relate to the seven segments of a digit. Each of the seven output signals go to a MPS-A42 transistor, which is part of a circuit called a segment driver. This transistor acts like a switch to turn the segments on or off. The outputs of the seven segment drivers go to the seven segment pins of the display glass. This is how the computer tells the display driver which segments to light. The MPU has a four-signal data path that goes to all five displays (or seven for Six Million Dollar Man). These four signals provide the 4-bit input into the decoder, and remember, all four signals go to ALL of the displays. Please see the diagram of how the segments are labeled, and a truth table showing the 15 possible inputs and outputs to the decoder. For those of you that don't know about binary arithmetic, you can get 15 possible combinations of on/off with 4 digits. (e.g., "0000", "0001", "0010", ..., "0111", "1111"). This is also how you count in binary, or base-2. Remember that the display driver is only interested in 10 of the 15 possible combinations, the ones that represent the numbers 0 - 9, or "0000" - "1001". Any other input combinations will result in unpredictable outputs from the decoder, so we label these as "don't cares", since we know they will never happen under normal circumstances.

The digit driver circuit consists of an MPS-A42 transistor and a 2N5401 transistor connected in a circuit that acts as a switch. Normally with no input signal applied, the switch is off, keeping the high voltage supplied by the HV Regulator away from the display. There are 6 digit signals provided by the MPU, one for each digit. The MPU will enable one signal at a time, telling the display driver which digit to operate. This signal will then turn on the "switch" for the digit, allowing the high voltage a connection to the proper pins of the glass display to energize the desired digit. These signals from the MPU are simply 6 wires and the MPU will activate one of them at a time. Like the segment signals, the six digit signals go to all of the displays in a daisy chain fashion.

Before explaining how the computer makes all this work, let's summarize: A display driver has four inputs from the MPU. The first is a collection of six signals used to tell the display driver which digit to energize (six digits in the display, six digit signals). Only one of these signals are on at a time. The second input is a collection of four signals that tell the display driver which number to display, in the form of segments. These four signals provide a binary pattern that is interpreted as a binary number from 0 to 9. The third input is a strobe signal, which tells the decoder to read it's inputs, and the fourth signal is a blanking signal, which tells the decoder to turn off all it's outputs. Also, but not mentioned above, are various voltages from the power supply and high-voltage regulator. Each display driver is supplied with +5VDC from the 5-volt regulator on the solenoid driver board. This is used to drive the logic circuits of the decoder. There is also +190VDC* applied to the display driver from the high-voltage regulator on the solenoid board. Finally there is a connection to ground, which brings all the voltages to the proper reference point.

*For brand new displays, this voltage should be at +190VDC in order to "burn in" the display. Once a display has become used, this voltage may be backed down to +170VDC, which will work just fine and will help prolong the life of the display.

3.12.1.2 How The Computer Controls The Display

The next thing to understand is how the computer in the pinball machine operates the displays. For this section, referenced MPU board will be the AS-2518-17 module, but all this also holds true for the "-35" MPU module as well. And again, the Bally AS-2518-21 and AS-2518-15 Display Drivers will be referenced. The Stern style display drivers operate the same, and the six digit Stern drivers are 100% swappable with the Bally displays. There are a few minor differences between the seven digit varieties of displays preventing a direct swap without modifications.

Display Signals from MPU Board Illustrated




As mentioned above, there are four sets of signal lines that go from the MPU module to the display driver modules. The first set is the BCD data set which carry the display segment BCD data to all the displays on 4 wires. They leave the MPU module (A4) at connector J1, pins 25-28 and visit every display driver module at connector J1 and pins 16-19 (D4 - D0). The next set of signals is the digit enable signals. These 6 wires carry signals to all the display driver modules with information telling it which digit to light. The third set of signals are 5 latch strobe signals. There is one separate signal for each display driver, and it is the signal that tells the driver's decoder to read the decoder inputs, and output the proper segment signals. The final signal is a single single that goes to all the display drivers called Display Blanking. The signal tells the display driver's decoder to turn off all segment outputs, thereby blanking out the display, or turning all segments off.

Once the machine has been turned on and has booted up, the processor on the MPU module is continuously running a program that is stored in the module's ROM chip(s). This program is responsible for controlling the game by reading all the switches, lighting all the lamps, activating all the solenoids, and controlling the displays. The program keeps a lot of information in RAM and uses this information to keep track of scores, switches, etc. An interrupt is a term for a section of computer program that interrupts the "main" program in order to execute a smaller program, sometimes referred to as a "service routine". We won't get into just how this actually happens, just be aware that the main program of a computer may be interrupted at any given time. And to make things even more complicated, interrupts themselves can be interrupted by higher priority interrupt service routines. There may be several different interrupts that occur in a pinball's computer program, but the one we want to study is the one that controls the displays. Keep in mind what was mentioned above, that at any given instant, only one digit is lit on any display. This is called multiplexing.

320 times a second, or once every 3-1/4 milliseconds (thousands of a second), the CPU is interrupted to service the displays. In memory, the CPU keeps track of all the information it needs to operate all the displays. This information includes a counter used to indicate which display digit is active, the BCD data for all the displays, etc. Here's what the display service routine actually does:

  • Determine which digit was updated last time - The MPU looks at the digit counter and adds 1 to this value. If the new value is 7, it is changes to 1, then the new value is stored back into memory. Let's assume the new value is 4, so we're going to update the 4th digit.
  • Blank out all the displays - The CPU raises the signal on the Blanking Line which causes all displays to go blank (the blanking signal tells all the decoders to turn off their segment outputs).
  • Fetch the BCD data from memory - The BCD data for the first display, 4th digit is fetched from memory.
  • Send the BCD data to the display driver - This BCD data is placed on the BCD data bus and display #1 is strobed. This will cause the display's decoder to latch onto the input signals (store them for future use).
  • Do it again - The previous two steps are repeated for the second, third, fourth, and fifth display.
  • Enable the digit - The MPU then enables the 4th digit and disables the other 5 digits by raising and lowering signals on the Digit Enable lines.
  • Finally, turn the digit on - The MPU lowers the signal on the Blanking Line, which causes the all of the decoders to output their proper segment signals and the 4th digit on each display is displayed.
  • All done! - The interrupt service routine then exits and control returns to the main program

As you can see, the display interrupt service routine only handled 1 digit for all displays. Every time it is invoked, it will process the "next" digit, resetting the counter back to 1 when necessary. The process of updating 1 digit for all displays takes about 500 microseconds, or 1/2 of a millisecond, to complete. Pretty cool, eh?

So, lets do some math. It takes 1/2 millisecond to update one digit, and since there are 6 digits, it takes 3 milliseconds to display all six digits. Since the interrupt runs 320 times a second, and it takes 6 interrupts to update the entire display, dividing 320 by 6 means that the displays are completely updated just over 54 times every second. That's fast enough to fool your eyes and brain into thinking the display is completely lit all the time. Also, since the interrupt routine takes about 1/2 millisecond to run, and it runs 320 times every second, that means about 160 milliseconds of every second of time is spent updating the displays, which is about 16 percent of the time.

3.13 Bally Auxiliary Lamp Driver Boards

Bally AS-2518-43 Aux. Lamp Driver Board
Midway A084-91614-A000 Aux. Lamp Driver Board - same as AS-2518-43)


To increase the total of controlled lamps used in a particular game, Bally implemented auxiliary lamp driver boards. These aux. lamp driver boards were either used for effect (chaser or infinity lamps used between backglasses in games such as Xenon or Space Invaders), additional controlled insert lamps, or both (top rollover insert lamps and Queen's Chamber chaser lamps used on Centaur).

There are essentially only two different boards used in games. The first is an AS-4518-43. This is the smaller footprint board, which has only one output connection. It is capable of driving 12 discrete circuits or a total of 24 lamps (12 sets of 2 lamps).

Outputs for AS-4518-43 Aux. Lamp Driver Board
J2 Header 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Output
  Q1  
  Q2  
  Q3  
  KEY  
  Q4  
  Q5  
  Q6  
  GND  
  GND  
  --  
  Q7  
  Q8  
  --  
  --  
  --  
  --  
  Q9  
  Q12  
  Q11  
  Q10  


Bally AS-2518-52 Aux. Lamp Driver Board



The second board is the AS-4518-52. This board is narrower, yet longer in construction. It is capable of driving 28 discrete circuits or a total of 56 lamps (28 sets of 2 lamps). Either aux. lamp driver board receives the same inputs from the CPU board. Likewise, these signals are identical to the signals sent to the primary lamp driver board. The only differing signal between the primary lamp driver board and either aux. lamp driver board is the lamp strobe. The primary lamp driver board receives lamp strobe 1, while the aux. lamp driver boards receive lamp strobe 2.

Outputs for AS-4518-52 Aux. Lamp Driver Board
J2 Header 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 -- --
Output
  --  
  --  
  KEY  
  Q2  
  Q7  
  Q6  
  Q1  
  Q3  
  Q5  
  Q4  
  Q9  
  Q14  
  Q13  
  Q8  
  Q10  
  --  
  Q12  
  Q11  
  --  
  --  
J3 Header 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Output
  --  
  KEY  
  Q16  
  Q21  
  GND  
  GND  
  Q20  
  Q15  
  Q17  
  Q19  
  Q18  
  Q23  
  Q28  
  Q27  
  Q22  
  Q24  
  Q26  
  Q25  
  GND  
  GND  


3.14 Bally Sound Boards

3.14.1 Bally AS-2518-32 Sound Board

Bally AS-2518-32 Sound Board


3.14.1.1 -32 Sound Board ROMs & Jumper Settings
Game U3 ROM Jumper Settings
Lost World  E729-18   B 
Six Million Dollar Man  E729-18   B 
Playboy  E729-18   B 
Voltan Escapes Cosmic Doom ? ?
Supersonic  E729-18   B 

Source: Bally / Midway 1982 parts catalog.

3.14.2 Bally AS-2518-50 Sound Board

Bally AS-2518-50 Sound Board


3.14.2.1 -50 Sound Board ROM & Jumper Settings
Game U3 ROM Jumper Settings
Star Trek  E729-18   B, D 
Kiss  E729-18   B, D 
Paragon  E729-51   B, C 
Harlem Globetrotters  E729-51   B, C 
Dolly Parton  E729-51   B, C 

Source: Bally / Midway 1982 parts catalog.

3.14.3 Bally AS-2518-51 Sound Board

Bally AS-2518-51 Sound Board


3.14.3.1 -51 Sound Board ROMs & Jumper Settings

Jumper A is used when U3 is a 6802 and U10 (6810) is not used.
Jumper B is used when U3 is a 6808 and U10 (6810) is used.
Jumper C is used when U4 is 2K in size (9316B or 2716).
Jumper D is used when U4 is 4K in size (2532 or 4732).


Game U4 ROM Jumper Settings
Future Spa  E781-2, E781-5 (or sub with E781-13)   B, C 
Nitro Groundshaker  E776-14, E776-15   B, D 
Silverball Mania  E786-8, E786-11   B, C 
Space Invaders  E792-2, E792-7   B, C 
Rolling Stones  E796-11, E796-19   B, C 
Mystic  E798-2, E798-5   B, C 
Hot Doggin  E809-2, E809-7   B, C 
Viking  E802-2, E802-7   B, C 
Skateball  E823-2, E823-14   B, C 
Frontier  E819-2, E819-9   B, C 
Speakeasy  E877-1   B, C 
BMX  E888-02   B, D 

Source: Bally / Midway 1982 parts catalog.

3.14.4 Sounds Plus Sound Board (AS-2518-56)

Placeholder for Bally AS-2518-56 Sounds Plus Board



The Sounds Plus board was only used in Xenon and very early Flash Gordons. The Sounds Plus board has a daughter card called the Vocalizer that contains the roms and circuitry needed to generate speech.

3.14.5 Vocalizer Speech Board (AS-2518-57)

Placeholder for Bally AS-2518-57 Vocalizer Board



The Vocalizer board was only used in Xenon and very early Flash Gordons. It is a complimentary board to the AS-2518-56 Sounds Plus board, and contains the numerous roms needed for an extensive speech vocabulary.

3.14.6 Squawk & Talk Sound Board (AS-2518-61)

The Squawk & Talk (S&T) sound board was the first to offer sound and speech in one board to Bally games. It is probably the most intuitive sound board Bally offered too. Similar to the Bally MPU boards, the S&T uses an LED flashing system during board initialization. There are at least 3 variations of the board: -61, -61a, and -61b.

Some variations of this board have a J3 header connection located at the top upper left edge of the board. This connection was never implemented. Likewise, some boards are populated with a 2 x 20 pin .100" header connection located on the left edge of the board. This connection is marked on the schematics for the use of a vocalizer, which was never implemented.

All of the different variations of this board should work on all games. The exceptions are Centaur and Centaur II. These two games must use the -61b board. The reason for this are the extra input / output connections at J2 (bottom right of board) needed for connection to the Say It Again board. A standard -61 or -61a has a total of 6 header pins at J2, while the -61b has a total of 10 header pins.

  • 3 Revisions of the AS-2518-61 Squawk and Talk Sound Board
  • AS-2518-61 Squawk & Talk Sound Board
  • AS-2518-61a Squawk & Talk Sound Board
  • AS-2518-61b Squawk & Talk Sound Board. Note expanded connector at J2 (lower right).



3.14.6.1 Squawk & Talk Jumper Settings
Bally Squawk & Talk Jumper Locations. Note that the -61B revision of the board contains a jumper FF located within the stack of resistors below the AD558. Neither the -61 or the -61a revision has this jumper.



The first table below is a list of the Squawk & Talk jumper settings by game as specified in the 1982 Bally / Midway parts catalog. These jumpers reflect the original ROM configuration when the game was released. If you've changed your sound ROMs to 2716s or from 2532s to 2732s, see the second table below.

  • If using a 6808 CPU chip with a 6810 chip, install L, remove K
  • If using a 6802 CPU chip, install K, remove L
  • The 5 EE jumpers must be installed when the AY-8912 chip is not present.
  • The -61B revision of the board has an FF jumper, located in the stack of resistors below the AD558. This jumper should be IN if the "Say it again" board (reverb) is not used or connected. With the "Say it again" board in use, jumper FF should be OUT.
Game Jumper Settings
Flash Gordon  C, D, E, G, Q, S, U, W, Y, AA, H, DD, N, and L or K 
Eight Ball Deluxe  C, D, E, G, Q, S, U, X, Y, AA, H, DD, N, and L or K 
Fireball II  C, D, E, G, Q, S, U, X, Y, AA, H, DD, N, and L or K 
Embryon  C, D, E, G, Q, S, U, X, Y, AA, H, DD, N, and L or K 
Fathom  C, D, E, G, Q, S, U, W, Y, AA, H, DD, N, and L or K 
Medusa  C, E, G, Q, S, U, W, Y, BB, D, H, DD, N, EE, and L or K 
Elektra  C, E, G, Q, S, U, W, Y, BB, D, H, DD, N, EE, and L or K 
Centaur  C, E, G, Q, S, U, W, Y, BB, D, H, DD, N, EE, FF (if no SIA board), and L or K 
Rapid Fire  C, E, G, Q, S, U, W, Y, AA, H, EE, CC, M 
Mr. & Mrs. Pac-Man  C, D, E, G, Q, S, W, Y, U, AA, H, DD, N, and L or K 
Spectrum  C, D, E, G, Q, S, U, W, Y, BB, H, DD, N, EE, and L or K 
Vector  C, D, E, G, Q, S, U, W, Y, AA, H, DD, N, and L or K 

Source: Bally / Midway 1982 parts catalog.

ROM Size/Location      U2         U3         U4         U5    
 2716
D, P
Q, T
U, X
Y, BB
 2532
D, G
Q, S
U, W
Y, AA
 2732
F, P
R, T
V, X
Z, BB


3.14.7 Say It Again Board

File:AS-2518-81-2.jpg
Bally AS-2518-81 Say It Again Sound Board


The "Say It Again" sound board is a board used to add reverb to a Squawk & Talk sound and speech board. This board was only used on Centaur and Centaur II.

3.14.8 Cheap Squeak Sound Board

Bally Cheap Squeak Sound Board A080-91603-XXXX or Midway P/N M-051-00114-XXXX

The Cheap Squeak was designed as a lesser expensive sound board. It only utilizes a 6803 microprocessor, which allows it to function without 6821 PIAs and external ram memory. This sound board is only capable of simple tones and sounds and no speech.

The following -35 games use the Cheap Squeak:

  • Black Pyramid
  • Cybernaut
  • Fireball Classic
  • Kings Of Steel
  • Spy Hunter
  • X's & O's

Lady Luck, a 6803 based game, also uses this sound board.

At power up, the Cheap Squeak's LED will flicker briefly, then flash, flash again, then turn on and stay on. Once the LED stays on, it seems to turn itself off for certain sounds, and then turn back on. Likewise, it appears to idle with the LED on, when no sounds are playing. The LED May turn on and off when sounds are playing or idle, but this is not always the case. With some games (Kings of Steel, X's and O's, and Black Pyramid), after the sound board successfully boots, the LED remains on.

There are differing results when pressing the self test button depending on the sound ROMs. For example, pressing the self test with Black Pyramid ROMs results in a differing tone played every time. While pressing self test with Kings of Steel or X's and O's ROMs, causes the sound board to play the same sound each time.

Board Theory of Operations
The 6803 (U1) microprocessor multiplexes A0-A7 with D0-D7, calling those signals AD0-AD7. The processor fetches information from the sound ROMs (both code to execute and sound clips) by placing address information on AD0-AD7 and strobing the processor's AS (address strobe) signal to the 74LS373 (U2), thereby latching the lower 8 bits of the address bus in the LS373. A8-A15 are used along with jumpers JW1 through JW12, to implement a memory mapped I/O scheme to address the 2 sound ROMs which can be 2532s, 2732s, or 2764s. A14 and A15 control the 74LS10 (a triple 3-input nand gate) to assert device selects to the ROMs.

Note that the processor does not use the traditional R/W signal as it never "writes" to memory. Besides placing instruction address and data on the address and data busses, the processor reads sound selects via P20-P24 and writes sound data to the DAC via P10-P17. Think of these lines as PA1-PA7 of a 6821 or 6532.

The 6803 is initialized by the MPU at power up into 6803 mode N. Once initialized and running, the sound ROM code running in the 6803 accepts sound signal commands and merely reads pre-formatted sound "clips" from the sound ROMs and then writes the data to the ZN429 (U6) digital-to-analog converter (DAC) 8 bits at a time. The DAC converts the digital data to an analog level which is presented to the amplifiers for output to the speaker(s).

The board creates 5VDC on board by regulating 12VDC down to 5VDC. Unregulated 12VDC enters the board at J1-10. It is filtered by C8, C9, and C10. The inductor at L1 smooths the voltage somewhat. D6 (VR332, equivalent to a 1N5402), D7, and D8 drop the voltage by .5 - .7 volts (normal voltage drop across a diode). The 7805 at U9 further regulates the voltage down to 5VDC which can be measured at TP2 (TP3 is ground).

This 5VDC is used as a reference voltage by the amplifiers as well as to power the TTL logic ICs. The ZN429 DAC also uses this 5VDC as a voltage reference. To prevent the sound volume from fluctuating over the range of operating temperatures, the reference voltage is held constant by a "voltage divider biasing circuit" comprised of resistors at R22, R23, and R24, and a 2N5305 NPN transistor at Q7. This reference voltage is presented at pin 5 of the DAC.

Test Points

  • TP1 should measure about 11VDC.
  • TP2 is 5VDC.
  • TP3 is ground.
  • TP4 is the clock signal, provided externally by the 6803 for the purpose of synchronizing address and data read cycles.
  • TP5 is the reset signal, which is also present on pin 6 of the 6803.
3.14.8.1 Cheap Squeak ROMs & Jumper Settings
Bally Cheap Squeak Sound Board with Jumpers Highlighted



 General EPROM Configuration   Jumpers Connected 
 U3 & U4 using 2532s  J6, J9, J12
 U3 & U4 using 2732s  J7, J10, J11
 U3 ONLY using 2764 (works only for Cybernaut and X's & O's)   J2, J4, J7, J11


Sound ROM Configuration and Behavior by Game

 Game   EPROM Configuration   Jumpers Connected   LED Boot Behavior   SW1 Tune 
 Cybernaut   U3 - 2764, U4 - nu   J2, J4, J7, J11   Flicker, Blink, Blink, On   Space Ship Landing 
 X's & O's   U3 - 2764, U4 - nu   J2, J4, J7, J11   Flicker, Blink, Blink, On   Single Chime that decays 

After pressing the test switch (SW1), a sound is played, and then the board reboots itself.
Note: Combining 2732 images into a single 2764 does not work for Black Pyramid, Fireball Classic, Kings Of Steel, or Spy Hunter.

3.14.9 Bally Sound Board Pinouts

Pin Connection AS-2518-32 AS-2518-50 AS-2518-51 AS-2518-56
Sounds Plus 		AS-2518-61
Squawk & Talk 		AS-2518-61A
Squawk & Talk 		AS-2518-61B
Squawk & Talk 		A080-91603-C000
Cheap Squeak
J1-1 Sol. Address A Sol. Address A Sol. Address A Sol. Address A Sound Select Sound Select Sound Select Sound Select
J1-2 Sol. Address B Sol. Address B Sol. Address B Sol. Address B Sound Select Sound Select Sound Select Sound Select
J1-3 Sol. Address C Sol. Address C Sol. Address C Sol. Address C Sound Select Sound Select Sound Select Sound Select
J1-4 Sol. Address D Sol. Address D Sol. Address D Sol. Address D Sound Select Sound Select Sound Select Sound Select
J1-5 +5VDC +5VDC +5VDC +5VDC N/U N/U N/U N/U
J1-6 Ground Ground Ground Ground Ground Ground Ground Logic Ground
J1-7 N/U N/U N/U N/U Gen. Ill. Bus (6VAC) Gen. Ill. Bus (6VAC) Gen. Ill. Bus (6VAC) N/U
J1-8 Sol. Bank Select Sol. Bank Select Sound Interrupt Sound Interrupt Sound Interrupt Sound Interrupt Sound Interrupt Sound Interrupt
J1-9 +43V +43V N/U N/U N/U N/U N/U N/U
J1-10 N/U N/U +12V Unregulated +12V Unregulated +12V Unregulated +12V Unregulated +12V Unregulated +12V Unregulated
J1-11 Key Key Key Key Key Key Key Key
J1-12 Sol. Address E Sol. Address E Sol. Address E Sol. Address E Sound Select Sound Select Sound Select N/U
J1-13 N/U N/U N/U (Spare Address Line) N/U (Spare Address Line) N/U N/U N/U Earth Ground
J1-14 Ground Ground Ground N/U (Ground) Ground Ground Ground Logic Ground
J1-15 +43V Return (Sol. Ground) +43V Return (Sol. Ground) +12V Unregulated Return +12V Unregulated Return +12V Unregulated Return +12V Unregulated Return +12V Unregulated Return +12V Unregulated Return
J1-16 n/a n/a n/a n/a n/a +12V Unregulated Return +12V Unregulated Return n/a
J1-17 n/a n/a n/a n/a n/a +12V Unregulated +12V Unregulated n/a
J1-18 n/a n/a n/a n/a n/a N/U N/U n/a
Pin Connection AS-2518-32 AS-2518-50 AS-2518-51 AS-2518-56
Sounds Plus 		AS-2518-61
Squawk & Talk 		AS-2518-61A
Squawk & Talk 		AS-2518-61B
Squawk & Talk 		A080-91603-C000
Cheap Squeak
J2-1 Speaker + Speaker + Speaker - Speaker - Speaker - Speaker - Speaker - Speaker -
J2-2 Speaker - Speaker - Speaker + Speaker + Speaker + Speaker + Speaker + Speaker +
J2-3 n/a n/a n/a n/a Key Key Key n/a
J2-4 n/a n/a n/a n/a Remote Volume Return Remote Volume Return Remote Volume Return n/a
J2-5 n/a n/a n/a n/a Speech Volume Speech Volume Speech Volume n/a
J2-6 n/a n/a n/a n/a Sound Volume Sound Volume Sound Volume n/a
J2-7 n/a n/a n/a n/a n/a n/a Reverb Audio In n/a
J2-8 n/a n/a n/a n/a n/a n/a Shield Ground n/a
J2-9 n/a n/a n/a n/a n/a n/a Audio Out n/a
J2-10 n/a n/a n/a n/a n/a n/a Shield Ground n/a
Pin Connection AS-2518-32 AS-2518-50 AS-2518-51 AS-2518-56
Sounds Plus 		AS-2518-61
Squawk & Talk 		AS-2518-61A
Squawk & Talk 		AS-2518-61B
Squawk & Talk 		A080-91603-C000
Cheap Squeak
J3-1 n/a n/a n/a n/a n/a n/a Ground n/a
J3-2 n/a n/a n/a n/a n/a n/a Data n/a
J3-3 n/a n/a n/a n/a n/a n/a Clock n/a
J3-4 n/a n/a n/a n/a n/a n/a +5VDC n/a
J3-5 n/a n/a n/a n/a n/a n/a Key n/a
J3-6 n/a n/a n/a n/a n/a n/a PB3 n/a
J3-7 n/a n/a n/a n/a n/a n/a PB2 n/a

3.15 Stern Sound Boards

3.15.1 Stern SB-100 Sound Board

Stern SB-100 Sound Board - 1st Generation



The first generation SB-100 sound board was used on the earliest Stern games with electronic sound. These boards were fully populated, because in addition to electronic sounds, there was a (simulated) chimes option (typically dipswitch setting 23 on M-100 MPU boards). Although they will have higher pitched sounds, the first generation boards are compatible with all Stern games using the M-100 MPU board and electronic sounds. With some modifications, this board can be made to sound like the lower pitched 3rd generation board.

Stern SB-100 Sound Board - 2nd Generation (less populated)



The second generation SB-100 sound board was used on Stern games with electronic sound and no chime sound option (typically dipswitch setting 23 on M-100 MPU boards). Due to the lack of the optional chime sounds option, this board is less populated. Although they will have higher pitched sounds, the second generation boards are compatible with all Stern games using the M-100 MPU board and electronic sounds, which did not use the chime option. With some modifications, this board can be made to sound like the lower pitched third generation board.

Stern SB-100 Sound Board - 3rd Generation (C-1 - less populated)


The third generation SB-100 sound board was used on some of the last Stern M-100 games with electronic sound and no chime sound option. Like the second generation board, due to the lack of the optional chime sounds, this board is less populated. The difference with this board and the second generation board is that the component markings and through holes have been removed where components are no longer used. Although they will have lower pitched sounds, the third generation boards are compatible with all Stern games using the M-100 MPU board, which did not use the chime option. This board also shipped in the "standard" green PCB variety. This board is easily identified by the large "C-1" notation. Some test points are located in different positions compared to the first two generations, and TP9 is omitted.

3.15.2 Stern SB-300 Sound Board

Stern SB-300 Sound Board



The Stern SB-300 sound board was used in all games which used the M-200 MPU board.

3.15.2.1 Stern VSU-100 Speech Board
Placeholder for Stern VSU-100 Speech Board



The Stern VSU-100 speech board was used as a complimentary board to the SB-300 sound board. In other words, Stern never designed a sound board which was capable of both sound and speech.

3.16 Stern MA-100 ROM Board

Stern MA-100 ROM Board


For the first Stern solid state game, Pinball, a ROM board was initially used. This board was mounted to the backbox and a harness connected the board to the MPU board at the top connector J5.

3.17 Jumper/ROM Info

Jumpers are small zero ohm resistors or pieces of bare or insulated wire used to connect various points of an MPU board together to allow the board to utilize varying sizes and types of ROM/EPROM chips. Many of the logic gate chips on the MPU boards are used to steer address signals to the proper pins on the various U1-U6 chip sockets. If you have changed the jumpers and you cannot make the board boot with known good working roms, it is possible you have an addressing problem caused by one of the logic gate support chips. Always double check your jumper work by following the trace or schematic to the next point after the jumper; i.e. do not just measure continuity across the jumper. Go to the next point where the jumper connects and test between those points.

Be careful soldering to the pads, as they can be fragile. It is best to remove all the solder from the pad and insert the jumper fully into the via rather than just solder to the top. Soldering the jumper top and bottom will help with any cracked vias. Solder the jumpers to the top of the board where the jumper silk screening is printed rather than the back so that you can easily spot what type of rom chips the board is jumpered for.

All software for Bally/Stern boards can be reformatted into a 2x2732 configuration. It helps to understand how the original software is formatted to understand how to combine and pad the software to get different formats. Any board jumpered to take 2x2732 has its address ranges of U2 as $1000-$17FF (lower half) $5000-$57FF (upper half). The U6 ranges are $1800-$1FFF (lower half) and $5800-$5FFF (upper half).

If the game was originally 4x2716, U1=$1000-$17FF, U2=$5000-$57FF, U5=$1800-$1FFF, and U6=$5800-$5FFF. Thus, to combine this setup to work on a 2x2732 configuration, you would combine U1+U2 into U2.732 and U5+U6 into U6.732 using either your burner software, or the DOS command copy /b u1.716 + u2.716 u2.732.

If you need to use 2716 eproms but don't have any, you can use a 2732 eprom in its place by duplicating the data. Either use the DOS command copy /b rom.716 + rom.716 rom.732 or load the data twice into your burner; depending on its software, you might change the buffer address by $800 (so the duplicate data loads after the first segment) or it might ask if you want to append the data to the existing buffer.

3.17.1 Bally AS-2518-35 or -133 Jumper Info

The following is a list or various ROM and EPROM combinations that can be used with the Bally -35 or -133 board and the associated jumpers and jumper cuts. The letter "E" has been eliminated on every jumper number after the first to save space. 9332 ROMS are the "Black" masked ROMS originally supplied by the factory and can be replaced with 2532 EPROMS without any modification to the original board, provided that board has not been altered.

U1 ROM U2 ROM U6 ROM MPU Jumpers in Numeric Order Jumper CUTS
9316 9316 9316 E1-4,2-6,7-8,9-11,12-36,13-15,16a-19,31-32,33-34
2716 74S474 74S474 E1-3,2-6,9-11,12-36,13-15,16a-18,31-32,33-35
2532 or 9332 2532 or 9332 E4-12, 7-8, 10-11, 13a-14, 16a-34, 29-33, 31-32 E13-15
2532 or 9332 2732 E4-12,7-8,10-11,13a-14,16a-29,31-32,33-35 E13-15
9316 9316 E2-6,7-8,9-11,12-36,13-15,16a-19,31-32,33-34
9316 9316 2716 E1-4,2-6,7-8,9-11,12-36,13a-19,16a-18,31-32,33-35 E13-15
2716 9316 E1-5,2-4,7-8,10-12,11-29,13a-14,16a-19,31-32,33-34 E13-15
2716 2716 E1-5,2-4,7-8,10-12,11-29,13a-14,16a-18,31-32,33-35 E13-15
2716 2716 2716 E1-5,2-4,7-8,10-12,11-25,13a-14,16a-18,31-32,33-35 E13-15
2716 2716 9316 E1-5,2-4,7-8,10-12,11-25,13a-14,16a-19,31-32,33-34 E13-15
2716 2716 2532 or 9332 E1-5,2-4,7-8,10-12,11-25,13a-14,16a-34,29-33,31-32 E13-15
2732 2716 E4-13a, 7-8, 10-11, 12-GND, 16a-18, 31-32, 33-35 E13-15
2732 2732 E4-13a, 7-8, 10-11, 12-GND, 16a-29, 31-32, 33-35 E13-15
U1 ROM U2 ROM U6 ROM MPU Jumpers in Numeric Order Jumper CUTS

The -133 board was used in 3 games: Baby Pacman, Granny and the Gators, and Grand Slam. If you want to use a -133 board in a game that uses another board, you must change CR52 (a 1n4148 diode near J4) to a 2k resistor. If you want to use a -35 board in a game that requires a -133, change resistor R113 to a 1n4148 diode, banded end farthest away from J4. IMPORTANT: Before plugging the board in, measure the voltage at the MPU J4-15. If it is 6.3 VAC, this game requires a -133. If it is 43 VDC, the game requires the -35 configuration.

3.17.2 Bally AS-2518-17 and Stern MPU-100 Jumper Info

The following is a list of various ROM and EPROM combinations that can be used with the Bally -17 or STERN MPU-100 board and the associated jumpers and jumper cuts. The letter "E" has been eliminated on every jumper number after the first to save space. '9316 ROMS are the "Black" masked ROMS originally supplied by the factory. The "-DASH" between numbers means to jump one point to the other, e.g. E 1-2 means jump E1 to E2. Some jumps are very close to each other, others are much farther away. Some cuts in the traces must also be made on the solder or component side of the MPU board, so read the NOTES section below carefully.

U1 ROM U2 ROM U6 ROM MPU Jumpers in Numeric Order Jumper CUTS
9316 9316 E1-2, 3-4, 6-7, 8-10 Usual Factory Setting
2716 9316 E1-2, 3-4, 6-7, 8-10 Cut & Jumper See Note A
2716 2716 E1-2, 3-4, 6-7, 8-10 Cut & Jumper See Note B
2732 NONE E6-7, 8-10 Cut & Jumper See Note C
2732 2732 E1-2, 3-5, 6-7, 8-10 Cut & Jumper See Note D
74S474 74S474 2716 or 9316 1-2, 3-4, 8-9 Factory Setting for Freedom or Night Rider ONLY
2716 2716 See Note E Freedom/Night Rider ONLY
U1 ROM U2 ROM U6 ROM MPU Jumpers in Numeric Order Jumper CUTS
3.17.2.1 Notes to above Cut and Jumper Section

If a jumper combination is NOT listed in the chart above, this jumper must be cut or removed! Double check your work and verify that someone else has not modified the board before you begin the process. Incorrect jumper combinations will prevent the MPU from booting, showing a locked on MPU LED. This is a very common fault.

Note A, 2716 in U2, 9316 in U6
In addition to the jumpers listed above, you must also make the following cuts and jumps to use this configuration.

  1. On the component side of the -17 or MPU-100 board, cut the trace that runs from U2 pin 18 to U3 pin 18. Do this where the trace passes between sockets U2 and U3.
  2. On the solder side of the -17 board, run a jumper from U2 pin 18, to U17 pin 11.
  3. On the solder side of the -17 board, cut the trace going to U2 pin 21.
  4. On the solder side of the -17 board, run a jumper from U2 pin 21 to U2 pin 24.

Note B, 2716 in U2, 2716 in U6
In addition to the jumpers listed above, you must also make the following cuts and jumps to use this configuration.

Make sure jumpers E1-2, E3-4, E6-7, and E8-10 are in place.

  1. On the solder side of the -17 board, cut the trace leading to U18 pin 4.
  2. On the solder side of the -17 board, connect U18 pin 5 to the trace you cut leading to U18 pin 4. It's easiest to run the wire from U18 pin 5 to the via ("trace thru dot") that connects to this trace.
  3. On the solder side of the board, cut the trace leading to U2 pin 21.
  4. On the solder side of the board, cut the trace leading to U6 pin 21.
  5. On the solder side of the board, jumper from U2 pin 21 to U2 pin 24.
  6. On the solder side of the board, jumper from U6 pin 21 to U6 pin 24.

Note C, Single 2732 at U2 for the early 1977 to 1979 games.
This modification combines the two original 9316 ROMs at U2 and U6 into a single 2732 EPROM at location U2. This configuration works for the following games only.

  • Black Jack
  • Bobby Orr Power Play
  • Eight Ball
  • Evel Knievel
  • Mata Hari
  • Night Rider

To combine the original 9316 (or 2716) U2 and U6 ROM images into a single 2732 U2 image, use this DOS command:

COPY /B U2ROM.716 + U6ROM.716 U2COMBO.732

Make sure to use the "/b" switch when copying, as shown above. The command combines the two files into a single binary file.

Add the following cuts and jumpers to the -17 MPU to use this configuration.

Reference the left picture.

  1. Verify that jumpers E6-E7 and E8-E10 are in place. Remove any jumpers at E1, E2, E3, E4, or E5. That is, remove all jumpers except E6-E7 and E8-E10.

Reference the center picture.

  1. On the solder side of the -17 board, find U18 pin 4. Cut the trace close to pin 4 as shown below. In the picture below, a blue/black dot marks the cut made with a Dremel "ball cutter" bit.
  2. Scrape the solder mask from the trace leading to pin 4.
  3. Tack solder one end of a short length of wire (or resistor lead) to the bare trace. Solder the other end to U18 pin 5. This connects U18 pin 5 to U2 pin 18. You may choose an alternate method. Just be sure to connect the trace from the "trace through hole dot" to U18 pin 5.

Reference the right picture.

  1. On the component side of the board, locate U2 pin 13 (top right hand corner of U2). Slightly higher and to the right is a via "trace through hole" with a trace running straight down. Cut this trace to separate the via from the trace. In the picture below, the blue/black dot marks the location to cut.
  2. On the component side, jumper from the via noted in step 5 to pad E4 (red wire in picture below). This connects jumper pad E4 to U2 pin 21.
  3. On the component side, notice the large GROUND trace that runs down the board just to the right of the ROM sockets. To the right of the U2 ROM socket, scrape the solder mask from this large ground trace and add a jumper wire from this trace to pad E3 (orange wire/white tracer in picture below. Note that in this instance, U3 was also removed and the ground pad of U3 is used as a jumper point). This connects U2 pin 20 to ground.
  4. Use your multimeter to verify that continuity exists between jumpered points.
  • Note C Cuts and Jumpers
  • Component Side Jumper Locations
  • Solder Side Trace Reroute at U18 and "through hole" to U2, pin 18
  • Component Side Cut (black dot) and Jumper Detail

Note D, 2732 in both U2 & U6
This is a popular modification to the -17 & MPU-100 board as it maximizes the ROM space available and can accommodate almost every game from this period.

In addition to the jumpers listed above,make the following cuts and jumps to use this configuration. Make sure jumpers E1-2, E3-5, E6-7, and E8-10 are in place, & NO others are connected. It is a good idea to continuity check each of the necessary jumpers, BEFORE beginning work.

  1. On the solder side of the -17 or MPU-100 board, cut the trace that runs to U2 pin 21.
  2. On the component side of the board, cut the trace that runs to U2 pin 18. Best place to do this is where the trace passes between sockets U2 and U3. Use a mulitmeter continuity setting to figure out the trace to cut, and mark it with a Sharpie to avoid confusion and cutting of the wrong trace.
  3. On the solder side, jump a wire from U2 pin 18 to U2 pin 12.
  4. On the solder side, jump a wire from U2 pin 21 to U9 (6800CPU) pin 24.
  5. On the solder side of the board, cut the trace that runs to U6 pin 21.
  6. On the component side of the board, cut the trace that runs to U6 pin 18. A good place to do this is where the trace passes between sockets U6 and U5. Use your DMM's continuity setting to figure out the trace to cut.
  7. On the solder side, add a jumper from U6 pin 18 to U6 pin 12. On the solder side, jump a wire from U6 pin 21 to U2 pin 21 (this connects both U2 and U6 pins 21 to U9 pin 24).
  8. On the solder side, cut the trace that runs to U17 pin 2.
  9. On the solder side, cut the trace that runs to U18 pin 4.
  10. On the solder side, add a jumper from U17 pin 2 to U18 pin 4.

Note E Bally Freedom and Night Rider ONLY
These notes are for the Bally Freedom and Night Rider ROMs

These two games used an unique set of ROMs at U1 and U2. These are 74S474 or 7461 (512 byte) ROMs at U1 and U2, and a 9316 or 2716 (2K byte) at U6. According to Williams tech, they state that a U1 2716 EPROM and a U6 2716 EPROM can be used for these two games.If using a -17 or M-100 MPU with a U1 2716 EPROM and U6 2716 EPROM, there are some cuts and jumps required:

  1. Cut the trace from U1 pin 18 to U2 pin 18.
  2. Cut the trace from U1 pin 21 to pad E7.
  3. On the solder side, add a jumper from U1 pin 21 to U1 pin 24.
  4. On the solder side, add a jumper from U1 pin 18 to U17 pin 11.
  5. On the solder side, add a jumper from U1 pin 22 to U2 pin 22.

3.17.3 Stern MPU-200 Jumper Info

The following is a list or various ROM and EPROM combinations that can be used with the Stern MPU-200 board and the associated jumpers and jumper cuts. 9316 ROMS are the "Black" masked ROMS originally supplied by the factory, used on the first few MPU-200 games. Stern switched almost exclusively to a 4x2716 configuration sometime around Seawitch through the end of their run and various boards have been spotted in the wild that seem factory with 2x2732 configuration. Galaxy has been spotted with an oddball 3x9316 + 1x2716 configuration.

 U1 ROM   U2 ROM   U5 ROM   U6 ROM   MPU Jumpers in Numeric Order  Jumper CUTS 
 (empty)  2716  (empty)  2716  2-3, 5-7, 13-14, 16-18, 23-25, 32-33, 34-35  All others
 9316  9316  9316  9316  1-5, 2-6, 8-9, 12-13, 16-18, 19-20, 22-25, 26-28, 29-31, 32-33, 34-35  All others
 9316  9316  9316  2716  1-5, 2-6, 8-9, 13-14, 16-18, 19-20, 23-25, 26-28, 29-31, 32-33  All others
 2716  2716  2716  2716  2-3, 5-7, 9-10, 13-14, 16-18, 19-21, 23-25, 27-28, 29-30, 32-33, 34-35   All others
 (empty)  2732  (empty)  2732  1-2, 4-5, 13-15, 16-18, 24-25, 32-33, 34-35  All others
 U1 ROM  U2 ROM  U5 ROM  U6 ROM  MPU Jumpers in Numeric Order  Jumper CUTS 

To combine the 9316 or 2716 ROM images into the proper format 2732 images, use the following MSDOS commands.

  • copy /b U1ROM.716 + U2ROM.716 U2Combined.732
  • copy /b U5ROM.716 + U6ROM.716 U6Combined.732

Note that jumpers 32-33 and 34-35 control the clock speed of the MPU-200. To use an MPU-200 in a game that normally uses a Bally -17/-35 or a Stern MPU-100 board, you should remove these jumpers as they will cause the software to run faster. This isn't really a problem, but timings of sound events might be affected as well as other unforeseen consequences. If using a MPU-200 board in a Bally -17 or -35 or a Stern MPU-100 game, you can remove the 5101 RAM at U13, as it will not be accessed. If the chip or socket at U13 is bad, it can be ignored. This however is a waste of resources, as the MPU-200 board is hard to find and cannot be substituted by a -17 or -35 board without some modifications.

Additionally, if all 32 dip switches are turned off upon boot with an MPU-200, it will flash seven times and jump into self-test mode. This will toggle alternatively every solenoid, flashing controlled lamps, and test each digit on the score displays. At least one DIP switch must be on to avoid this. This is not a function of the board itself, just the software used. For example, if you put earlier stern software (previous to Meteor) or any Bally software in an MPU-200, it will not enter self-test with all dips off.

  • To down-grade the MPU-200 to the older MPU-100, remove jumpers 32-33 and 34-35 (used on all Stern MPU-200 EPROM configurations), and remove the 5101 RAM chip from U13.
  • The MPU-200, when jumpered for four 2716 EPROMS (U1,U2,U5,U6), will run Bally games using three 2716 (U1,U2,U6) ERPOMs.
  • On a MPU-200 configured for four 2716 EPROMs (U1,U2,U5,U6), to run Bally games using two 2716 (U2,U6) EPROMs, change the MPU-200's jumpers E13-14 to E13-15, and E5-7 to E1-5.


3.17.4 Bally -35 2764 EPROM Conversion

These instructions will convert a Bally -35 MPU to use a single 2764 EPROM. Colors in the table will match the wire colors in the example photos.

Bally -35 MPU - 2764 EPROM Conversion Instructions
1. Remove any PROMs or EPROMs from U1, U2, U3, U4, U5 and U6.
2.

Bend out pins 1, 2, 27, and 28 on a 28 pin EPROM socket so we can solder wires to the pins and install it in socket U2 on the MPU. Pin 26 of the EPROM is not connected. I usually remove this pin from the socket but you can just bend it out and not connect anything to it.

3.

Ensure that there are no jumpers connected to E4, E13, E13a, E11, E9, E12, and E35.

4.

Connect jumpers E7 and E8. This connects EPROM A9 to CPU A9.

5.

Connect EPROM socket pins 1, 27 and 28 to +5V. I chose to use pin 24 of U3 as the source for +5V. This wire is RED in my examples.

6.

Connect EPROM pin 2 (A12) to jumper location E4 (A14). This wire is pink in my examples.

7.

Connect jumper location E12 to ground. I use the large ground trace closest to E12. That will connect EPROM CE to ground. This wire is BLACK in my examples.

8.

Connect jumper positions E9 and E13. That will connect EPROM A11 to CPU A11. This wire is green in my examples.

9.

Connect jumper position E11 to the side of resistor R14 closest to the EPROM socket. That will connect EPROM OE to the output of the gate at U17A. That is an enable that combines CPU A12, VMA and phase 2 of the clock creating an enable that we can use for our EPROM. We could also use jumper location E35 but R14 is closer and makes for a cleaner installation, in my opinion. This wire is yellow in my examples.


  • Bally -35 2764 EPROM Conversion
  • PinWiki dash 35 2764 conversion 1.jpg
  • PinWiki dash 35 2764 conversion 2.jpg
  • PinWiki dash 35 2764 conversion 3.jpg
  • PinWiki dash 35 2764 conversion 4.jpg
  • PinWiki dash 35 2764 conversion 5.jpg


3.17.5 Bally -17 And Stern MPU-100 2764 EPROM Conversion

These instructions will convert a Bally -17 or Stern MPU-100 MPU to use a single 2764 EPROM. Colors in the table will match the wire colors in the example photos.

Bally -17 And Stern MPU-100 - 2764 EPROM Conversion Instructions
1. Remove any PROMs or EPROMs from U1, U2, U3, U4, U5 and U6.
2.

Bend out pins 1, 2, 20, 23, 27, and 28 on a 28 pin EPROM socket so we can solder wires to the pins and install it in socket U2 on the MPU. Pin 26 of the EPROM is not connected. I usually remove this pin from the socket but you can just bend it out and not connect anything to it.

3.

Ensure that there are no jumpers connected to E3, E4, and E8.

4.

Connect EPROM socket pins 1, 27 and 28 to +5V. I chose to use pin 24 of U3 as the source for +5V. This wire is RED in my examples.

5.

Next we need to connect EPROM A12 (pin 2) to CPU A14 (pin 24 of U9). The problem is that there are no jumper locations on the -17 and MPU-100 that are connected to A14 so we have to connect directly to pin 24 of the Motorola 6800 (U9). One option is to run the wire over the top of the board to the back (solder side) but I wanted to make my conversions look as clean as possible so I use pin 20 of U3 as a through hole to get to the other side of the board. So we connect pin 2 of the EPROM to pin 20 of U3 on the components side of the board then connect pin 20 of U3 to pin 24 of U9 on the solder side of the board. This makes for a clean looking conversion on the components side of the MPU. If you're going to use this method you need to ensure that there are no jumpers connected to E8. This wire is pink in my examples.

6.

Connect EPROM pin 20 (CE) to ground. I like to use the large ground trace closest to the EPROM socket. This wire is black in my examples.

7.

Connect jumper E3 to the side of resistor R14 closest to the EPROM socket. That will connect EPROM OE to the output of the gate at U17A. That is an enable that combines CPU A12, VMA and phase 2 of the clock creating an enable that we can use for our EPROM.

8.

Connect EPROM pin 23 to jumper location E4. That will connect EPROM A11 to CPU A11. This wire is green in my examples.


  • Bally -17 and Stern MPU-100 2764 EPROM Conversion
  • PinWiki dash 17 2764 conversion 1.jpg
  • PinWiki dash 17 2764 conversion 2.jpg
  • PinWiki dash 17 2764 conversion 3.jpg
  • PinWiki dash 17 2764 conversion 4.jpg
  • PinWiki dash 17 2764 conversion 5.jpg
  • PinWiki dash 17 2764 conversion 6.jpg


3.17.6 Stern MPU-200 2764 EPROM Conversion

These instructions will convert a Stern MPU-200 to use a single 2764 EPROM. Colors in the table will match the wire colors in the example photos.

Stern MPU-200 - 2764 EPROM Conversion Instructions
1. Remove any PROMs or EPROMs from U1, U2, U3, U4, U5 and U6.
2.

Bend out pins 1, 27, and 28 on a 28 pin EPROM socket so we can solder wires to the pins and install it in socket U2 on the MPU. Pin 26 of the EPROM is not connected. I usually remove this pin from the socket but you can just bend it out and not connect anything to it.

3.

Ensure that there are no jumpers connected to E2, E4, E5, E6, E10, E17, and E18.

4.

Connect EPROM socket pins 1, 27 and 28 to +5V. I chose to use pin 24 of U3 as the source for +5V. This wire is RED in my examples.

5.

Connect EPROM pin 2 to jumper location E10. This wire is pink in my examples.

6.

Connect jumper location E5 to E4.

7.

Connect jumper location E17 to E18.

8.

Connect jumper location E2 to E6


  • Stern MPU-200 2764 EPROM Conversion
  • PinWiki MPU 200 2764 conversion 1.jpg
  • PinWiki MPU 200 2764 conversion 2.jpg
  • PinWiki MPU 200 2764 conversion 3.jpg

3.17.7 Bally/Stern ROM Memory Map

ROM images based on 2732 EPROMs are available on the net for all Bally and Stern machines so I'll give an example of the ROM memory map using these as the example. This is important to consider when doing single EPROM conversions because you will need to create a custom ROM image from original game code.

ROM image Memory Location (Hex) Memory Location (Decimal)
U2 First half:$1000-$17FF(Decimal: 4096-6143)
U2 Second half:$5000-$57FF(Decimal: 20480-22527)
U6 First half:$1800-$1FFF(Decimal: 6144-8191)
U6 Second half:$5800-$5FFF(Decimal: 22528-24575)


3.17.8 Creating A 2764 ROM Image From 2732/2532 ROM files

This is one way to create a 2764 EPROM image.

1. Download the game code. Most should be available from ipdb.org.
2. Download Oliver's Vector home ROM files here. The archive will contain fsp.exe which we will use below to split the ROM images in half.
3. Open a command (DOS) prompt and navigate to the directory where you unzipped the Vector home ROM file downloaded above. You will also want to have your 2732 game code images in this directory. What we are going to do is split the ROM images in half creating 4 new files. Here is the command syntax:

fsp U2_2732_image.bin U2_split fsp U6_2732_image.bin U6_split

Where U2_2732_image.bin and U6_2732_image.bin are the names of the U2 and U6 ROM images and U2_split and U6_split are the prefixes for the output files.

We should now have files named U2_split-0, U2_split-1, U6_split-0 and U6_split-1.

4. Now we can issue this command to create our image:

copy /b U2_split-0 + U6_split-0 + U2_split-1 + U6_split-1 2764_ROM_Image.bin

The command above must be entered as one string.

This will create a file called 2764_ROM_Image.bin. Burn that to your EPROM and you're ready to go!


3.18 Attract Mode Test Points

3.18.1 Rectifier Board Test Point Values

Model TP1 TP2 TP3 TP4 TP5
AS-2518-18 +5.4 +/-.8VDC +230 +/-27.4VDC +11.9 +/- 1.40VDC 7.3 +/-.9VAC +43 +/-5.4VDC
AS-2518-49 +5.4 +/-.8VDC +230 +/-27.4VDC +11.9 +/- 1.40VDC 7.3 +/-.9VAC +43 +/-5.4VDC
AS-2518-54
AS-2518-132

Source: Bally Electronic Pinball Games Repair Procedures, F.O. 560-3

3.18.2 MPU Test Point Values

Model TP1 TP2 TP3 TP4 TP5
AS-2518-17 +5 +/-.25VDC +11.9 +/- 1.40VDC +21.5 +/- 2.7VDC GND +5 +/-.25VDC
AS-2518-35 +5 +/-.25VDC +11.9 +/- 1.40VDC +21.5 +/- 2.7VDC GND +5 +/-.25VDC
AS-2518-133 +5 +/-.25VDC +11.9 +/- 1.40VDC GND +5 +/-.25VDC

Source: Bally Electronic Pinball Games Repair Procedures, F.O. 560-3

3.18.3 Lamp Driver Test Point Values

Model Board Type TP1 TP2 TP3 TP4 TP5
AS-2518-14 Lamp Driver +5 +/-.25VDC GND
AS-2518-23 Lamp Driver +5 +/-.25VDC GND
AS-2518-43 Aux Lamp Driver +5 +/-.25VDC GND
AS-2518-52 Aux Lamp Driver +5 +/-.25VDC GND

Source: Bally Electronic Pinball Games Repair Procedures, F.O. 560-3

3.18.4 Solenoid Driver/Regulator Test Point Values

Model Board Type TP1 TP2 TP3 TP4 TP5
AS-2518-16 Solenoid Driver +5 +/-.25VDC +190 +/-5VDC +5 +/-.25VDC +230 +/-27.4VDC +11.9 +/- 1.40VDC
AS-2518-22 Solenoid Driver +5 +/-.25VDC +190 +/-5VDC +5 +/-.25VDC +230 +/-27.4VDC +11.9 +/- 1.40VDC
AS-2518-107 Combo Lamp/Solenoid Driver

Source: Bally Electronic Pinball Games Repair Procedures, F.O. 560-3

3.18.5 Sound Test Point Values

Model Board Type TP1 TP2 TP3 TP4 TP5 TP6 TP7 TP8 TP9 TP10 TP11 TP12 TP13
AS-2518-32 Sound +5 +/-.25VDC GND +12.5 +/-1.3VDC +43 +/-5.4VDC SOL RET N/A N/A N/A N/A N/A N/A N/A N/A
AS-2518-50 Sound +5 +/-.25VDC GND +12.5 +/-1.3VDC +43 +/-5.4VDC SOL RET N/A N/A N/A N/A N/A N/A N/A N/A
AS-2518-51 Sound +11.9 +/- 2.5VDC +5 +/-.25VDC 0VDC (No Sound), 2.5+/-.2VDC (Sound) +2.5 +/-.2VDC 0VAC (No Sound), .35 +/-.1VAC (Sound) N/A N/A N/A N/A N/A N/A N/A
AS-2518-56 Sound +11.9 +/- 2.5VDC +5 +/-.25VDC GND 0VDC (No Sound), 2.5VDC (Sound) +2.5 +/-.2VDC 0VAC (No Sound), .35 +/-.1VAC (Sound) N/A N/A N/A N/A N/A N/A N/A
AS-2518-57 Vocalizer GND +5 +/-.25VDC Analog Output Digital Input Speech Clock N/A N/A N/A N/A N/A N/A N/A N/A
AS-2518-61 Sound GND +5 +/-.25VDC +11.5VDC -5VDC Speech Volume Control Voltage Sound Volume Control Voltage AY3-8912 Output E TMS5200 Output VMA TMS5200 Clock Reset
AS-2518-81 Reverb GND Audio In +11.9VDC +12VDC +4VDC to +8VDC
M-051-00114-B045 Sound

Source: Bally Electronic Pinball Games Repair Procedures, F.O. 560-3

Model Board Type TP1 TP2 TP3 TP4 TP5 TP6 TP7 TP8 TP9
SB-100 Sound +5VDC Frequency adjustment for R6 pot Frequency adjustment for R2 pot GND Frequency adjustment for R13 pot Vdd +11.6VDC +6.2VDC +10VDC
SB-300 Sound Analog GND +12VDC +10VDC +5VDC Clock input for U18 GND N/A N/A N/A

3.18.6 Display Module Test Point Values

Model Board Type TP1 TP2 TP3
AS-2518-15 Display Module +5 +/-.25VDC +190 +/-5VDC GND
AS-2518-21 Display Module +5 +/-.25VDC +190 +/-5VDC GND
AS-2518-58 Display Module +5 +/-.25VDC +190 +/-5VDC GND
AS-2518-121 Vidiot

Source: Bally Electronic Pinball Games Repair Procedures, F.O. 560-3

3.19 Fuse Values

Model Board Type F1 F2 F3 F4 F5 F6
AS-2518-18 Power Supply 10A, 32V 3AG 3/4A, 250V 3AG SB 4A, 32V 3AG 5A, 32V 3AG 20A, 32V 3AG 3A, 32V 3AG SB
AS-2518-22 Solenoid Driver 1/4A, 250V 8AG N/A N/A N/A N/A N/A
AS-2518-49 Power Supply 20A, 32V 3AG 3/4A, 250V 3AG SB 4A, 32V 3AG 5A, 32V 3AG 20A, 32V 3AG N/A
AS-2518-54 Power Supply 20A, 32V 3AG 3/4A, 250V 3AG SB 4A, 32V 3AG 5A, 32V 3AG 20A, 32V 3AG N/A
AS-2518-107 Combo Lamp/Solenoid Driver
AS-2518-132 Power Supply

Source: Bally Electronic Pinball Games Repair Procedures, F.O. 560-3

Unless noted otherwise, all fuses are fast blow.

Various Bally and Stern games have under playfield fuses as well. A common symptom of a blown playfield fuse is flippers operational, but no other on playfield solenoids work. This is because the flippers draw too much current for the under playfield fuse (which is usually 1 or 2 amp) so they are connected before the fuse is in the circuit. The main solenoid fuse on the rectifier board is used to protect against a shorted flipper coil instead.

Sometimes a large under playfield mechanism will be fused separately. Gottlieb did this often, but it is rarer in Bally/Stern games. If you have a solenoid not working it doesn't hurt to check and see if it has a fuse, or if someone decided to add one in the past. The under playfield solenoid fuse is a very small value as for the most part Bally/Stern games are only able to fire one solenoid at a time. This is a limitation of the hardware design, as the 4-to-16 decoder chip on the solenoid driver board can only decode one of 15 signals to fire an associated solenoid.

Some games do utilize the remaining 4 continuous drivers for a momentary coil. (One example is the ball walker area in Flight 2000 - the 2 kickers are activated by the continuous solenoids, for a momentary period). You should identify which under playfield solenoids if any are used as such in your game and add a fuse to the activation (single, thinner wire) side of the coil to prevent a software glitch from locking on that coil. Simply splice it inline with the single wire and use a 1 amp fast blow fuse to protect the driver circuit and coil in the case of a lockup. It would only be necessary to add the fuse for large loaded coils - relay or high resistance coils like on a flag gate or pop-up post coil relay do not place such a great load on the driving transistor that it is in any danger of burning out.

Weak playfield solenoids can be caused by a weak connection on the under playfield fuse holder. Remove the fuse and check for proper tension on the fuse clips for any tarnishing. It is best to just replace suspect fuse holders with new vs. attempting to clean the originals.

3.20 Proprietary Numbers on Chips

Some Bally IC chips are marked with proprietary Bally part numbers, and no other markings are present. Below is a list of some of the most common chips found on Bally boards, and the more commonly referred to chip.

  • 620-28 - 6800 CPU chip
  • 620-29 - 6820 PIA chip
  • 620-30 - 6810 RAM chip
  • 620-37 - 14514 / 4514 CMOS 4-Bit Latch / 4-to-16 Line Decoder chip
  • 620-38 - 14534 / 4543 CMOS BCD-to-Seven-Segment Latch / Decoder / Driver chip
  • 620-39 - 74L154 4 to 16 decoder chip (can sub with 74HCT154)
  • 620-125 - 6808 CPU chip



3.21 Flippers

3.21.1 Identification of Different Flipper Assemblies

  • Bally & Stern Flipper Assemblies
  • Early Bally Flipper mechanism from a 1978 Star Trek
  • Later Bally Flipper mechanism from a 1981 Medusa. Commonly referred to as "linear flippers". Note-Double EOS switch for Upper Zipper Flipper
  • Early Stern Flipper mechanism from a 1979 Dracula. Note-Double EOS switch for Upper Flipper
  • Later Stern Flipper mechanism from a 1981 Star Gazer



3.22 Accessing Bookkeeping and Diagnostic Modes

  • Bally and Stern Coin Door Test Buttons
  • Bally -17 and -35 Coin Door Test Button
  • Stern -100 Coin Door Test Button
  • Stern -200 Coin Door Buttons



4 Problems and Solutions

4.1 Connectors

Vibration, heat, poor storing conditions, and in some cases alkali damage all take their toll on the connectors used in a pinball machine. Both the male header pins and crimp connectors within plastic housings, (in rare instances, IDC connectors too), are susceptible to the aforementioned conditions. Likewise, some previous person may have performed work on the game which got the job done, but was not necessarily the proper or most desirable way to do it.

Since the backbone to a properly and reliably functioning game is the connectors, spend some time inspecting all of the connectors in the game, before turning it on for the first time. If any connectors appear to be suspect, repair or replace the connector involved. The best practice is to replace both the male headers and the crimp connectors, if a specific area has seen some sort of damage from heat, oxidation, or alkali damage. In most every case, the plastic or nylon connector housings can be reused, if the crimp connectors are carefully removed.

4.1.1 The Importance of Keying Plugs

Connector as found on a Stern Rectifier Board J3 Connector
Connector as found on a Stern Rectifier Board J1 Connector
Connector repinned, keying plug installed, and proper housing for Stern Rectifier Board J3 Connector

In addition to connectors, keying plugs play a large role in a game. Every .100" and .156" header connection has one pin removed. The purpose of this practice is to differentiate between connectors with minimal effort. The compliment housing will have a keying plug installed in the position where the header pin is absent. This was done originally at the factory. However, if a connector was replaced or repinned, a person who previously worked on the machine may not have installed a new keying plug.

Two examples of connectors without keying plugs can be seen in the pics on the left. This game had its wiring harness removed, so the connectors were not placed in their proper positions, and had to be reinstalled. The problem with the first pic is these two .156" 10-pin housings were being used in lieu of the proper, single .156" 20-pin housing at the J3 connection of the rectifier board. The potential to incorrectly plug these two connectors onto the J3 header is extremely high. Because neither housing had a key installed, plus two housings were used instead of one, there are at least four different ways that this connector could be connected. Incorrectly plugging the connector onto the header may have caused extreme unnecessary damage to several of the circuit boards.

In the lower pic to the left, the connector which goes onto J1 of the rectifier boards was missing a key too. In addition to possibly being plugged in upside down, this connector just as easily could have been plugged in one pin off. If the connector was installed incorrectly, the results would not have been good.

Considering the overall cost of a keying plug (around $0.25) it is well worth the small cost and effort it takes to install one. It can potentially save the user from unnecessary and costly repairs to the game. If you simply do not have a keying plug and can't remove the old one from the connector, you can use a square toothpick or a Qtip shaft cut down slightly to make one. It is a hack but it's better than destroying good boards and components.

4.1.2 So, Just What is this Green Slime?

Typical Blue-Green Slime, sometimes found on Bally/Stern header pins. This was sometimes used in an effort to improve conductivity.


Sometimes, a blue-green slime seems to be "bleeding" from a header pin connector. This is most likely some sort of conductive agent, used in an attempt to improve conductivity across damaged header pins, either the male or the female side. The best course of action, for long term reliability, is to repin both the male and female sides of the connector, getting rid of the slime as you go.

4.2 Powering up the first time

Thanks to Bally's decision to modularize their system, there is a technique you can use to save yourself some headaches when you first power up a machine. This method connects one section/board at a time and allows you to test each board thoroughly before moving onto the next so you can isolate any issues to one board. This method concentrates on the -18, -49, and all stern games rectifier boards. The -54 rectifier board on later Bally games would employ a slightly different technique.

Disconnect all connectors from the mpu, lamp, sound (if present) and solenoid driver boards. Disconnect J1 and J3 from the rectifier board. Leave J2 connected. Check all the fuses for proper values on the rectifier board. Power on the game, watching to see if any fuses blow. Use a meter and test each test point on the rectifier board to ensure proper voltage is present (remember some of the voltages are AC). Get the rectifier board working 100%.

Turn the game off and reconnect J3 to the solenoid driver board. Connect J3 to the rectifier board as well. Power on the game and repeat the voltage tests on all the test points on the rectifier and solenoid driver board. Investigate any suspect voltages and correct any faults found. Keep in mind that the displays use both the high voltage and +5 voltage so it might be a good idea to remove the displays from the circuit, also.

Once the solenoid driver board tests ok, connect J4 to the mpu board. Do not reconnect any other connectors to any other boards at this point. Power on the game; if all is well, you will see the mpu board start its LED flash sequence. This is where the majority of repairs will take place; there's no point in hooking other boards up until you have a solid rectifier, solenoid driver/power regulator, mpu board combination working.

After/if the mpu board is booting, add the rest of the connectors back in. When you add the playfield's connectors back in and power up, listen for any solenoids locking on at power-up. Repair the solenoid circuits before proceeding if any do lock on.

At each step in the chain, be sure to inspect and repair any and all connectors in the circuits. Often the connectors get tarnished and do not conduct well enough for reliable game operation. Do not try to clean or sand any suspect connectors - this is a temporary fix at best and only delays the inevitable task of replacement.

Using this method for powering up the game for the first time ensures that any issues you encounter will be isolated to the last piece you added in. Diagnosing and repairing one board at a time is much simpler than multiples.

4.3 Rectifier Board Issues

4.3.1 -18, Stern Rectifier boards

The rectifier board takes the AC voltage from the transformer and uses bridge rectification to convert those voltages to DC. No filtering is done on the rectifier board (that's the solenoid driver/power supply board's job). Depending on the generation of board, there are varying amounts of discrete diodes or bridge rectifiers that produce the DC voltages needed. Additionally, all of the circuits have a fuse for protection in the event of a short on this board.

Putting a meter on the test points across the top of the board will tell you if you're missing any voltages. Refer to the schematics or the charts above to determine the proper voltages at each test point, and remember that some of the voltage outputs are AC volts. If there are any missing voltages investigate further to determine the cause. The main issues with the rectifier board are cracked header pins, bad bridges, poor fuse clips, and poor connections on through hole vias.

If you do nothing else to your power supply, replace the header pins and the connectors that connect to them. The pins are usually burnt beyond usage as are the connector pins. (Often, operators chopped the harness connectors off and soldered the wires directly to the pins as a "fix".) You can get 10 amp header pins from various suppliers that are more robust than the original 7 amp versions. Often just this change alone will fix many lamp "wavering" problems you see on games with lots of feature lamps.

When you replace the header pins consider prying up the plastic insulator piece slightly that keeps the pins in place so you can solder to the top and bottom of each pin. This technique will cure any problems with vias you have and obviate the need for jumper wires that accomplish the same thing. After soldering you can push the insulator piece back down.

Many times the fuse clips are weak or aren't making good contact with the fuses. If you inspect the fuse clips and they are tarnished at all, replace them. Any fuse rated at 4 amps or more use a high current fuse clip to eliminate issues in the future with that circuit. Lower value fuses can use lower current clips with no problem. Solder the fuse clips bottom and top where appropriate for maximal mechanical strength and conductivity.

It's a good idea to replace the discrete diode bridge for the high voltage score display with 1N4007 diodes mounted slightly off the board (about 1/4") so that air can circulate all around them. Solder the leads top and bottom where appropriate. The power resistors should be replaced also with a slight air gap below them to help with cooling. It is possible to approximately double the size of the power resistors (25 ohm to 47 ohm, 600 ohm to 1.3k ohm) to help with the heat dissipated Resistor/temperature discussion although the final test results were not posted.

If you need to replace any bridges, the original style VJ248 bridge can be replaced on the -18 board with a wire lead 25 or 35 amp bridge rectifier. The notch in the bridge is the positive + output; diagonally opposite this lead is the DC negative -. The other 2 leads are the AC inputs and are interchangeable. You have to bend the leads slightly to get them to fit in the smaller VJ248 solder pads. Mark the top of the board with the positive, negative, and AC leads so you don't solder the bridge off by mistake.

If you decide to use the original style bridge, make sure it's not thicker than the other bridges in any other position. The 3 bridges must lie flat against the heat sink slug for proper cooling. The best way to install a replacement VJ248 of the same thickness is to put the bridge in place, then bolt the slug back onto all 3 bridges. Solder the replacement bridge in place. Reinstall the slug with heat sink compound (available at computer supply stores or Radio Shack) smeared in a single thin layer on each bridge surface, and the slug surfaces. Use the least amount possible to get a good even layer; more is not better.

After replacing any parts, test your work with only the J2 cabinet connector installed. Never over fuse a circuit unless the manual for the game specifies; for instance, games with more than 2 flippers often recommend a larger fuse for the solenoid circuit to handle the additional load multiple flipper coils place on the fuse.

4.3.2 Bally -54 Rectifier board

Bally -54 Rectifier Board (Component Side)
Bally -54 Rectifier Board (Solder Side)


The weak link of this generation of rectifier board are the CR1-CR4 diodes (originally 1n4004, replace with 1n4007) and CR5-CR8 (6 amp at least 50v diodes). The original bridges are much hardier on this generation of rectifier board. If you do need to replace a bridge, the best way is to install it and bolt it onto the bottom mounting plate, then solder it into place. The bottom mounting plate acts as a heat sink for the bridges.

BR1 is for the feature lamps on the game, BR2 is for the solenoids. The bridge formed by CR5-CR8 forms the +5 vdc, and the one formed by CR1-CR4 supplies the high voltage for the displays. Mount the discrete diodes slightly off the board for better cooling. While not a large problem on this board, inspect the fuse clips as well to ensure they are not tarnished.

4.4 The Single Largest Culprit in MPU death

Typical Alkaline Damage to a Bally MPU board



By far the largest reason the MPU board stops working is because of alkaline corrosion from the on-board rechargeable batteries. The original battery used on the board is a 3.6 volt nickel cadmium rechargeable battery; over time, this type of battery leaks corrosion into the traces surrounding it, affecting the 5101 memory, the reset circuit, the 6810 memory, the LED area, and all the traces around it. One of the worst boards had corrosion stretching throughout the entire ground plane of the board.

4.4.1 Get rid of that battery now!

Some Options to Eliminate the Ni-Cad Battery on the board



If a battery is on the board, it is recommended to remove it immediately. There are several different ways to replace the battery; in order of preference: 5101 ram eliminator, memory capacitor, nothing, lithium battery, remote mounted AA battery pack, exact replacement. The different options with pros and cons are listed below.

5101 Ram Eliminator

A 5101 ram eliminator doesn't eliminate the ram per se; it replaces it with a more modern ram that is either flash ram or has an internal battery built into it.
Advantages: eliminating a static sensitive obsolete part, eliminating any type of battery that could leak, high reliability, very long retention time (10-99 years)
Disadvantages: relatively high cost, need to remove the 5101 (if not socketed) and replace or add a socket, stressing relatively frail mpu board traces, flash ram type can wear out (although unlikely)

Memory Capacitor

A small capacitor (5.5 volt 1.5 farad works well) is added to the board in place of the original battery. The charging circuit for the rechargeable battery works just fine in maintaining enough of a voltage to enable the capacitor to act in place of the battery.
Advantages: relatively low cost, easy installation, no risk of battery leakage, high reliability
Disadvantages: large initial charge time, some 5101s draw too much current, must turn machine on every couple of months to top off capacitor

Nothing

Removing the battery and cleaning the board up, you do have the option to replace the battery with nothing. If you don't care about audits, settings, or high scores, this is a valid option. Some software might not like having random garbage in its memory range, though. If you have a stern mpu-200 based game, do not leave the battery out as the random garbage will cause problems with stern's software.
Advantages: No risk of leakage, ever
Disadvantages: No high scores save, no audits, garbage in ram can cause issues with some software

Lithium Battery

Replacing the original nickel cadmium battery with a lithium battery is possible; however it is imperative that you add a blocking diode to prevent the battery from being charged.
Advantages: Very long life (10 years on average), cheap, can be mounted far from boards so any leakage would not leak onto boards
Disadvantages: Must use blocking diode, can leak (unlikely), any leakage difficult to clean up

Remote Mounted AA Battery Pack

A popular and inexpensive option is to replace the original battery with a small AA pack, with a blocking diode. Like the lithium replacement option, it is essential to have a blocking diode (usually incorporated into the pack) so the mpu doesn't try and charge the AA batteries. If you get lithium AA batteries you can get very long life. This type of battery pack needs to be checked periodically to ensure no leakage is occurring.
Advantages: Low cost, can be moved far from boards so if batteries so leak they do not leak directly onto boards
Disadvantages: Periodic maintenance (replacing/checking), can leak

Exact Replacement

You can replace the NiCad with a NiCad. Not recommended as all of the original pitfalls of the original battery are still present, but it will work. Many people use cordless phone batteries to do this; just make sure the voltage and amp hour rating are similar to the original battery. (3.6 volt 150 milliamp hours)
Advantages: Low cost, can also be mounted far from boards so any leakage does not leak directly onto boards
Disadvantages: Can leak, needs periodic maintenance (checking for corrosion)

Note that all of the replacement options that include batteries are subject to the same conditions causing leakage as the original battery was. It is strongly recommended that if you decide to use a solution that uses a battery as its power source to remotely mount the battery so that any future leakage does not drip onto the boards. The bottom of the head is a good choice as is the sidewall in cases where there are no boards mounted there. Corrosion can and does travel through the wiring used to relocate the batteries however, so periodic inspection of the batteries and board are necessary to ensure that no further corrosion is occurring.

4.4.2 Installation of Various Battery Eliminators

4.4.2.1 AA Battery pack
4 AA cell battery holder & diode


One inexpensive option is to replace the Ni-Cad rechargeable battery pack with a 4 AA battery pack plus diode using ordinary alkaline batteries. You must install a blocking diode (1N5817 or 1N4004 work fine) to prevent the game from trying to charge the battery and causing damage. Install the diode in the 1st bay with the banded side soldered to the "+" or RED terminal and the non-banded end to the "-" BLACK negative spring. Having a battery backup will save high scores & credits, and some sound settings. The holder can be mounted in a convenient spot in the backbox, and leads soldered to the MPU and a Molex .062 2 terminal pin and socket connected to the battery holder for easy servicing of the MPU. Mouser P/N 12BH348-GR for the 4 AA pack.

4.4.2.2 Lithium Battery with Holder
Lithium Battery with Holder Installed on Stern M-200 Board


Adding a 3v lithium button battery is an option. It is recommended to install a lithium battery holder. Adding a holder is beneficial for two reasons. First, heating a lithium battery is dangerous. There is potential for explosion, if the battery is overheated. Secondly, adding a holder makes future replacement of the battery much easier.

If adding a lithium battery, a blocking diode must be installed to keep the MPU from charging the battery. Although it cannot be seen in the pic to the left, a blocking diode is installed on the component side under the battery holder. The M-200 has 4 through holes located just above the bottom ground trace. These through holes are used to install an alternate block style Ni-Cad battery. The bottom left through hole is for the battery's positive terminal. The upper left through hole is used only to support the battery (the pad does not go to any of the board's circuitry).

In this example, the banded side of the diode is soldered to the lower left through hole. The non-banded side is soldered to the upper left through hole. Additionally, the positive lead of the lithium battery holder is placed in the same upper left through hole. The negative terminal of the battery holder is soldered into the center larger through hole located along the large ground trace (bottom perimeter of board).

Once the battery holder is installed, install the battery. A CR2032 lithium battery was used in this case. Once the battery is installed, use a voltmeter to make certain that the battery voltage is getting to the 5101 RAM chips. Place the black lead of the voltmeter on any of the ground traces, and the red lead on pin 22 of U8 and U13. A reading of ~3vdc should be seen.

4.4.2.3 Memory Capacitor
Prepping an MPU Board for Memory Cap Installation



Remove original battery, neutralize and clean any corrosion left behind/inspect parts in area, replace as needed. Drill a small hole to the left of the original batteries' negative terminal. Install the capacitor with the negative lead in the leftmost hole of the negative terminal, soldering it into place. Solder a lead to the positive terminal of the capacitor and bend the leg over to help hold it in place. Solder the other end of the lead to either the positive hole from the original battery, or to the bottom lead of R12. Double check your work in reference to the negative lead of the capacitor. Turn the machine on an let the capacitor charge for between 8-12 hours. Once the cap is charged, turning the machine on for about an hour a month will keep it topped up.

4.4.2.4 Original style battery or cordless phone battery
Remove original battery, neutralize and clean any corrosion left behind/inspect parts in area, replace as needed. Solder new battery in, or better yet, attach fly leads to battery and remote mount. Turn machine on for large initial charge to battery.


4.4.3 Repairing Alkaline Damage

Should a leaky battery damage your MPU, the first decision to make is this: is the board worth repairing? The extent of the alkaline damage combined with your PCB repair skills, the value you place on your time, and the quality of the result you want to obtain may drive you to replace the board entirely with either an OEM board or one of several aftermarket replacements. Alkaline cleanup, component sourcing and replacement, and socketing troublesome old IC sockets can take 3 to 8 hours.

Should you decide to repair the board, component kits to replace everything in the "corrosion zone" are available from several sources for about $10.

Note: Most alkaline damage repair kits do not include the L1 and L2 inductors which are often damaged. Since these inductors are so small in value, simply replace them with a jumper.

Once you've decided to proceed, to neutralize alkaline, you need an acid. The most common way to accomplish this is to wash the affected area of the board with a mild vinegar solution (50% vinegar to 50% water). Use a toothbrush to scrub the board's affected area and outside the affected area, front and back. After scrubbing, rinse the board with water, then use isopropyl alcohol (the highest percentage you can get - 99% or 91%) in a wave action down the board to displace the water. Let the board air dry, or use a hair dryer without heat to speed the drying process.

After the board is dry, replace any components that were affected by the corrosion damage. The parts will be harder to remove and solder as the combination of corrosion and cleaning makes the solder difficult to heat. Adding new solder before desoldering will help in this process, as will boosting your temperature controlled soldering station up by about 50 degrees. Be careful as the pads are very fragile at this point and can lift easily.

Alternate methods of cleaning the affected areas include bead blasting, and sanding. If the damage is very slight, sanding might work best but you remove all the conformal coating when you do so (that's the green masking material applied to the traces on the board). You have to either replace this coating, or use an alternate material to protect the bare traces from oxidation. Clear nail polish has been used to do this, but isn't a great substitute. You can also tin the affected traces with new solder which will protect the copper. It is also possible to purchase conformal coating but it is toxic and messy to work with. **** Note when sanding your MPU you should wear a mask. The board has glass fibers that are bad for the lungs.

Bead blasting is a technique that uses small glass beads on a sand-blaster type setup to blast away the corrosion. Again, the conformal coating is removed and should be replaced. Some critics of the bead blasting technique indicate that the corrosion actually gets embedded in the board by this method; however, it is an accepted repair practice in electronics repair. Most people do not have the equipment needed to do this.

Regardless of the method used, any bare copper traces should be at minimum tinned with solder. Often after removing the components, you will need to sand the remaining pad to get a good soldering surface. A small fiberglass sanding pen is ideal for this purpose (available at auto parts stores). It allows precise sanding on small surfaces. Sand right before you solder as copper starts to oxidize immediately. Use a can of compressed air to blow any fiberglass particles out of your work area and be careful using the fiberglass pen as the fibers will become embedded in your skin causing irritation. Wear eye protection as well as you really do not want these particles flying into your eyes!

Use a meter to test your work, following point by point as you replace components to ensure you have a clean circuit path. Add jumpers as needed, and solder replaced components top and bottom to ensure good via continuity. Use a high quality socket when you replace the 5101 ram and be very careful around this area, as it is right next to the battery zone and is usually heavily damaged. All of the traces in this area are part of the address and data bus for all the roms, pias, and cpu, so any shorts or damage here will affect the rest of the board.

Finally, remember also that the alkaline will travel both down and up from the original position of the battery. Inspect the lamp driver board (right below the MPU) to ensure it hasn't become corroded also. If the corrosion was really bad, inspect the J4 connector on the MPU too. Corrosion will travel into the wires in this connector. If this has happened, you must cut the wires back far enough so the corrosion is not seen; otherwise it will leach back into your board and continue to spread through the wiring harness. Hopefully, the harness will still have enough slack to make it to the J4 connector. If not, you can extend the wires neatly by soldering and heat-shrinking new wires onto the harness. Having a parts machine to pull this type of wiring from enables you to match closely the wire markings so prevent confusion in the future.

Another thing about the J4 connector is that on most boards the two ground pins only connect from the top of the board. If battery corrosion made it to that connector it is a good idea to add a jumper on the solder side of the board from J4 Pin 18 and 19 to ground that runs along the outside of the board.

  • Bally/Stern MPU Alkaline Damage Abatement
  • The front side of a Stern MPU-200 with alkaline damage removed.
  • The back side of a Stern MPU-200 with alkaline damage removed.
  • An MPU-200, "corrosion zone" cleaned up.
  • A Bally -17 MPU, "corrosion zone" cleaned up.

4.4.4 Using the Dallas/Maxim DS1811 Reset Generator on Bally and Stern MPUs

Like the technique employed to repair alkaline damage on Gottlieb System 80 MPUs, the Dallis/Maxim DS1811 reset generator may be used on Bally and Stern MPUs also. The DS1811-10 has a typical trip point of 4.35VDC. You may also use the Microchip Technology equivalent, part number MCP130-460DI/TO, available from Great Plains Electronics.

Unlike Gottlieb System 80 MPUs, using the DS1811 on Bally and Stern boards doesn't save you much work. It replaces only 10 easily sourced components. And since you've done the hard work of cleaning up the alkaline damage already, using the DS1811 isn't really worth the trouble. But, if you'd like to use the DS1811, the procedure is described below.

After you've cleaned up the alkaline damage, replace all components except these 10:

  • VR1
  • R1, R2, and R3
  • R112
  • R138, R139, and R140
  • Q1 and Q5

With the board oriented as it would be in the game (J5 at top), install a jumper from the top through-hole of where R139 was, to the bottom through-hole of where R138 was.

There are several ways to form the legs of the DS1811 for a Bally or Stern MPU. However, with the flat side of the DS1811 facing left (i.e. at J4), the legs of the DS1811 should be placed in the prior location of Q5 such that:

  • Pin 3 of reset generator (top pin) is where the upper, right lead of where Q5 was
  • Pin 2 of reset generator (middle pin) is where the lower, right lead of Q5 was
  • Pin 1 of reset generator (bottom pin) is where the left lead of Q5 was

Add a 1K, 1/4W resistor from pin 1 of the reset generator to +5VDC. This signal can be picked up conveniently at the banded end of CR5 (a 1N4148 diode).

  • Using the Dallas/Maxim 1811 Reset Generator

  • An MPU-200 MPU with alkaline damage abated, and reset components replaced with the DS1811 reset generator.

  • The same image, but with the single jumper indicated (yellow dot), where to install the DS1811 flat side facing left, and the added 1K resistor.

  • Illustrated picture of components to remove, keep, and jumpers to add.

  • Closeup of the DS1811 equivalent installed.


4.5 MPU boot issues

Non-booting Bally/Stern MPU boards fall into a couple different categories. If the board's LED is good and turns off at bootup, you're part of the way there. Refer to the flash sequence section to figure out where your issue lies to narrow down the problem area.

You must have a good set of roms in a game as well as a known good working 6800 cpu chip. Suspect any sockets that have a brown color or closed frame - often the spring tension on these sockets is poor necessitating replacement. Be very careful soldering on the mpu boards as the traces and pads are very fragile.

A "cheat sheet" comprised of the signals and pinouts for the primary chips and all of the header connections used with a Bally -35 MPU board is available here. The PDF is specifically for a -35 board, however, Bally -17 and Stern M100 boards are similar. The obvious differences are the ROM jumper settings and connection J5 (J5 on a -35 board has one more pin).

4.5.1 LED locked on

Remove all ICs except for U6, U9, U11, and U2 if it is a Stern. I recommend Leon's test rom for tricky MPUs. This allows you to leave out U11 and check most address and data lines.

Short pins 40 and 39 momentarily on the cpu chip; after doing so, see if the LED goes off. If it does, concentrate your efforts on repairing the reset circuit as it's not holding the reset low on powerup for the minimum required 50 milliseconds. If it doesn't, it's best to pull the board from the machine and put it on the bench, using the benchtop power supply details to keep working on the board outside the machine.

Ensure that there's not simply a problem with Q2 or the LED itself (i.e. the board isn't actually booting up anyway, just with the LED locked on). A logic probe on pin 18 of the U11 6821 PIA will tell you if the LED signal is changing.

An easy quick test for the reset section is putting your DMM on pin 40 of the cpu. It should read 5v, and in most cases if it does the reset is good. Attach a logic probe to the output of Q5 - on power on, you should see this circuit start low and then go high approximately 50 milliseconds later. If it goes high immediately, at a minimum replace Q5, then Q1. Make sure the 2 watt resistor is not leaning on Q5. The heat can kill the transistor.

Next put the probe on U9 pin 3, 36, and 37 to see if you get pulsing - these are all clock signals and you should see pulsing on all 3 pins. If you have an oscilloscope you can visually see the signals, or even on a multimeter, the voltage will show between 2.5-2.9 volts. If you see zero volts or 5 volts, your clock signal is bad. The CPU could be dragging down the clock. Remove U9 and see if the clock signal is now good. When clock signal is bad suspect U15 or U16. Also check C15 and C14. U15 is failed most often.

Check U9 Pin 2. This is the HALT line. You should see 5v, if not replace U9

Check U9 Pin 5. This is the VMA line. It is a pulsing signal and reads about 2.8v with a DMM. If incorrect first try a new U9. Next check U14D, U15C, U19B. U15 is the most likely IC to have failed.

Still locked on? After double checking all your jumper changes, roms, and rams in another board if possible, time to start suspecting other issues such as bad sockets, traces, or addressing chips. Bad sockets are quite common.

4.5.2 Reset Circuit

The purpose of the reset circuit is to ensure the +5 vdc is stable before allowing the system to start. At startup, the reset signal is held low via pull-down resistor R139 until the +12 vdc line rises above the zener diode VR1's value (8.2 or 9.1 volts depending on your board). At this time, the input voltage threshold for the +5 regulator on the solenoid driver board has been met with some headroom. Q1 starts conducting, turning on Q5, which provides the actual reset signal. All of this happens in approximately 50 milliseconds.

Most of the reset circuit is in the corrosion zone (lower left corner of the mpu board) and consists of most of the components commonly included in "repair kits". If the reset circuit is not working, the LED will not turn off. If Q1 or Q5 fails, conducting all the time, the cpu chip will never come out of reset.

The reset circuit continuously monitors the +12 volt line; if it falls below VR1's threshold voltage, the game will reset. If a Bally/Stern game is resetting, concentrate on why the +12 volts is dropping below 8.2/9.1 volts. (VR1 sets the threshold at which the reset signal turns on, and also when it would turn off in a rebooting scenario.)

Note that Q1 and Q5 actually form a switching power regulator for the reset signal only. The normal +5 supplied to the rest of the board is derived entirely from the solenoid driver board's regulator. The reset signal regulated power is used to charge the battery, provide the reset signal, and to power the 5101 chip(s) on the board. Normal +5 vdc is blocked from entering the reset circuit by the 1N4148 diode CR7. Diode CR5 (1N4148) blocks the battery voltage from powering the entire board via CR7.

4.5.3 Bally / Stern MPU Board LED Never Lights or is Locked On

If the LED never lights, either the +12v (TP2, J4 pin 12) is missing or the LED is bad. By default, the LED is ON until the software tells U11 to turn it OFF.

If the LED lights solid, there's some digging to do. First off, if the board has any corrosion damage at all, it needs to be cleaned and neutralized before attempting any repairs. While shot-gunning components might fix the board, it won't be 100% reliable if the corrosion isn't addressed.

Make sure you are getting a solid power path from the rectifier board through the solenoid driver / power regulator board to the MPU board. A locked on LED can be caused by a poor connection anywhere in this chain. Sometimes reseating connector J4 on the MPU board (lower left) will 'clean' a connector well enough to make a better connection. While this may be a short term fix, be aware that any connector, which seems to work better after being reseated, really should be repinned and have its header pins replaced.

Next, put your DMM or a logic probe on pin 40 of the 6800 cpu chip (U9). Power on the board. You should see the voltage remain low / at zero, and then approx 1/10 of a second later, rise to about +5 vdc (high). This is the reset signal, which originates from the components in the lower left corner of the board, and is sent throughout the board to U9, U10, and U11. What the reset section (called the 'valid power detector') does is not allow the MPU to boot until the +12 volts are stable over the value of ZR1 (a zener diode, usually either 8.2 or 9.1 volts). This delay ensures that the +5 voltage is stable enough to run the MPU board reliably (the +5 volts is derived from the +12 volts on the solenoid driver / power regulator board).

The 6800 CPU chip will not 'unlock' and start program execution until it sees a transition from a low (0 volts) to high (~5 volts) signal. This is the purpose of the power on reset delay. The reset delay and signal must be present at all three of the reset inputs at U9 (pin 40), U10 (pin 34), and U11 (pin 34). If the signal starts out immediately at a high level, the MPU will not start to boot until the transition takes place. If you have a locked on MPU, you can take a screwdriver or your meter probe and short pins 39 and 40 together for a brief moment on U9. If the game starts to boot after doing this, it's a safe assumption that the reset circuit is to blame. Shorting the pins together simulates what the reset circuit does.

If you need to rebuild the reset circuit, full kits are available from specialty suppliers such as Great Plains Electronics or Big Daddy Enterprises. The kits include all replacement components in the corrosion zone. Bare bones, component replacements are Q1 (2n3904 or 2n4401) and Q5 (2n4403 or 2n3906), but it is a good idea to go ahead and replace all the parts that come in the kit. Replace components one at a time to ensure that you do not mix any up. Note there are a some components that are polarized in their installation, which include VR1, CR5, Q1, and Q5. Look carefully at the board to see if there are traces on the top and bottom of a component. The continuity at a through hole can be compromised due to alkaline corrosion. Therefore, it is recommended to solder these components from the top and bottom of the board to ensure that a good connection is maintained.

The following is the list of parts for the reset section that should be replaced. Parts listed with more than one type are equivalent and can be substituted freely. It is also possible that the inductors L1 and L2 need to be replaced as well, however this is very rare. If there is heavy corrosion on them they should be replaced.

Transistors:

Q1 - 2N3904/2N4401 (lower left area)

Q2 - 2N3904/2N4401 (near LED)

Q5 - 2N3906/2N4403 (lower left area)

Diodes:

VR1 - zener 1N9598/1N4738A

CR44 - 1N4004 rectifier diode

CR5, CR7 - 1N4148 switching diode

CR8 - LED

Capacitors:

C1, C2 - 820pF ceramic capacitor

C3 - 0.01uF ceramic capacitor

C5 - 4.7uF tantalum capacitor

C13, C80 - 0.01uF ceramic capacitor

Resistors: (1/4 w unless noted)

R1, R3, R24, R28 - 8.2k

R2 - 120k

R11 - 82 ohm/2 watt

R12 - 270 ohm

R16 - 2k

R16 - 2.2k (stern mpu-200 only)

R17 - 150k

R29 - 470 ohm

R107 - 3.3k

R112 - 1k

R134 - 4.7k

R140 - 20k

If your reset circuit is operating as designed, yet the LED is still locked on, next step is to pull all the chips from the board except for U9, U11, and U6 (leave all chips U1-U6 installed on Stern MPU boards. Only U6 is required on Bally boards to perform the initial LED turn off). It helps to have a known working U6 from a Bally game to use as a test chip for this purpose. Be aware that you need to know/have the board jumpered for the correct type of chip you're inserting.

See if the board starts and turns off the LED with just the chips above installed - if it does, add these chips back in this order to see which might be bad: U10 PIA, U1-U5 program chips, U7 6810 ram, U8 5101 ram. Often a bad ram/rom can cause the entire system to lock up. Bad chip sockets can be a factor as well; the early Bally -17 boards have a closed type brown socket that's especially prone to failure.

Double check the jumpers to ensure they match the ROM chips you have available, and change the ROM chips to known working ones for testing. A final thing to check if the machine won't boot is the clock circuit. It is fairly robust, and far more common that the reset circuit itself or chip sockets are the issue. To check the clock circuit you need a logic probe or oscilloscope. A multimeter might show the average voltage on a clock circuit, or it might just show meaningless constantly changing numbers. The clock signal is pin 3 on the CPU chip, and the shifted clock signal goes to pins 36 and 37. The frequency is about 500 kilohertz for Bally -17, -35 and Stern MPU-100 boards, and approximately 850 kilohertz for the Stern MPU-200 board.

If the clock signal is missing, pull U9 first to make sure the CPU chip isn't damaged, and test it again.

It can be frustrating to track down a locked on LED problem, but breaking the problem up and testing each section individually helps. Just remember that if the LED turns on and then off, most of the battle is won. The board has booted far enough that the software was able to start and turn off the LED. Proceed onto the LED flash testing to determine what needs to be fixed beyond that.

4.5.4 Bally / Stern MPU Board LED Flash Sequence

Upon start up, Bally and Stern boards have an LED that flashes. The LED is used to convey the results of specific tests conducted on various parts of the system. This section explains what is being tested and how, according to information from the Bally "FO-561-2 Theory of Operation rev. 5-1982" manual, and the Stern manual "Theory of Operation, Stern's Microprocessor Controlled Solid-State Games".

When power is first applied to the MPU board, the LED by default is ON. The very first set of valid instructions in every Bally / Stern game is to turn the LED off. This is more of a flicker than a flash, so is not counted as a flash in the 7 flash sequence.

4.5.4.1 Quick summary

Flicker: MPU reset good, program booted.
1st Flash: ROM Checksums OK
2nd Flash: U7 6810 ram OK
3rd Flash: U8 5101 ram OK (U8 & U13 on mpu-200)
4th Flash: U10 PIA OK (see details for caveats)
5th Flash: U11 PIA OK (see details for caveats)
6th Flash: U12 555 Display interrupt timer OK
7th Flash: Zero crossing interrupt detector OK (solenoid voltage present)

4.5.4.2 LED flicker:

The LED flicker tells you the CPU chip was able to start a valid program stored in the EPROMs, and that the reset circuit itself is good.

4.5.4.3 First flash:

After the program is running, it performs a checksum of all programmed chips U1-U6. Most Bally games' programming is split between an operating system chip U6 and a game ROM chip at U2. Stern games were a little looser. The operating system and game code are freely interspersed.

Bally checksums are calculated by summing each byte, and discarding any carries. Most games check their code in $0400 blocks, so it would be possible to determine down to the chip which chip failed this checksum. (A 2716/9316 chip has hex $0800 space available in it - 2732 sized images are $1000 in size. The smallest chip used was a 474 PROM which has $0200 bytes available) However, to do so would require a way to read the X register from the 6800 CPU chip at the time of checksum failure. So if you do not get the first flash, it is best to replace the U6 chip first, then move onto the other chips - U5, U2, and U1 (if present).

Stern checksums are calculated similarly, but not in chunks. The entire program space is summed and must equal $00 for the first flash to occur.

Regardless of whether the manufacturer is Bally or Stern, after the checksum is passed, the first flash occurs.

4.5.4.4 Second flash:

Next, the program tests the 6810 RAM chip at U7, by writing the data $00 to each memory location contained in the RAM ($00-$7F). It then reads back each location to ensure that $00 is returned. It increments the data to $01 and repeats this test. This continues until the data read back is $FF (256), which is the maximum value of any one byte stored in the RAM. It then increments the memory location being tested, and repeats the $00-$FF data storage test. If any of the tested locations return an unexpected result, the program stops, and alerts you to a problem with U7 (since you got the first ROM checksum flash, but not the 2nd U7 OK flash).

4.5.4.5 Third flash:

Now, the program tests the non-volatile 5101 RAM chip at U8 (U8 AND U13 on Stern MPU-200 boards). The 5101(s) store(s) bookkeeping data, game parameters, high scores, replay levels, etc. The program tests this RAM ($200-$2FF) by reading the original nibble / byte (see sidebar), and saves it in a temporary location. Then, it stores a test pattern in the location similar to the U7 test. After the byte successfully passes the test, the original data is returned to the location, and the program loops onto the next byte.

LEARN MORE: How does a 128 byte 5101 RAM occupy 256 memory locations?

If you look at a pinout of the 5101 memory, you will notice it is a 128 byte device. Yet, it is addressed by the MPU via 256 memory locations ($200-$2FF). This is because the 5101 is actually a 256 nibble device - a nibble is a half-byte (4 bits). So data stored to a 5101 in a pinball machine actually only stores half of the data byte being sent to it. Which half it stores is dependent on the board design. Bally and Stern use the upper nibble for storage, and Williams used the lower nibble. Stern MPU-200 boards have an additional 5101 at U13. This stores the lower nibble in conjunction with U8 storing the upper nibble of a byte saved to $200-$2FF. This allows MPU-200 games to store more data, and avoid doing some fancy processing by getting the data in and out of the non-volatile ram area.

For example, here's some pseudo-machine code for what happens:

  • LOAD #$24 (the data you want to store is 24)
  • STORE $231 (you want to store the data 24 at memory location $231)
  • READ $231 (you want to read back the data you just stored)

The data returned is not #$24 as expected, but rather #$2F. The lower nibble was never stored, because the 5101 memory does not store data as bytes but rather as nibbles. To store #$24 properly would require splitting the byte into its nibbles '2' and '4'. The 2 would be stored in one memory location, while the 4 would be shifted, and stored in another memory location.

Showing the byte as binary might be helpful to visualize what's involved. The hex #$24 in binary is %00100100. Split into nibbles is %0010 (the 2) and %0100 (the 4). The upper nibble is the one the 5101 is able to store directly, but the position in the byte of the lower nibble prevents it from being stored. A shift operation is performed 4 times on the byte to reposition the lower nibble as the upper nibble, which enables it to be stored to the 5101. Each shift moves the binary pattern to the left one bit - here's the full sequence:

  • Start=%00100100
  • Shift left=%0100100x
  • Shift left=%100100xx
  • Shift left=%00100xxx
  • Shift left=%0100xxxx

This gives the #$4 in the high nibble. All byte data has to be split this way to be saved, and recombined upon reading from boards (Bally -17 and -35, Stern MPU-100) with a single 5101 RAM chip. You can see why Stern added the second 5101 RAM to their boards. It makes programming much easier!

4.5.4.6 Continuity Chart for U8 5101 RAM in a Bally AS2518-17, -35 or Stern MPU-100 Board
IC & Pin# Data Address Continuity Points
U8 pin 1 (A3) U7 pin 20, U6 pin 5, U9 pin 12
U8 pin 2 (A2) U7 pin 21, U6 pin 6, U9 pin 11
U8 pin 3 (A1) U7 pin 22, U6 pin 7, U9 pin 10, U11 pin 35
U8 pin 4 (A0) U7 pin 23, U6 pin 8, U9 pin 9, U11 pin 36
U8 pin 5 (A5) U7 pin 18, U6 pin 3, U9 pin 14
U8 pin 6 (A6) U7 pin 17, U6 pin 2, U9 pin 15
U8 pin 7 (A7) U7 pin 15, U6 pin 1, U9 pin 16, U11 pin 24
U8 pin 8 GROUND
U8 pins 9&10 (D10&D00) U7 pin 6, U6 pin 14, U9 pin 29, U11 pin 29 (* pins 9 & 10 siamesed together)
U8 pins 11&12 (D11&D01) U7 pin 7, U6 pin 15, U9 pin 28, U11 pin 28 (* pins 11 & 12 siamesed together)
U8 pins 13&14 (D12& D02) U7 pin 8, U6 pin 16, U9 pin 27, U11 pin 27 (*pins 13 &14 siamesed together)
U8 pins 15&16 (D13&D03) U7 pin 9, U6 pin 17, U9 pin 26, U11 pin 26 (* pins 15&16 siamesed together)
U8 pin 17 (CE2) Q5 Right Upper leg, U9 pin 40, U11 pin 34
U8 pin 18 (OD) U18 pin 6
U8 pin 19 (CE1) U17 pin 8
U8 pin 20 (R/W) U7 pin 16, U9 pin 34, U11 pin 21, U18 pin 7
U8 pin 21 (A4) U7 pin 19, U6 pin 4, U11 pin 22
U8 pin 22 (Vcc) C13 Left Leg, R12 upper Leg, CR5 Lower Leg


4.5.4.7 U8 & U13 5101 RAM Continuity Chart for Stern MPU-200 ONLY
IC & Pin# Data Address Continuity Points
U8 pin 1 (A3) U13 pin 1, U7 pin 20, U6 pin 5, U9 pin 12
U8 pin 2 (A2) U13 pin 2, U7 pin 21, U6 pin 6, U9 pin 11
U8 pin 3 (A1) U13 pin 3, U7 pin 22, U6 pin 7, U9 pin 10, U11 pin 35
U8 pin 4 (A0) U13 pin 4, U7 pin 23, U6 pin 8, U9 pin 9, U11 pin 36
U8 pin 5 (A5) U13 pin 5, U7 pin 18, U6 pin 3, U9 pin 14
U8 pin 6 (A6) U13 pin 6, U7 pin 17, U6 pin 2, U9 pin 15
U8 pin 7 (A7) U13 pin 7, U7 pin 15, U6 pin 1, U9 pin 16, U11 pin 24
U8 pin 8 U13 pin 8, GROUND
U8 pins 9&10 (D10&D00) U13 pins 9 & 10, U7 pin 6, U6 pin 14, U9 pin 29, U11 pin 29 (* pins 9 & 10, siamesed together)
U8 pins 11&12 (D11&D01) U13 pins 11&12, U7 pin 7, U6 pin 15, U9 pin 28, U11 pin 28 (* pins 11 & 12 siamesed together)
U13 pins 11&12 (D11&D01) U7 pin 3, U6 pin 10, U9 pin 32, U11 pin 32
U8 pins13&14 (D12&D02) U13 pins 13 & 14, U7 pin 8, U6 pin 16, U9 pin 27, U11 pin 27 (*pins 13 &14 siamesed together)
U13 pins 13&14 (D12&D02) U7 pin 4, U6 pin 11, U9 pin 31, U11 pin 31
U8 pin 15&16 (D13&D03) U7 pin 9, U6 pin 17, U9 pin 26, U11 pin 26 (* pins 15&16 siamesed together)
U13 pins 15&16 (D13&D03) U7 pin 5, U6 pin 13, U9 pin 30, U11 pin 30 (* pins 15&16 siamesed together)
U8 pin 17 (CE2) U13 pin 17, Q5 Right Upper leg, U9 pin 40, U11 pin 34
U8 pin 18 (OD) U13 pin 18, U14 pin 9
U8 pin 19 (CE1) U13 pin 19, U17 pin 8,
U8 pin 20 (R/W) U13 pin 20, U7 pin 16, U9 pin 34, U11 pin 21, U14 pin 10
U8 pin 21 (A4) U13 pin 21, U7 pin 19, U6 pin 4, U11 pin 22
U8 pin 22 (Vcc) U13 pin 22, C13 Left Leg, R12 upper Leg, CR5 Lower Leg


4.5.4.8 Issues with 5101 RAM chips

The socket at U8 or U13 is very close to the corrosion effects of the alkali of a leaking battery. This is often the cause of boards not showing a 3rd flash. There are traces on the component and solder side of the board, and the socket can hide corrosion to these traces. In turn, the corrosion can affect the 5101 chip's contact with the socket pins.

Unless the board is unusually clean, a rebuild of a non-working board should include replacing the socket at U8 (and U13, for MPU-200). The chip is an oddball configuration with 22 pins which makes machine pin sockets hard to find. Use 2 strips of machine sockets, so that you can solder both above and below the board. Inspect the traces closely on the top; it is best to avoid soldering on the top of a board unless you have to as it makes it extremely difficult to desolder the sockets in the future. Take extreme care not to make a solder bridge of the traces on the component side, as it is easy to do. Performing a continuity test to adjacent socket legs after soldering is a good practice. Also, note that some of the pins are shorted together. (see above)

Failure to achieve the third flash can also be attributed to the following: If the U8 and U13 (MPU-200) show correct continuity as charted above, and the 5101 RAM is known to be good, the problem could lie with the U19 (4011) chip. On Stern MPU-200, replace U14 (4572) instead. Finally, although rare, the 6800 CPU at U9 could be bad.

The speed of the 5101 RAM chip can make a difference in the functioning of an otherwise good MPU board. If a slow chip is put into a good MPU, strange behavior such as inconsistent boot ups, incorrect score display behavior, among other things can result from the RAM not being able to keep up with the demand from the CPU. Below is a list of 5101 RAM chips and their speed. A lower number means a faster chip. A Stern MPU-200 board needs 2 chips of at least 450ns, while the Bally AS 2518-17 or -35 and the Stern MPU-100 can work correctly with a slower chip of 650ns. The stern mpu-200 board should have matching speeds for the 2 rams as they are selected and accessed simultaneously. A faster chip is fine for a replacement, but there will not be a performance enhancement.

  • PCD5101P (Philips manu) 150ns
  • 5101-1 450ns
  • 5101L-1 450ns
  • 5101L-2 450ns
  • 5101-2 450ns
  • 5101 650ns
  • 5101L 650ns
  • 5101-3 650ns
  • 5101L-3 650ns
  • 5101-8 800ns (too slow for any board)

The 5101 RAM chip is especially sensitive to static discharge that will damage it, so take extra precaution in handling. The memory RAM replacement chip sold by Tom Callahan at pin-logic.com uses a 6116 and special adapter, and makes a good replacement. Pin-logic also sells the static ram chips that do not require batteries for backup; although he advertises that these do not work in the Stern MPU-200 board, 2 of them did work on a test board. Another source for varying types of 5101 and 6116 replacements is a blog site WarpZoneArcade. There is information how to roll-your-own 5101 replacement adapters. Or, visit its accompanying site, PinForge, and purchase pre-made RAM adapters.

4.5.4.9 Fourth and Fifth flashes:

Next, the program tests each of the 2 6820/6821 peripheral interface adapters (PIAs) at U10 and U11, starting with U10. The PIAs are set to a known state, then data is stored and read back from them to verify their registers are functioning properly. It is important to note that it is not possible for the PIA to be 100% tested with this test, as external data would have to be fed in to do so. However, the test will at least test the internal registers. It would be possible for a PIA to pass the self-test, but still not work properly with external inputs.

Assuming the PIAs pass, the fourth (U10) and fifth (U11) LED flashes occur. The LED itself is connected to the U11 PIA. So if the LED is locked on, U11 might be bad. It's worth letting a locked on LED board 'sit' for a minute or so to see if the game boots all the way up without flashing each test step. This is an example of how a PIA can pass self test but still be bad. The LED control pin has no feedback as to if the LED is in fact flashing.

4.5.4.10 Sixth flash:

The sixth flash waits for an external input on U11 pin 40 from the display interrupt generator circuit, which occurs 320 times a second. If you're missing the sixth flash, there may be a problem with either the U12 circuit, OR the input pin on the PIA. A logic probe, oscilloscope, or a multimeter on pin 40 can help you determine which is at fault. A logic probe will pulse if the display circuit is operating; the scope will show you the signal's waveform; and the multimeter should settle on a voltage somewhat between 0-5 volts.

One definite reason for lack of a sixth flash is a poor connection on the ground side of C16, the film capacitor. If there isn't a sixth flash, but all other tests and measurements are good, it's a safe bet that U11 is bad and needs to be replaced.

Note that the sixth flash does NOT check for the proper frequency of operation of the display generation circuitry. As long as there is a pulsing signal (technically, ONE state change), the test is marked good and the program allowed to continue.

4.5.4.11 Seventh flash:

The last flash waits for an external input on U10 pin 18 from the zero crossing detector circuit, which occurs 120 times a second (as the AC waveform passes or "crosses" 0 volts). Diagnosis of issues with the 7th flash are similar to the 6th flash. You can measure the input to pin 18 to determine if the signal is present or not. A signal present, but no flash could mean a bad U10 PIA. A missing signal usually points to missing solenoid voltage. The source of the zero crossing signal is derived from the solenoid voltage delivered from the rectifier board. Equally, if a signal is present, there may be an issue with the zero crossing detection circuit itself.

Note again that the seventh flash does NOT check for the proper frequency from the zero crossing detector. It simply checks for a pulsing signal, and only checks for ONE transition.

After the 7th flash, the program does some background setup: reads dip switches, enables the displays, attract modes, switch scanning etc. in a 'game over' mode, waiting for player input. The LED will sometimes be dimly glowing or even pulse as this happens, which is not a cause for alarm. You can rebuild the LED circuit around Q2 if this worries you, but it is harmless.

One interesting anomaly is that the 7th flash will not occur, if there is a bad 1N4004 diode (CR3) used on -32 / -50 sound boards. On these two sound boards, the +12VDC used on the boards is derived from the +43VDC solenoid voltage, and isolated by CR3.

Bally MPU with burned R113 and R16




If you have +43VDC solenoid voltage, but no 7th flash. Check R113 and R16 on the MPU. These two resistors take the 43v and form a voltage divider. If these resistors burn they can stop the 7th flash from happening. Test point 3 should show approximately 21.5 pulsating DC if the resistors are ok.

Still no 7th flash and you have 21.5 vDC on TP3, check CB1 which is on U10 pia pin 18. It is the logic side of the zero crossing and should be pulsing. If it is stuck low or stuck high replace u14.

4.5.5 Chip Sockets

Reliable socket connections are a requirement for any printed circuit card to work as designed. Old sockets, as discussed below, should be replaced. Use extreme care in desoldering the old sockets, the traces and pads on Bally Mpu-17 boards are easily lifted. It is possible to lift other boards' traces and pads as well, especially if any battery corrosion is in the area.

The chip sockets on old Bally and Stern boards (also most any board of this era) are long past their reliable lifetimes. They may work, but they may also cause intermittent connections that will have you chasing your tail tracking down odd problems with your game. Like the 40-pin interconnect used in Williams System 3-7 games, these sockets should always be replaced. On Bally/Stern MPUs, these include U2, U6 (more game ROMs if you don't combine the ROM images), U7 RAM, U8 5101 RAM (and U13 5101 RAM if a Stern MPU-200), U9 CPU, U10 and U11 6820/6821 Peripheral Interface Adapters.

Perhaps the most maligned socket brand, and rightfully so, is the Scanbe brand. In the picture below, you can see why. These 30+ year old sockets passed the point of reliability many years ago. Included in the Scanbe socket picture below are two pins pulled from a Scanbe socket. The pins were designed to grip the SIDES of the IC legs, unlike the design of modern sockets that grip the front and back faces of the IC leg. Get rid of them now.

  • Poor quality and old sockets found on many early pinball machine circuit boards
  • It's hard to believe that these "brown" sockets ever worked.
  • The spring strength of the connectors in these AUGAT sockets eroded over time.
  • Closeup of the infamous "Scanbe" brand socket. "Scanbe" is molded into the top of the socket face.


4.5.6 ROMs/ICs

this is a stub

The Motorola part number equivalencies that appear on the top of U9, U10, and U11 are...

  • SC44216P = 6800 microprocessor
  • SC44067P = 6820 PIA

4.5.7 Clock Signal

replacement 9602 board: http://www.homepin.com/9602.html

replace with a 6802: http://pinballeon.com/6802/e6802.htm

4.6 How to make a Benchtop power supply for the MPU board

ATX Power Supply connector modification
Bench Top Power Supply hooked up to MPU board

Being able to test the MPU board on the bench is a great advantage in trouble shooting. A benchtop power supply is quite easy and inexpensive to put together. All that is required is a computer supply and some mods to the connectors. An ATX or old AT computer power supply will supply the + 5 vdc and +12 VDC that is needed to run the first 6 flashes. The 7th and final flash requires +43 vdc and cannot be supplied by the ATX power supply, but the MPU can be "fooled" into thinking it is present.
An ordinary ATX computer can be had used or new for under $20. Exact wattage is unimportant. In order to be able to switch the power on and off, the 20 pin connector on a ATX supply must be modded. This is not necessary on an older AT supply. Because modern motherboards use a soft power on/off, the green pwr-ON at pin 14 must be tied to the black COM ground at pin 13 or 15 thru 17. This can be done easily done with a short length of wire and a .062 pin attached to each end and shoved into the 20 pin connector at pin 14 and 15. Or, simply cut the wires free from the connector and solder them together.<bt>

Take one of the molex 4 pin connectors which have 1 RED wire (+5 vdc), 1 YELLOW (+12 vdc) and 2 BLACK ground wires. Attach an alligator clip to the RED, YELLOW and one BLACK lead. A good tip is to use the color coded rubber boots to protect from accidental shorts and keep it easier to identify the leads at a glance. If you should accidentally short the +12 volts to +5 via you will kill all the ICs on the board!

To use the power supply, with the power OFF, clip the black lead to TP 4 at the top right of the board. Clip the RED +5 vdc lead to TP 5 at the bottom right, near the battery. Finally clip the YELLOW +12 vdc to TP 2 on the bottom left of the board. Make certain that this connector is not on or shorted to TP3 which is nearby. This error would damage the board, so double check this before turning on the power. Connect the power supply to 120vdc outlet and flip the switch to boot.

Upon power up, the brief flicker should be seen, then 6 (not 7) more flashes for a good board. The 7th flash cannot be achieved without +43vdc present. On some occasions, the LED will lock on and not go into the flash sequence. This COULD BE because the MPU is sensitive to the exact voltage supplied and will not boot with a computer supply. If the board successfully boots in the game, this could be the cause. This condition is rare, but possible. Also, if the LED locks on immediately, try doing a manual reset by briefly shorting together pins 39 and 40 of the U9 CPU with a small screwdriver. This forces the pin 40 to go low and begin the boot process. It is also possible to short the junction of resistors R1 and R3, on the right side of R1, to board GROUND with a jumper clip to accomplish the same effect.


4.7 Solenoid Driver Board Issues

4.7.1 Testing and Replacing Transistors

Before and After of Replaced Drive Transistor, Diode, and Resistor


To test a transistor set the DMM to diode test mode. With the game turned off, place the black lead on the metal tab of the transistor. Probe the two outer legs of the transistor with the red lead. The DMM should read between .4v and .6v (some DMM will show 4xx - 6xx). The center leg should be a dead short to the metal tab. If either outer leg reads anything outside of the .4v to .6v range, the transistor more than likely needs to be replaced. Use a TIP-102 as the replacement transistor.

If a transistor needs to be replaced it is usually a good idea to check / replace the 1n4004 diode and 330 ohm resistor associated with that transistor as these will often burn / short. Also check the diode on the associated coil, it is likely to be shorted as well.

4.7.2 Replacing a failed Solenoid Transistor

If any coil has locked on, or any SE9302 transistor has failed the diode test with your multimeter, replace the following components on the Solenoid Driver board as a set. In addition, also test the playfield coil disconnected from the game wires, with the ohm setting. The coil should have a value higher than 3 ohms up to around 15 ohms. Lower than 3 ohms is a dead short and will heat up and burn up components if the fuse does not blow. The coil diodes should also be replaced if the coil has locked on. Since the diode must be disconnected from the lugs to test it, it just makes sense to clip it off, replace the diodes, and not bother to test them. 1N4004 diodes or better, (1N4005,6 or 7) should be used. You cannot test the diode in place. The diode band (cathode) goes to the power lug of the coil, usually a double wire connection, because the coils are daisy chained. If it is a single wire, look at another nearby coil and note the color of the double wire.

Replace any suspect SE9302 with a TIP 102 transistor. They are used in many pinball machines, so having a small supply of them is highly recommended. The TIP 102 is a more robust transistor and can handle more current. On the SDB replace the associated diode, and resistor for any driver transistor you replace. Check the associated pin for signs of burning and replace the header pins and connector pins if they have turned brown or look corroded. If a diode and/or resistor has actually burnt, check for continuity and make sure the circuit board traces have not also burnt up. This is a common problem when someone has over-fused the game.

4.7.3 Transistor Mapping

Bally Transistor Mapping


To view a chart of what specific transistor controls a specific coil on a particular Bally game (transistor mapping), click here.

4.7.4 Solenoid Driver Board Transistor, Resistor, Diode & associated Connector Chart

TIP 102 1N4004 Diode 330 ohm Resistor SDB Connector
Q1 CR1 R9 J1-pin 2, J2-pin 9
Q2 CR2 R6 J1-3, J2-4
Q3 CR3 R16 J2-5, J3-4
Q4 CR4 R18 J1-5
Q5 CR5 R10 J2-10
Q6 CR6 R12 J2-11
Q7 CR7 R14 J2-12
Q8 CR8 R20 J5-10
Q9 CR9 R26 J5-9
Q10 CR10 R28 J5-15
Q11 CR11 R32 J5-14
Q12 CR12 R30 J5-13
Q13 CR13 R22 J5-12
Q14 CR14 R24 J5-11
Q15 CR15 R39 Flipper Relay
Q16 CR16 R34 J2-6, J3-7, J5-8
Q17 CR17 R42 J5-7
Q18 CR18 R45 J2-15, J3-9, J5-3
Q19 CR19 R47 J2-8
TIP 102 1N4004 diode 330 ohm Resistor SDB Connector

4.7.5 Solenoid Driver Upgrades

Visual of Solenoid Driver Upgrades

There are several upgrades which can be performed to reduce stress on connectors.

The two 5v test points on the SDB can be tied together. On the solder side of the driver board jump TP1 and TP3 together. Be careful to apply this to the correct test points. This eliminates a possible failure point on J3 pins 13 and 25. Note that TP1 is for the raw +5vdc coming directly from the regulator. TP3 is for the voltage that feeds all the +5vdc circuits in the driver section. If there's a problem in the driver section, it can be isolated by removing this jumper, and removing pin 13 or 25 from J3.

Next the C23 capacitor needs a common ground with the rest of the SDB. Jump the negative side of C23 to a convenient ground trace. On most boards there is a ground trace in the vicinity of the C23 negative lead. Other boards might require a longer jumper. Scrape the solder mask off the trace to get a good solder surface to solder the jumper. If replacing the C23 capacitor, (a good idea, they can be 30+ years old at this point), it may be possible to leave the negative lead "long" on the new capacitor. Then, bend the leg down to the ground trace, and solder it in both the mounting hole and the trace.

The C26 capacitor needs a ground upgrade similar C23 mentioned above. Tie the negative side of C26 to the ground trace located at the perimeter of the board. This connects both C23's and C26's negative side to a common ground on the SDB. The goal of these modifications is to provide redundant grounds with the least resistance possible.

4.7.6 How to Rebuild the Bally/Stern Solenoid Driver Board

The Solenoid Driver board is critical to the operation of the electronics of the game. The Solenoid Driver Board (SDB) comes in several types for Bally and Stern, and are completely interchangeable for any game of the 6800 MPU type. The SDB supplies the game with the voltages for the MPU, coils and high voltage for the displays. Recommendations are given below for replacing components to ensure proper operation and reliability. In all cases, do the ground modifications as shown in another part of this Wiki.

Minimum Recommendation for a Working board

At a minimum, replace both large capacitors on a working or non-working board, especially if they look original. Original caps are often metallic blue or metallic silver, and are at least 30 years old and due to fail, owing to the electrolytic chemicals in the capacitor drying up. The capacitor at C 23 has a factory value of 11,000 uf and 20 volts, but these values are not easily found these days. An electrolytic capacitor of between 11,000uf and 16,000 uf and 25 volts or greater can be safely substituted. A screw terminal capacitor makes for a easy installation, but a snap cap with leads can be used as well. Recent prices for screw terminal caps have increased greatly so this may be a factor in your decision.
The high voltage capacitor at C26 has an original spec of 160 uf and 350 volts and is an axial electrolytic capacitor. Once again, this part is difficult to encounter with these exact specs, but an axial or radial cap from 150uf to 180 uf and at least 350 volts can substitute. 400 v or 450v caps can be had in a radial format and size that will fit. With a radial cap you will have to make leads that bend back to the negative (-) solder pad.

Preferred recommendation for a working board

Besides the above, replace ALL the .100 and .156 molex header pins and the connector terminals in the nylon housings with Molex TRIFURCON Phosphor Bronze tin plated crimp terminals (Molex Part# 08-52-0113 for 18-20 ga. Wire, and 08-52-0125 for 22-26 ga.) for the .156 connectors, and Molex .100 tin plated Phosphor Bronze crimp terminals, (Molex Part#08-52-0123). Do NOT skimp on this step and just do one or the other. The receptacles can be re-used if not burnt. If replacing the receptacles, the locking or non- locking ramp type are fine. Often, the connector at J5 does not cover the last few pins. I don’t like this personally, although no harm can be done if the key is in place.
Check every resistor that is usually covered by the plastic shield between the two big heat sinks for correct value and no sign of burn. Replace any that are suspect. Use the diode function on your multimeter to check the zener diode at VR1 and the 1N4004 diode. The zener diode can be difficult to source. See below for recommendation.

Gold Standard for Working or NON-WORKING board

It doesn’t make too much sense trying to trace down the exact problem and replacing only the bad components on a NON-WORKING board. Better to replace every possible bad component and start fresh. To ensure long life and proper operation of a working board, in ADDITION TO THE ABOVE, it is recommended that the components below are replaced regardless. Below is a list of components that effect the HV and +5 volt logic circuits on the SDB. Replace them all! * The Stern Revision J board is rather different, & a component list for that board follows. It can be identified by the extensive silk screening of each component and its function as seen in the 4th photo above, and by the HUGE 2 Watt and (2) large 1 Watt resistors. It is most often found on late date Stern games like Flight 2000 or Viper, etc.

Component replacement list for Bally AS2518-16 or -22 and Stern SDU100 SDBs, except * revision J

Resistors

  • R51 22k ohm 1/2Watt
  • R52 390 ohm 1/4 Watt
  • R54 8.2k ohm 1/4 Watt
  • R55 1.2k ohm 1/4 Watt
  • R56 82k ohm 1/2 Watt
  • R35 100k ohm 1 Watt

Diodes

  • CR21 1N4004 400PIV 1 amp or better (1N4007, etc.)
  • VR1 Zener diode 1N5275A 140 volts, 1/2 Watt. Can Sub 1N5275B or NTE 5099A

Transistors

  • Q21 2N3584 250volts, 2 amp, TO-66 NPN
  • Q22&23 2N3440 250 volts 1 amp TO-39 NPN
  • Q20 LM323K (original 78H05KC or LAS1405)

Capacitors

  • C27&28 .01 uf 400 vdc metal polyester capacitor
  • RT1 25k ohm potentiometer (2 types) a 15mm black one, Piher Part# PT15LH06-253A2020 or a 6mm blue one BournsPart#3306P-1-253

Stern SDU100 revision J

  • R35 100k ohm 1 Watt
  • R51 33k ohm 2 Watt
  • R52 390 ohm 1/4 W
  • R53 2.4k ohm 1/4 W
  • R54 8.2k ohm 1/4 W
  • R55 1.2k ohm 1/4 W
  • R56 82k ohm 1/2 Watt
  • R73 3.3 ohm 1/4 W
  • R74 470 ohm 1/4 W
  • R75 100k ohm 1 Watt
  • CR22 1N4004 or better
  • Q24 2N3904 Transistor

Other diodes, transistors and capacitors as above for AS2518-18

4.7.7 Modify the Display Fuse holder

alternative text
Fuse adapter for the SDB allowing use of a common fuse




Some versions of the solenoid driver board have a small fuse present for the displays. This fuse is a difficult to find, expensive, type 8AG 3/16 amp fast blow fuse. It would be best when rebuilding the SDB to replace the fuse holder with new clips and convert it to a more commonly available 3AG type. The original value of 3/16 amp can be substituted with a more common 1/4 amp fast blow fuse without worry of over fusing the circuit, due to inherent variations of tolerance by the manufacturer.

Perform the following to replace the existing 8AG fuse holder:

  • Bend up or clip off the upper solder lead of the clip
  • Move the TOP fuse holder up one hole
  • Solder a 20 ga. wire from the base of the clip back down to the lower hole.

Be careful not to solder the clips with the inner "ears" facing in the wrong direction, or the work will have to be redone. It may be helpful to put an old 3AG fuse in the clips to aid in spacing, and assure that the "ears" are positioned correctly. The fuse will also hold the clips in place while soldering. See the photo above. Of course, an inline fuse holder can also be used instead, but looks less professional.

4.8 Solenoid problems

Before proceeding to diagnose solenoid or problems, see this section: How coils are turned on

4.9 Lamp problems

4.9.1 Lamp Sockets

Most lamp problems stem from the sockets themselves. The recommended best practice for problem sockets is to replace them. If you want to try and get them working reliably, here are some things you can try.

First off, clean the inside of the socket. There are nice dremel tool tips to help, but even a screw driver can scratch off some crud from inside a socket. Sometimes a light squeeze on the socket with pliers can make the bulb stay in nice and snug and be less likely to move and stop lighting.

Repairing Single Lamp Sockets

The bare wire running under the playfield for controlled lamp power carries 5-6 vdc and is usually soldered onto the sockets mounting bracket. The mounting bracket then connects to the outside of the socket. After 30 years or so the connection from the mounting bracket to the lamp socket's outside can break. To remedy this there are two things you can try. You can solder the barrel of the lamp socket directly to the base of the socket (so it can no longer swivel). This can be kind of tricky without a higher power soldering iron - if you use paste flux it is easier. Radio Shack also sells a solder called "Crystal Clear" flux which works well. To do this well, you need to either sand or file (the file works best and saves your fingers) the area to be soldered on the base and the barrel. The other option is to run a small jumper wire that connects the bare wire directly to the outside of the socket, thus eliminating the poorer connection between the mounting bracket and barrel.

Another less common issue is the connection between the lamp socket's solder lug (insulated wire to the driver board connects here), and the back tip of the socket (what moves out/in and is spring loaded to hold bulb in place). What you can do here is to move the insulated wire off of the lamp socket's solder lug and solder it directly to the back of the socket tip. This eliminates any potential connection issues with the solder lug/tab and the back pin.

Before soldering onto a socket, either the barrel or the spring terminal, it must be sanded or filed clean or the solder will not stick. Paste flux or the crystal clear flux solder recommended above will help. If you have a temperature controlled soldering station crank it up above what you would normally use for boardwork to about 800 degrees.

Repairing Multiple Lamp Sockets Tied Together



Another issue is the multiple lamp mounting boards/brackets like those used in Stern Big Game, Meteor, and others. The lamp sockets are riveted to these mounting boards and are not replaceable. These mounting boards suffer from a bad connection to the bare controlled lamp wire running under the playfield. You can either solder each lamp barrel to the base plate or run a daisy chain wire from barrel to barrel to help fix issues with the sockets. Again, lamp sockets need to be sanded (200 grit works well) or filed before solder will stick.


4.9.2 Failed Lamps SCRs

MCR106 Used in Place of a 2N5060

If the lamp is always stuck on usually the SCR driving it is bad. Bally/Stern use two different styles of SCRs to control lamps. The MCR106 is able to drive up to two lamps at the same time. The smaller 2n5060 will only drive one lamp. These SCRs do fail from over sinking of current, from old blackened bulbs (which draw more current) or from being shorted to something under the playfield carrying a high voltage (solenoids). If you accidentally short several lamps together, this would cause the SCR to fail also.

If you are in a bind it is possible to use an MCR106 in place of a 2n5060 with one caveat. Two legs of the MCR106 SCR need to be reversed to be used as a replacement for a 2n5060. As you are looking at the board the top and middle leg of the MCR106 need to be reversed.
**Note: a 2n5060 CANNOT be used instead of an MCR106

Still can't get your controlled lamp to light up and you're sure it's wired up right and getting power? A lamp that never lights might be a bad SCR that needs to be replaced or the connection that runs from the lamp board to the lamp. Use your DMM on continuity test to follow the path from the SCR to the lamp itself to find out if it is a connector issue.

On some lamp boards there are buffer and decoder chips. A bad buffer or decoder chip can prevent a lamp from lighting. If you have multiple lamps out and you have tested the wiring for each, look back to the chips to see if the lamps out have a chip in common. Refer to the game's schematics. If so, and you have tested the SCR's as good, the chip is possibly bad. The .100 headers on the lamp driver boards are subject to a lot of stress and should be resoldered to ensure there are no hairline cracks in the connectors.

Bally Lamp Driver board using MCR-106 SCRs
Stern Lamp Driver board using MCR-106 SCRs
A Bally/Stern lamp driver board, showing proper orientation of smaller MCR-106 SCRs. Note the position of the metal part of the component relative to the silkscreen.


Pay close attention to the orientation of an MCR106 when replacement is necessary. Bally and Stern lamp driver boards have the MCR-106s installed opposite of one another. T106 SCRs in a TO-202 package (the style with an extended metal tab) are much easier to tell how they are oriented.

4.9.3 All Controlled Lamps not Working

If all of the controlled lamps are not working properly in the backbox, on the playfield, or both, this could be due to several reasons. Bad / marginal connectors, either on the rectifier board itself (male header pins) or the female housing that connects to it. A bad lamp bridge rectifier or blown controlled lamp fuse on the rectifier board could cause this too. The easiest way to determine if the controlled lamps are receiving the proper voltage on the playfield or backbox is to probe the bare controlled lamp bus wire (make certain to probe the controlled lamp bus and not the general illumination bus) on either or both areas with the red lead of a DMM. Then, connect the black lead of the DMM to a known, solid ground. Voltage should read approximately in the 6-8vdc range.

Additionally, if the +5vdc is missing on the lamp driver board, no lamps will light. Only one incoming pin (pin 3) on the J4 connection (upper left connection on board) of the lamp driver board carries the +5vdc. Check for the presence of +5vdc at TP1 of the lamp driver board.

Finally, the lack of a solid ground on the lamp driver board will also cause all lamps not to light. Most all of the different variations of the lamp driver board have the grounds secured via the screws which fasten the board to the backbox. However, the second generation of the Bally lamp driver board, the AS-2518-23, does not have these extra ground points. Since all of the lamp driver boards are interchangeable, there is the potential to have any one of the boards in the backbox. Pins 1, 2, and 11 of the J4 connection (upper left connection on board) carry the incoming grounds for the lamp driver board. The grounds originate from pins 3, 4, and 14 at connector J3 of the rectifier board.

4.9.4 Dim controlled lamps

The biggest cause of dim controlled lamps is a weak connection via the power chain's connectors. The original lamp driver bridge rectifier is slightly undersized on the rectifier board at 8 amps; either changing to #47 bulbs can alleviate some of the load placed on this bridge, or it can be upgraded to a larger capacity (25 amp or higher). Changing the header pins on the rectifier board to a 10 amp capable pin plus changing the connector pins to Trifurcon connectors will help the power chain. Application of a dielectric to prevent oxidation can help prevent new pins from prematurely wearing out, but do not apply any chemical to old, tarnished pins.

The fuse clips on the rectifier board can get tarnished, too. Replacement with high current phosphor-bronze clips will help dimming issues. The less resistance in the power chain the better and brighter your lamps will be.

4.9.5 Lamp Driver Board Attacked by Battery Corrosion

Lamp Board Attacked by Corrosion



Lamp Board Traces Attacked by a Leaky MPU battery

Corrosion can drip down from the battery on the MPU board to the lamp board. If this happens replace components with corrosion on them.

Although it is not nearly as common, a leaky battery can destroy the traces on the solder side of the lamp driver board. If everything else has been checked regarding some lamps not functioning, remove the lamp driver board from the game, turn the board over, and inspect the board traces. Primarily the top horizontal traces are the most susceptible to alkali damage. Since alkali damage is very unpredictable, any of the other areas of the board may be damaged.

4.10 Switch problems

Bally / Stern 5 x 8 Switch Matrix







Bally/Stern games use a switch matrix arranged as a 5x8 matrix for a total capability of 40 switches. Some games (Medusa, Spectrum, and others) have a sixth column of switches making a total of 48 switches in a 6x8 matrix. This guide will concentrate on the "normal" 5x8 matrix but the information applies also to the 6x8 matrix; the sixth column signal wire is merely in a different location, and for switch matrix troubleshooting purposes the information is interchangeable.

Columns 0 and 1 are shared by the playfield and cabinet switches. Any problems with switches in these columns can be further diagnosed by disconnecting the J2 connector (playfield) or J3 connector (cabinet) to isolate the playfield, cabinet, or both from switch tests. Switch test on Bally/Stern machines is entered by pressing the self-test button on the coin door 4 or 5 times, depending on the game. The stock software in each game only shows the lowest switch number that's closed; multiple closures or an entire row/column closed will still only show the lowest switch number. (There is a special test rom available that will walk through all the closed switches one at a time).

Start a switch diagnostic test by removing all balls from a game and placing all drop targets into an upright position. Enter switch test and you should have a zero flashing on the match display, signifying no closed switches. Use the chart from the manual in your game if you see a closed switch to investigate why it's showing closed.

Probable reasons for a stuck switch are:

  • points or blades adjusted too closely/touching
  • switch diode touching lamps/other wiring
  • solder splashes/conductive debris on a switch
  • shorted switch capacitor, if used

Here is where disconnecting the playfield will help. If you disconnect the switch connector for the playfield from the MPU board (J2) and the stuck switch stays, you know the problem is not on the playfield. If the switch is in column 0 or 1, you might have a problem with the switch matrix going to the cabinet. It is common for switches on the coin door to become mangled, either being stuck closed or shorted to the coin door in some way. You can disconnect the coin door separately while in switch test to see if the switch faults clear.

If you disconnect both J2 and J3 isolating all switches in the game from the MPU board, you should always see the flashing 0 indicating no switches closed in switch test. If you do not, there is a board fault that should be investigated and corrected. All of the signals for pulsing and reading the switch matrix originate and terminate at the U10 6821 PIA chip. As a quick test for this chip, you can swap it with the U11 PIA to see if the problem's symptoms change. If they do or the switch matrix starts to work correctly, you have a probably failed PIA chip and should replace it. Be aware that the U11 PIA chip controls the solenoids in the game, so if the U10 chip is bad in any way, you might have locked on solenoids causing more damage if your fuse doesn't blow. Before swapping the PIA chips, remove the solenoid driver board's .156 connectors to the solenoids to prevent damage. You could also just remove the J4 connector that feeds the solenoid driver board's input signals. (Lower right connector on the SDB).

Sometimes due to wiring error a switch diode is reversed in the switch matrix, either from previous repair work, or even from the factory. When this happens, you will see the wrong switch closing. Test for this by entering switch test and activating each switch one at a time referencing the switch chart in the manual, ensuring that each switch reports the correct switch number. This means that each drop bank will need to be reset by hand after each drop target test. If the game in question has a spinner, test the spinner with the playfield down and then place a piece of paper between its switch points so testing other switches with the playfield up doesn't show a false positive on the spinner from movement.

Momentary contact switches will sometimes have a switch capacitor installed. The purpose of this capacitor is to ensure the MPU board "sees" very quick hits to a tilt, pop bumper, sling, or standup target. Switch capacitors should not be installed on rollover, saucer, spinner, outhole, or trough switches. There are a few games that do have capacitors installed on these switches. You can cut these off safely and not worry about replacing them.

A symptom of a failed (shorted) switch capacitor will be a stuck switch showing in the switch test, and no scoring/scoring once/multiple repeated scoring in gameplay. As a test, you can cut one leg of the capacitor off a suspect switch and see if the problem goes away. If it does, the capacitor was probably bad and you can replace it. (A 0.047 mfd 16 volt or higher non-polarized ceramic disc replacement works well.)

Quick hits to switches that do not have capacitors installed can have a capacitor installed to improve performance, or if you have a pop or sling that activates when hit but does not score. You can add the 0.047 mfd capacitor to these switches on the two lugs that do not have the diode installed; one end of the capacitor goes to the blade lug that has one end of the diode but not the matrix wiring, and the other end of the capacitor goes to the switch lug that only has the matrix wires without the diode. If you do add a capacitor to a sling or pop bumper, be aware that it is possible for radio frequency interference from a coil's collapsing magnetic field to be picked up by the capacitor, causing the switch to activate. This is most common with flipper coils - holding a flipper up and releasing generates a short pulse of radio frequency static.

It is good preventative maintenance on machines that have switch capacitors to methodically replace each and every one with a new one. Adding a switch capacitor to pops and slings is a good idea also as you can run a slightly larger switch gap on the activation switches, which helps to prevent switch bounce and repeat, unwanted, activations.


4.10.1 Leaf Switch Theory,Operation and Adjustment

Leaf switches in pinball games come in 4 formats: Normally Open (N.O.), Normally Closed (N.C.), Break-Make, & Make-Make, the last 2 of which are really combinations of a N.O. and/or a N.C. switch. Regardless of their function, all switches are in fact, a N.O. or N.C. type.

Leaf switches in electronic games are of 2 compositions,at least 2 bronze blades each holding a gold flashed contact or a Tungsten contact. Since gold is a very high conductor of electricity, it is used on most switches, with the exception of the flipper cabinet and End of Stroke (EOS) contacts, which are tungsten. Gold will not tarnish or turn black, but sometimes gets dirty from solenoid dust, and this may effect its conductivity. Tungsten flipper contacts must be redressed with an ignition-type flat file as used in automotive repair, as the contacts are too hard to be smoothed by sandpaper or flex file abrasive. Gold flashed contacts must NEVER be cleaned with a file or sandpaper. A business card, or thin cardboard, is all that is necessary to clean electronic gold flashed contacts. If they seems to have a foreign substance on them, a Q-Tip with a degreaser like Mean Green, or alcohol can be used and then wiped off. Contact cleaner must NOT be sprayed liberally on the switch, but can be applied with a swab.

4.10.2 Normally Open (N.O.) Switch
Normally Open Switch in resting state
Schematic rendering of N.O. & N.C. switches

This is a leaf switch that has 2 contacts that complete a circuit when pushed closed by some mechanical device. The switch re-opens when the device moves away from the moveable contact, and the springiness of the blade opens the circuit.

4.10.3 Normally Closed (N.C.)Switch
Normally Closed (N.C.) switch in resting state with optional nylon spacer

This type of switch opens a circuit when a mechanical device pushes against the blade.

4.10.4 Break-Make switch
Break-Make switch in resting state

This switch is really a composite of a N.O. and a N.C. switch with just three blades instead of four. The movable center blade has contacts on BOTH sides. Proper adjustment insures that when a device moves the center blade towards the open contact, the CLOSED switch OPENS BEFORE the center contact touches the open switch’s contact. When the pressure is released, the center blade returns to its N.C. side and re-makes contact. This type of switch is often used on relays. A Make-Break switch is really the same, just upside-down, or in reverse, of the order of actions of a Break-Make switch. Special care should be taken with a Make-Break switch that the three blades are not at any time shorted together. This can happen often when the blade stiffener is maladjusted, and contacts the other blade of the pair. Note that this type of switch is rarely seen in Bally/Stern solid state machines.

4.10.5 Make-Make switch
Make-Make switch in resting state

A make-make switch is a composite switch consisting of two N.O. switches using only 3 blades. When mechanical pressure is applied to the blade, the switch “makes” or closes the circuit, and continues to move until a second switch contact is closed, tying all 3 blades together.

4.10.6 Adjusting Switch Gaps

Proper switch clearance gaps are crucial for correct operation of any pinball game. Very many problems in games come down to a problem with incorrect operation of a switch. The switch blades are made up of phosphor bronze, which has high conductivity, resilience to form and excellent soldering characteristics, but has the drawback of being brittle compared to copper. Switch blades therefore, are commonly broken by the game, or careless actions by the repair person. A switch adjusting tool, purchased or homemade, is a necessity. Pliers or screwdrivers might be used in special circumstances, or in extremis, but are not best practice.

Switches in games are designed to have a degree of over-travel, which creates a wiping motion that helps clean the switch. The contacts must meet, stay together and slide across the face of the contact slightly, to be perfectly adjusted. They also must be gapped wide enough that they do not bounce, causing multiple switch closures. This problem is seen often in electronic games, when the rapid nature of the switch closures cause machine-gunning of the slingshots .

Before adjusting switches, make certain the screws holding the switch stacks are down tightly. Bakelite spacers in the switch stacks, due to excessive moisture, have occasionally shrunk by drying out, causing poor adjustment. After tightening a switch stack, be sure to recheck the gap of each switch.

To adjust a leaf switch, first make sure the switch stack of bakelite or plastic spacers and blades are tightly packed against each other by giving the 2 screws a twist of your screwdriver. As long as there is no screw movement with reasonable pressure, you can procede. Some switches have a regular nut on the backside instead of a speednut, which will require holding it as you turn.

Using your switch adjusting tool, put twisting pressure on the moveable switch blade as close to the switch stack as possible, and include the blade stiffener(if present) in the adjuster slot. Try to use as little force and change of gap as possible, and incrementally reach your gap goal. Using too much force and a back and forth gap readjustment, tends to bend and weaken the switch blade, and can make a deformed switch blade difficult to set for correct operation. This is often seen in games that have been “hacked”, and sometimes the only way to remedy the situation is to replace the entire switch, or disassemble it entirely. Cycle and recycle the mechanical device that activates the switch and observe the switch for proper gap, wiping action and release.

4.10.7 Recommended Gap Specifications

Kicking rubber, or slingshot switches usually have a generous gap, at least 1/16", due to the rubber ring pushing back against the switch, and a bit of trial and error will be necessary to find the ideal gap setting. Pop bumper switches and scoring “dead” bumper switches, should have a gap of 1/64" for the actuating switch under the spoon, and a 1/32" gap for the score switch when open. Flipper cabinet switch have a generous gap which can be adjusted to suit the owner of the machine to provide a good "feel". All other types of switches will probably function well with a 1/32" gap, and this would be a starting point, modifying the gap on a case by case basis.

4.10.8 The Importance of Diodes and Capacitors on Switches
Orientation of Diodes
A ceramic capacitor of the type used on switch. NOTE: The number is NOT the correct value for switches. Use 0.047 mfd for replacement


On all Bally and Stern solid-state (SS) pinball games, there MUST be a diode connected to each pair of switch blades. The diode’s purpose is to isolate the switch from the other switches in the switch matrix. A shorted or reversed diode will cause the MPU to read the strobe signal on the wrong return line, causing a switch read error.

Bally (and Stern) used a matrix of 5 columns (strobes, designated 0-4) and 8 rows (returns, designated 0-7) allowing for 40 switches. Some Bally games have a sixth column allowing for 48 total switches. The sixth column games re-use a signal pin from the earlier games that was the sound board address "E" pin. Stern uses this same pin for the 7th digit enable on their displays, and never used it for their sound boards. No Stern game has a sixth column of switches.

Bally and Stern both state 1N4004s as original, but 1N4148 diodes can be used, and in some cases, were used from the factory.

4.10.9 Switch Capacitor Purpose and Failure

Capacitors are used on switches that are normally subjected to very quick hits, to lengthen the switch pulse that the MPU reads. Not all switches will have capacitors; standups, pop bumpers, slings, and tilt mechanisms are the usual ones that benefit from them. All switches that need a quick reaction can have switch capacitors added to them (such as rollover buttons). These are ceramic non-polarized capacitors of a value of .047 or .050 uf and 16 volts or greater. A commonly available cap is the .047 uf 50 volt through Mouser P/N 140-50U5-473M-RC.

The factory installed caps are often damaged by the high wattage industrial soldering irons used in manufacturing the games, or they succumb to the vibration over time and no longer serve their purpose of lengthening the switch closures so the MPU can read them. Often a repairman will clip the cap off to diagnose a stuck switch problem and not bother to replace it. The switch will operate in this fashion, but for more sensitive play, the capacitor should be replaced. A failing cap can also make a switch bounce, and cause machine-gunning of a slingshot or pop bumper.

Sometimes the operation of the flippers or other solenoids will cause a phantom switch close on a pop bumper or slingshot - this is due to the way the software reads the fast react switches on those mechanisms. The RF produced by the flipper coil's collapsing EMF field is picked up by the switch capacitor, causing the pop or sling to fire. In most cases, no score is added, the mechanism just operates. Keeping the leads short on the switch capacitors might reduce their tendency to pick up this stray RF in the machine.

4.10.10 Proper Location of Wiring, Diodes, & Capacitors

Color coded wiring of the playfield switches will vary by game model and manufacturer, but the following should hold true for any Bally or Stern game made from at least 1977 to 1985 . Switches may have two or three lugs, depending on function. If the switch has 3 lugs, one of them is a DEAD LUG, which is electrically isolated from the other switch blades and ground. It may be that the dead lug is off to the side, or positioned the same as the other lug, and it may be difficult to visually verify which is the dead lug. The Dead Lug does not have a blade. The purpose of the dead lug is to have a isolated soldering point for the COLUMN wire(s) and the NON-banded (anode) end of the diode.

Switch capacitor is wired as shown, some switches will not have one.
A stand up target with capacitor & diode


A 3 lug switch will have the following connections.

  • A switch capacitor (if required) between the 2 switch blades.
  • A ROW wire(s), (with a capacitor leg, if needed) to one end of either switch blade
  • A COLUMN wire (usually daisy chained) plus the NON-banded end of the diode to the DEAD LUG
  • The BANDED (cathode) end of the diode to the opposing switch blade, either by itself, or sharing the lug with a leg of the capacitor


Remember that switches are usually daisy-chained together, so except for the last switch in a group, there will be a common COLUMN wire (sometimes NOT insulated) between each switch, along with the NON-banded diode. Any break is this wire will cause all the switches below it to not function. Keep in mind that the schematics show a logic progression of switches which may or may not match the physical chain under the playfield. A failed or leaky diode can only be tested for by desoldering one end of the diode & checking with the diode test function of your multimeter. It is very little more work to just simply replace the diode with new if it is suspected.

4.10.11 Troubleshooting multi- ball games that will not start

Bally produced multi-ball games which require ALL balls to be in the correct position before a game will start. If a pinball is stuck on the playfield, missing, or the switch does not register, is broken or maladjusted, then the game will not start. The symptoms each game shows varies. After the game boots, a few insert lights may flash repeatedly, or the game will not coin up, the general illumination will not light and credits will not count down, etc. Check the chart below for specific games.

These switches can be checked in the game diagnostic switch test see if they are stuck closed, or manually operated to see if they register. Remember that only the LOWEST number closed switch will appear on the display, so if other switches are closed or have a fault, they must be cleared to check for the higher switch number.

Game # of balls Symptoms Check switch
Centaur 5 All zeros on; displays flicker #01 Ball trough 4 
#02 Ball trough 5
#08 Outhole (will start with #01 or #02)
Elektra 2 Will not start; displays blank #01 Outhole 
#02 Left/Right of Outhole (Starts with #01 or #02)
Embryon 2 Always starts #05 Left/Right of Outhole 
 #08 Outhole
Fathom 3 Won't start; no GI, 2 insert lights flash, displays blank #01 Outhole 
 #02 Left of Outhole 1
 #03 Left of Outhole 2 (Starts with #1& 2, OR #1&3)
Fireball II 3 Won't start; displays blank #01 Outhole 
 #02 Left of Outhole
 #03 Left of Outhole
 #18 Right of Outhole
Vector 3 Won't start; displays normal #01 Left of Outhole 2 
 #02 Left of Outhole 2
 #03 Outhole
Xenon 2 Won't start; displays blank #02 Ball release 1 
 #28 Ball release 2


4.10.12 Electrical short troubleshooting Fuse helper
alternative text
Circuit breaker with a blown fuse soldered in position

As an aid to finding the cause for an electrical short, you can make a circuit breaker tool to eliminate the need to constantly replace a fuse, while you investigate the cause. This could save you money in the long run. It is better to under-fuse the connection by a small amount, then to use the rated fuse rating, so a 3 amp circuit breaker could be used on a 5 amp fuse holder, or a 10 amp breaker for a 20 amp G.I. fuse holder. The very small GMA fuses (5x20mm) will probably need some kind of jumper wire setup.

  • a burned out fuse of any amperage or voltage (you were saving them just for such a thing!)
  • a circuit breaker of 1, 3, 5, or 10 amps, whatever amperage you require for the circuit you are troubleshooting. Buy ones like Mouser P/N 691-CMB10311C3NBA (Carling) or 655-W57-XB7A4A10-10 (Tyco).

Solder the burned out fuse to the spade terminals of the circuit breaker as shown in the photo. Alternatively, make a two wire female spade mini harness, and solder the bare wires to each ends of the bad fuse and attach the circuit breaker that way.

4.11 Display problems

4.11.1 Display High Voltage Section

Bally High Voltage Section Repair

WARNING: This circuit uses high voltages. Don't continue, unless you are confident in your diagnostic abilities.

If all displays are blank, your high voltage (HV) section may not be working. On the Solenoid Driver Board (SDB), use a DMM to measure volts DC on test points TP2 and TP4, to determine if this section needs repair:

TP2 = 150 to 190VDC
TP4 = 230VDC +/- 25VDC

If TP2 is less than about 150VDC or over 200VDC, the HV section is malfunctioning. It is worth mentioning that when the source display voltage rises above the core acceptable levels, the displays will still operate. However, the increased voltage will strain the display circuitry. In turn, the displays glass life expectancy will decrease dramatically. If TP4 is 0VDC, then power is not getting from the Power Supply board, and the problem is located either on the Power Supply, or in the wiring/connectors carrying power to the SDB, so you must correct this before working on the HV section.

Replacing the components in the HV Section
(TP2=0VDC or 230VDC AND TP4=230VDC)
If you prefer to replace only the failed components, follow page 50 of the Bally repair manual, F.O. 560, dtd 20 June 1977.

Since there are not that many components, it is may be less time consuming to replace all the components in the HV section. Most of these components are located between the two large heat sinks. There is a school of thought when working with high voltage regulator circuitry that dictates replacing ALL the components, as a baked, tired, component can take out new components as well. This is one area where shotgunning the section might not be a bad idea.
HVSection.png


Check the Parts Suppliers section of the wiki to find suppliers, and get the following replacement parts (1/2 watt resistors can be substituted for 1/4 watt).

Part Part No. Location
Transistor 2N3584,250V,2A,NPN Q21
Transistor 2N3440,250V,1A,NPN Q22,Q23(heatsink)
Zener diode 1N5275A,140V,1/2W,10% or 1N5275B,140V,1/2W,5% VR1
Diode 1N4004,400PIV,1A CR21
Fuse 3/16A,8AG F1(-22 only)
Resistor 22k Ohm,1/2W,5% R51
Resistor 1.2k Ohm,1/4W,5% R55
Resistor 82k Ohm,1/2W,5% R56
Resistor 8.2k Ohm,1/4W,5% R54
Resistor 390 Ohm,1/4W,5% R52
Resistor 100k Ohm,1W,5%,metal film R35
Capacitor 0.01 uF, 400V metal polyester C27,C28
Capacitor 150 uF, 350V (if original, replace) C26
Potentiometer 25k Ohm, PC-mount RT1


Remove and replace components

  • Clip the old components from the board (make sure you have new ones first).
  • Use one of the desoldering methods to remove solder from the holes.
  • Stuff board with new components.
  • Check for correct orientation on transistors, diodes and the large capacitor if you replace it.
  • Leave a little space under components for air flow.
  • Bend leads on components so they won't fall out when board is inverted for soldering.
  • Double check that all the correct parts are in the correct places and properly oriented.
  • Solder the parts to the board
  • Clip excess off leads


Ready to test

Prior to powering up and testing, remove all display connectors, or test with F1 removed from the SDB. I recommend connecting your DMM with a clip lead to TP2 prior to powering the game, so you don't have to probe the HV section. Power up the machine and verify TP2 voltage is 150VDC to 190VDC. If the correct voltage is read, power down the machine and reconnect all the displays and put F1 back in. Leave your DMM connected to TP2.

Power up the machine and adjust RT1 so TP2 reads about 150VDC to 160VDC. If some of the displays no longer light, then adjust the voltage up until they are all lit.

New displays may require to be burned in at 190VDC for an hour or two, but then the voltage can be reduced. The lower voltage will prolong the life of the gas plasma displays.

If TP2 is not correct after your repair, you will need to recheck your work. If you cannot find a problem with your repair, then I recommend you follow the troubleshooting steps on page 50 of the Bally repair manual, F.O. 560, dtd 20 June 1977.

After you have repaired your HV section and verified that it is providing the correct voltage, the next part of the wiki will address how to correct problems with the displays.

4.11.2 Repairing Bally/Stern Displays

This page will help you repair your Bally 6- and 7-digit displays. The information is focused on the AS-2518-21 display. Since the AS-2518-15 display is interchangeable with the "-21" display, everything mentioned here will apply to both displays. Bally 7-digit displays (AS-2518-58) work the same way, they just contain additional components to support the additional digit, and lighting of commas. Some Stern DA-100 6-digit displays are very similar to the Bally versions, and the information here will also work for them. The Stern 7-digit display is slightly different, as its circuit board is physically deeper than any of the other displays and is not directly interchangeable with a Bally 7 digit display. Additionally, there are some input pin differences; however, if you have the schematic you can use the information here on the display, some transistors might be labeled differently.

First off, let's make sure your malfunctioning display is even repairable. If the glass display itself is shot, there's not much that can be done except to replace it. Here's a couple of photos of things that can go wrong with the glass display.

B-disp-d1.jpg
Burn spots on the digit segments - You can't fix these but you can live with them


B-disp-n1.jpg
Broken glass nipple - This display glass is ruined

Then there's the circuit board. Most if not all of the components can be replaced without much effort, but if there is damage to the circuit board itself, it may be impossible, or unreliable to repair.

B-disp-b1.jpg
Burn marks on the driver board


B-disp-b2.jpg
Underneath - The traces are a mess


So we're assuming you have displays that do not fall into the categories above. If your problem is with the digits not being displayed properly, and it's NOT the MPU's fault, then it can be fixed, and fairly easily too!

FIRST THING - Before you go any further, there are two things you should do to ALL your displays, even if they seem to be working fine. First, reflow the solder on the joints for the header pins. On the bottom side of the circuit board, just heat each header pin solder joint with your soldering iron, then add a little fresh solder. Be careful not to create any solder bridges between adjoining pins, unless there is already one there. Next, replace the six 100K ohm 1/4 watt resistors with 1/2 watters. They are the ones with the color code brown-black-yellow, and are labeled R1, R3, R5, R7, R9 and R11. You'll find seven of them, but only the ones I just listed need to be replaced. For 7-digit displays there will be eight of them, so replace R56 too, along with the six I just mentioned. 1/4 watt is just too small for their circuits and often they are overheated which causes them to change their resistance. On the photo of the burnt circuit board up above, the area that was burnt was under 3 of these resistors.

SECOND THING - Connectors, connectors, connectors! There are lots of signals going from the MPU to each display driver, and if these signals are not making it due to bad wiring or connectors, then your displays will misbehave. Check all the display driver connectors for loose or broken wires, burnt pins, busted connectors, and replace or repair anything that looks bad. Remember that some of the signals are daisy-chained from display to display, and a bad connection at one display can effect the others "downstream". Then check the J2 connector on the MPU, since this is where all the signals come from. Next check the J3 connector on the solenoid driver board. There's a high voltage regulator used by the displays on this board, and J3 is where it gets the 230 VDC from the power supply (pins 6 & 3) and feeds the regulated 190 VDC to the displays (pin 8). Finally, check the J3 connector on the rectifier board, since this is where the high voltage comes from. Connectors are always a big pain!

B-disp-hp1.jpg
Header pin solder joints - Notice the cracks around the last two pins on the right


B-disp-hp2.jpg
Header pin solder joints after being re-soldered - No cracks


B-disp-r1.jpg
Can you find the six 100K ohm resistors (brown-black-yellow)


B-disp-r2.jpg
Here they've been replaced them with 1/2 watt resistors of the same value (100K ohms)


Now we'll look at come common problems and briefly describe the cause, but I assume you've already replaced or checked the 100K Ohm resistors and re-soldered the header pins:

All displays are blank

Most likely cause of all blank displays is a lack of high voltage. First off, check F2 on the rectifier board. It's a 3/4 Amp slow blow fuse and it's test point on the rectifier board is TP2. You should read 230 VDC at this test point. If it's OK, then check the fuse on the solenoid driver board. The high voltage regulator's output is fused and if this fuse is blown then there's probably a short somewhere on one of your displays or the wiring harness. The high voltage regulator fuse is a 3/16 Amp, but it's a bit smaller (8AG) than a standard fuse, and a hard one to find. If the fuse is not blown, then the next thing to do is verify that at least 170 VDC is making it from the regulator to the displays. Check TP2 on the solenoid driver board. It should read between 170 and 190 VDC. If it does, then the regulator is working. While you're there, if your display glasses are not new, use the small trimmer pot on near the fuse and adjust the high voltage to be 170 VDC. Older displays work just fine on 170 VDC and it's less stressful.

Next thing to check is to be sure that the 170 volts is making it to the displays. Check connector pin J3-8 on the solenoid driver board, that's where the 170 VDC leaves. Check for a good connector, and check the solder joint on the header pin. The high voltage hits all of the displays at connector J1-1 and is daisy-chained from display to display. Check these connectors and the header pins too. On at least some early Stern games like Dracula, there is a fuse for the displays mounted on the back of the backbox door, often covered by a cardboard tag. It may be that these early games came without the fuse on the SDB. Check for proper amperage and continuity on this fuse holder if no voltage is reaching the displays.

Note - If there is any light at all on the display, then it is probably repairable. Some type of Bally display glasses will have some light on them, besides the digits. If there is light anywhere on the display, then the glass is most likely OK and your problems are elsewhere.

Dispglow.jpg
See how this display is glowing when it's first turned on? This indicates that the glass is good.

.

REMEMBER - 170 VDC WILL HURT!
TURN YOUR MACHINE OFF IF YOU'RE GOING TO MESS WITH IT BEYOND MEASURING VOLTAGES


If you have 170 VDC at all of the displays, and the decoder is still working, but the displays are still blank, then there is probably a problem on the MPU or the MPU connectors, such as a "stuck-on" blanking signal, etc.

If the blanking signal for the displays is stuck high or low on the MPU then the displays will remain blank. To check the blanking signal use your DMM or logic probe on MPU J1 P10. During the flash tests it should remain high (5v on DMM). Once attract mode begins it should begin pulsing (DMM will show somewhere between 1v and 4v). If the blanking signal is stuck high or low examine the blanking circuit including CR6, U14, U20. It could be the PIAs but unlikely as the power on flash test should check that the PIAs changes states. I have seen CR6 be open and cause displays to be blank.

A single display is blank

If you have one display that is blank, it could be the same problems as mentioned above (lack of 170VDC), something wrong on the circuit board, or something with the glass display itself. If you see any light glowing at all on the display, then the glass is most likely good and your problem is elsewhere.

The main cause of a single display being blank is a bad glass. If the glass is good, then your problem is most likely a bad 4543 decoder IC. The decoder on the display driver board is used to decode the digit data from the MPU into signals that light the proper displays. If the decoder is blown, then this could cause the display to be blank. A quick check with a logic probe will help you decide if the decoder is function properly.

Sometimes the solder joints on the display glass needs are cracked and need to be reflowed. Depending on which solder joints are bad, this could cause the display to be blank.

If you can verify there is 170VDc, and the decoder is working, then it may be just a bad display. If there is some light anywhere, then it's probably good. If not, it may or may not be good. A totally blank display is the hardest thing to fix. If everything looks good, but the display is completely blank, then you may just have to chalk it up as a bad display glass. Unfortunately they can not be repaired.

All displays are missing the same digit

If all the displays have the same digit out, then the problem is most likely caused by a bad connector at the MPU. The MPU daisy-chains the digit enable signal to all the displays, and if the same digit is out on all, then it's either an amazing thing that they all have a problem w/ the same digit, or the digit enable signal is not present. Check the connectors on the MPU and all of the displays. If all seem well, then one-by-one, disconnect each display and see if the problem goes away. If it does, then there's a short or a bad connector problem on the board you just removed. Be sure to turn the power off between swap-outs, and wait 10 seconds for the high voltage filter capacitor to discharge before you remove any connectors.

One display is missing the same segments on all the digits

Each display has it's own decoder chip, which takes four inputs from the MPU and outputs seven signals to light the seven segments of a digit. If you're having problems with one or more segments on all the digits, then it's either the segment driver transistor or the decoder itself. I'm sure there's a way to test the transistor but I just go ahead and install a new one. If that doesn't fix it, then replacing the decoder will. I've not had a segment problem that was not fixed by replacing both segment drivers and decoders (other than MPU problems). Be sure to orient the transistor and/or IC in the proper position. You'll notice a flat side on the transistor and a small notch on one end of the decoder IC. Be sure they line up just like the one you removed, and that the wires of the transistor go into the same holes that the old one came out of. Also, if you're replacing the IC, use a socket, so it'll be easier to replace it next time. Here's a handy chart that shows which segment driver transistor drives which segment, and another diagram of how the segments are labeled. Just find the segment that's giving you trouble on the chart, then look up it's corresponding driver transistor. Then find this transistor on the circuit board and replace it.

B-decoder-chart.jpg

One display has a segment locked on all digits

The segment driver has failed. When this issue is seen a lot of times the display will be blank except for the locked on segment. Replace the segment driver (see chart above) and then retest the display. If the display now works except for the segment drive you replaced is now turned off then replace the decoder chip. When the segment driver locks on it usually blows out that segment on the decoder chip.


One digit is out on one of the displays

If you have a single digit out, then it's probably the the level shifter and digit driver transistors for that digit. Just to be sure, check the connector on the display at pins J1-4 through J1-9. These are the connectors that supply the digit-enable signals from the MPU. Your problem may be that the display driver is not getting the enable signal from the MPU. If it is, then just replace the two transistors and you should be back in business. Here's a handy chart that shows you which level shifter and digit driver are associated with each digit. You'll find the transistor labels printed on the circuit board, or look at the diagram below. Just look at this chart, find the digit that's blank, and then locate the two transistors listed for that digit. Find them on the circuit board and replace them.

B-digit-chart.jpg

One digit is very bright, and the rest are out

If you have one digit that's very bright and all the rest are out, simply replace the digit driver and level shifter transistors for that digit. Turns out sometimes a short transistor will draw so much current that there's not enough left to drive the remaining digits, and so they all go blank. Just use the chart above to determine which transistors to replace.

Display(s) flicker

Closeup of Cracked Header Pins on a Bally/Stern Display Module. These cracks caused rapid "flicker" of the display.

This is almost always caused by broken solder joints on the header pins. If you already re-flowed the solder, double check them, then look for other broken solder joints and check all connectors for tight fits. Something is loose that's making them flicker.

Another cause of flickering or strobing digits is a faulty decoder IC. If your digits are flickering or "wavy", try replacing the decoder IC.

Here is a link to a video of a display board with a faulty 4543 decoder IC. Keep in mind that failed / failing decoder chips do emit symptoms other than what is seen in the video. Please see the other symptoms discussed above in this section.

Conclusion

That covers the most popular problems. I hope it covers yours. If you don't have the manual for your pinball machine, click on any of the thumbnails below for a large picture of the component layout of the all three boards. The 100K Ohm resistors are the ones colored brown-black-yellow.

4.12 Sound problems

Regardless of the sound board, it is always best to first remove the existing solder from the header pins, and reflow new solder onto them. Due to vibration over time, the header pin connections can end up with cracked solder joints, which is very common on the Bally sound boards. The next order of business would to replace the volume pot and the secondary pot, if one is used on the board. The original, factory pots used were not sealed, therefore, dirt and other contaminants can cause the pot to have dead spots or fail in general. Once these two things are done initially, troubleshooting the sound board, if it's still not working will be a lot easier.

If the board is still not working after the above recommended procedures, the next thing would be to check all of the voltages used on the board. See the Sound Test Point Value section for the test point voltage values for each particular sound board.

If all of the voltages check out, it's time to move onto the appropriate sound board troubleshooting section below.

4.12.1 -32 & -50 Sound Board Troubleshooting

Typical cracked solder joints on the header pins of a -32 board

The first course of action, before performing any troubleshooting to the board, is to inspect the header pin connections on the reverse side of the board. It is common to have cracked joints, where the header pins are soldered to the board. If cracked solder joints are apparent, heat and remove the old solder, and reflow new solder onto the connections. This will ensure that incoming voltages, ground and sound signals are solid on the board itself. Inspection of the connectors located in the female housing, which connects to the header pins, is equally a good idea. Replace any connectors that appear to be marginal. Molex Trifurcon crimp connectors are recommended as replacements.

Minimal component replacement for -32 and -50 boards


There are several components used, which should be replaced on these boards to minimize troubleshooting. Both the -32 and -50 sound boards use at least two 10Kohm pots. One for volume and the other for sustain. It is recommended to replace both pots for reliability. Likewise, a 1N4004 diode (CR3) is used on the board at the incoming +43vdc. Should this diode fail, it can cause the CPU to halt at 6 flashes upon boot up.

Capacitor kits for both the -32 and -50 boards are available from GPE. Although it is not always necessary to replace the caps on these boards, it is recommended.

Should the 86L93 (U11) chip fail, the 74LS93 is a viable substitute.

Voltage to look for and required for the sound board to work. TP1 = 5v, TP3 = 12v, TP4 = 43v. If any values are missing, check the connectors. Specifically check the integrity of the header pins.

12v regulator section:
Check for 67 volts on the positive side of C17. If missing replace CR3. If positive side of C17 is the same as TP4(43v) replace C17 and CR3.

If 12v is too high, check Q1, R22, CR4, and C18.

Clock Generator:
U1(4049) can fail. Check for about 2.5vdc where R3 and R5 meet. If voltage incorrect replace U1 which is a 4049 hex inverter. If still incorrect suspect U4 which is a 4526B programmable 4 bit counter. The 4526 can be purchased from vendors such as Mouser or Digi-Key.

Amplification / no sound issues:
First, make certain that the cabinet speaker is present, connected, and functioning. If an audible hum is heard from the speaker, then the amp is working. Next, check both pots. It is best to replace them if they are the originals (typically blue or black thumb wheel pots). Measuring across the two outer legs of the pot is the total resistance (10k). Measuring from the middle leg gives a resistance with regards to how the pot is adjusted (a value from 0 ohms to 10 kohms depending on the pot's position).

To check the amplifier at U9 (LM741), look for about 2vdc at U9 pin 2. If voltage is missing, replace the amp.


4.12.2 -51 Sound Board Troubleshooting

Bally AS-2518-51 Sound Board


Although the -51 sound board uses a 6800 family of microprocessor, the lack of an on-board, boot status LED makes this board more difficult to troubleshoot than the Squawk and Talk sound board. On the plus side, this sound board utilizes some of the same ICs, which are commonly found on the game's MPU board. Fortunately, all of the larger ICs on this board are socketed too. However, the sockets used on this board are subject to failure. Replacement with new, high quality sockets is advised for long term reliability.

The first course of action, before performing any troubleshooting to the board, is to inspect the header pin connections on the reverse side of the board. It is common to have cracked joints, where the header pins are soldered to the board. If cracked solder joints are apparent, heat and remove the old solder, and reflow new solder onto the connections. This will ensure that incoming voltages, ground and sound signals are solid on the board itself. Inspection of the connectors located in the female housing, which connects to the header pins, is equally a good idea. Replace any connectors that appear to be marginal. Molex Trifurcon crimp connectors are recommended as replacements.

Like all sound boards of this age, the capacitors are likely to have dried out and no longer work. Capacitor kits for this board can be acquired at Great Plains Electronics.

If the board is producing no sound at all, start by checking the volume adjustment pot. When turning the pot, the volume of the hum should get quieter and louder. If this is not heard, the amp/pot may be bad. If the hum volume changes, it is likely that the amp and pot are working. However, the 1K volume adjustment pot is 30+ years old, and was an "open" design. It is recommended that this pot be replaced with a quality sealed 1K ohm pot, whether or not the original pot tests good.

Check the power at the board with a DMM. TP3 is ground. TP2 and TP1 should measure 5VDC and 12VDC respectively. These are nominal values. Minor variation from these values is acceptable.

Next, it is time to check the large ICs on the board itself. First, swap the 6821 PIA at U2 on the sound board with either U10 or U11 on the MPU, and retest. Second, swap the 6810 RAM at U10 on the sound board with the 6810 at U7 on the MPU and retest.

If the problem persists, swap in a known good sound ROM at U4 and retest. Pay particular attention to the legs of U4, if it is a factory, masked ROM. It is common for the legs of these masked ROMs to oxidize or tarnish, just like the masked ROMs used on the MPU board. Hence, the legs can be fragile, and are prone to breakage. Likewise, tarnished legs on U4 could potentially be responsible for the lack of sound. Evidence of oxidation is visible in the form of blackened chip legs (silver oxide). The oxidation can be removed in several manners. The most common methods are gently using a pencil eraser over the legs, or soaking the chip legs in Tarn-X, followed by rinsing the legs in clean water, and allowing to dry before re-installation. As mentioned previously, if the legs are oxidized, use extreme caution when removing the chip from its socket, and removing the oxidation. The chip legs are fragile, and prone to breakage.

If a known good 6808 (a 6802 can be used when jumper A is installed, and jumper B removed) is available, swap it for the processor at U3 and retest.

If a known good AY-3-8910 is available, swap it for the sound IC at U1 and retest.

The sound interrupt signal and the sound select signal IC should also be checked. Connect a logic probe to the board; red lead at TP2 (+5VDC), black lead at TP3 (GND). With J1 connected to the game, place the probe on pin 40 of the PIA at U2. The signal should start high. When the game's MPU completes it's 7th flash, it will command the sound board to make the boot sound. At this point, pin 40 will go low for a split second, then return to high. If all goes well, consider the sound interrupt signal good.

Test the function of the sound select inverter 4049 at U5. U5 is a simple hex inverter. In attract mode, with no sounds being produced, pin 3 should be low, pin 2 should be high. Pins 5/4, 7/6, and 14/15 should test the same as pins 3/2 respectively. Pins 9 and 11 may not be connected on the game. If so, there will be no signal on those pins. However, their "inverted" signals at pins 10 and 12 should be high. Pins 13 and 16 are not connected.

The 6808 / 6802, AY-3-8910, and 6821 ICs require a reset signal which is created via a 1N4004, 1Mohm resistor, and NAND gates at U6 (4011). Test the 1N4004 diode via normal diode test. U6 can be tested with a logic probe similarly to U5.

Although U7 is specified as a 555 timer IC on the schematics, it is not listed on the BOM, and was not installed on this board.

4.12.3 Sounds Plus Sound Board Troubleshooting

This is a stub.

4.12.4 Vocalizer Speech Board Troubleshooting

This is a stub.

4.12.5 Squawk & Talk Sound Board Troubleshooting

4.12.5.1 Basic Information
Bally Squawk & Talk Jumper Locations. Note that the -61B revision of the board contains a jumper FF located within the stack of resistors below the AD558. Neither the -61 or the -61a revision has this jumper.


The image at left indicates jumper and test point locations. Jumper settings for various game and ROM combinations can be found here.

Test Point Number and Functions

   TP     Function 
1
  Ground 
2
  +5VDC 
3
  +11.5VDC 
4
  -5VDC 
5
  Speech Volume Control Voltage 
6
  Sound Volume Control Voltage 
7
  AY3-8912 Output 
8
  E (external clock provided by the 6808/6802) 
9
  TMS5200 Output 
10
  VMA (valid memory address) 
11
  TMS5200 clock (not used. Would clock the TMS6100) 
12
  Reset 

Source: Bally AS-2518-61 Squawk and Talk Schematics.

Integrated Circuits used by the Squawk & Talk sound board.

  • U1 - Motorola 6802 or 6808 8-bit microprocessor
  • U2 - ROM
  • U3 - ROM
  • U4 - ROM
  • U5 - ROM
  • U6 - 6810 RAM (only used if a 6808 microprocessor is used. If a 6802 is used, the socket is empty)
  • U7 - 6821 PIA for controlling the speech IC - TMS5200 VSP and TMS6100 VSM phrase ROM
  • U8 - TMS 5200 VSP Speech IC (does not occupy address space)
  • U9 - TMS6100 VSM 16k byte Phrase ROM containing speech data (never populated). Again, it does not occupy address space
  • U10- AD558 DAC for converting digital wave data stored in the ROMs to audio
  • U11- 6821 PIA for controlling the AY3-8912 sound IC and the test LED
  • U12- General Instruments AY3-8912 PSG (Programmable Sound Generator) IC, which again, does not occupy address space
4.12.5.2 Troubleshooting

Troubleshooting the Bally Squawk & Talk board is much simpler than it might first seem. First, the board provides "blink codes" at power up, similar to the way that Bally MPUs do. These codes help isolate the failed component.

The Squawk & Talk sound board is itself a single board computer. At it's heart is the 6808 processor with external RAM, or the 6802 processor with integrated RAM. Two 6821 PIAs are used to interface the processor to peripheral devices. The 6821 at U11 provides an interface to the AY3-8912 (if used) or the U16, a 4049 inverter where sound selects are sent from the MPU. The 6821 at U7 provides the interface to the TMS5200 Speech Processor.

Up to 4 ROMs can populate the board with either 2716, 2532, or 2732 ROMs.

An AD558 DAC (digital to analog converter) is used to make traditional pinball beeps, squeeks, and tones.

A TMS5200 Speech Processor is used to convert digitally stored (and compressed) sound information in the ROMs to analog "speech" (which can also be sounds).

An AY3-8912 Sound Generator is sometimes populated at U12 to produce simple tone sounds.

Two LM3900 operational amplifiers (op amps) amplify the analog signals from both the speech and the sound lanes before they are summed and provided to the main amplifier.

At TDA2002 amplifier is used at the final stage of amplification. A TDA2003 may be used to replace this amp.

Variable 1KOhm pots at R69 and R70 control speech and sound volumes independently (respectively).

Provisions are available for controlling volume via external pots by connecting to J2, pins 5 and 6.

Techniques Power the board on the bench.
Recognize blink codes.
Check voltage regulation circuits.>br> Use the "self-test" button.
Isolate the problem to the "digital" section or the "analog" section.
Use a logic probe to "listen" at points in the analog section.

Powering the board on the bench

Powering the Bally Squawk & Talk Sound Board on the Bench.


To power the board on the bench, use an old computer power supply that is capable of providing -5 and +12VDC.

  • Connect +12VDC to J1 pin 10. The board will use the +12VDC and will also regulate it down to +5VDC
  • Connect -5VDC to TP4
  • Connect the power supply ground to TP1

Double check all of your connections before applying power.

If the board is a -61b revision, as used in Centaur for instance, install a temporary jumper at location "FF". This allows operation without attaching the "Say it again" board.

Blink Codes
Executive Overview of "Blink Codes" follows. For a more in depth discussion of these blink codes, see the excellent information developed by Clive Jones, here.

   Blink     Meaning 
 First "flicker"   The processor has found the program code in ROM, and has begun to configure the board. 
 First Blink   The 128 bytes of system RAM (either within the 6802, or external in the 6810 if configured with a 6808) have tested good. 
 Second Blink   The 6821 PIA at U7 (which controls speech) has passed test. 
 Third Blink   The 6821 PIA at U11 (which interfaces with the AY3-8912 and fields sound codes from the MPU) has passed test. 
 Fourth Blink   Communication with the AY3-8912 has passed. Note that the PIA at U11 must be communicating correctly with the AY3-8912 for this test to pass. If there is no fourth blink, first swap a known good 6821 into U11. 
 Fifth Blink   The TMS5200 Speech Chip has passed testing. 


Pressing the red diagnostic button on the Squawk and Talk board, should cause the board to play various speech calls (games like Medusa seem to go on forever, Spectrum plays only a short speech call) followed by a sound call. The speech is generated by the TMS5200. The lone sound call at the end of this test is generated via the AD558 digital-to-analog converter. At the conclusion of playing these various speech calls and sound call, the board then reboots itself, and the same blink codes should be seen.

Basic maintenance actions
There are 5 actions you can take to ease the process of debugging the S&T board.

  1. Ensure the correct jumpers are installed, matching the game and sound ROM type
  2. Replace ALL electrolytic capacitors on the board
  3. Replace both 1KOhm pots (one for speech, one for sound)
  4. Replace ALL IC sockets (in use)
  5. Replace .156 male headers at J1 and J2 (at a minimum, reflow the solder joints)

Completing each of the above items usually results in a working Squawk & Talk board.

Capacitor list and recommended modern replacements
C1 - Axial Electrolytic 47uF, 25 Volt
C14 - Axial Electrolytic 4700uF, 25 Volt
C15 - Axial Electrolytic 10uF, 50 Volt
C19, C24, C25, C28, C31, C34 and C42 (7 capacitors) - Axial Electrolytic 1uF, 50 Volt
C27 - Axial Electrolytic 1000uF, 25 Volt
C29 - Axial Electrolytic 470uF, 16 Volt
C36 and C43 (2 capacitors) - Axial Electrolytic 2.2uF, 50 Volt
C37 and C38 (2 capacitors) - Axial Electrolytic 330uF, 50 Volt

4.12.6 Say It Again Reverb Board Troubleshooting

This is a stub.

4.12.7 Cheap Squeak Sound Board Troubleshooting

Bally Cheap Squeak Sound Board A080-91603-XXXX or Midway P/N M-051-00114-XXXX


At power up, the Cheap Squeak's LED will flicker briefly, then flash, flash again, then turn on and stay on. Once the LED stays on, it seems to turn itself off for certain sounds, and then turn back on. Likewise, it appears to idle with the LED on, when no sounds are playing.

The Cheap Squeak uses a single 1Kohm volume pot. If repairing this board, replace the pot with a high quality sealed 1K pot.

Test Points

  • TP1 should measure about 11VDC.
  • TP2 is 5VDC.
  • TP3 is ground.
  • TP4 is the clock signal, provided externally by the 6803 for the purpose of synchronizing address and data read cycles.
  • TP5 is the reset signal, which is also present on pin 6 of the 6803.

Bench Testing
To power the board on the bench, provide 12VDC to J1, pin 10.
Provide ground from your power supply to J1 pins 14 and 15.
Connect a small speaker to the speaker output pins at J2 (2 pins).
The board generates it's own 5VDC via a 7805 voltage regulator at U9. Therefore, you do not have to connect 5VDC.

When powered on, the reset circuitry on the board holds pin 6 of the 6803 (~RESET) low for a split second, then allows it to go high.

Pressing SW1 should cause the board to generate a test sound, then the board reboots itself. Note that different sound ROMs may act differently. Generating a tone and rebooting is characteristic of the "X's and O's" sound ROM.

The test sound can be heard using an inexpensive logic probe like the Elenco LP-560 at some points in the analog sound path. Attach your logic probe to +5VDC (TP2) and Ground (TP3) on the board. Probe pin 4 of the ZN429 DAC then press SW1. You should hear some sort of tone briefly, then the board will reboot. The tone can also be heard at pins 1 and 4 of the TDA2002 amp. Unfortunately, between the amp and R25, a logic probe isn't adequate for cheaply hearing the tone. However, if you can hear the tone at pin 4 of the ZN429 and can not hear the tone at pin 1 of the amp, there is a good chance that the LM3900 has failed.

This link contains some additional info regarding the Cheap Squeak sound board.

Note that the Cheap Squeak schematics for Black Pyramid have some incorrect info regarding the connections for U3 pin 2 to ground and U3 pin 20 to U4 pin 10.

4.12.8 SB-100 Sound Board

Technical write up regarding the SB-100 sound board is located here.

Just some general tech notes. Please be advised of the following per the Stern SB-100 sound board schematics. A single 220mfd 50v capacitor can be installed at either position C47 and C48. Likewise, two 100mfd 50v capacitors at these two positions are acceptable substitutes. Even though the circuit board stencils are marked with a "+" on one of the cap leg positions, the capacitor(s) at these positions must bi-polar / non-polarized.

4.12.8.1 Adjusting the Sound Quality on a Stern SB-100 board

Earlier versions of the Stern SB-100 Sound board can have an unpleasant high pitched sound for all the different sounds. STERN advises the following changes to lower the frequency of the tones, which sound much better to my ears.

  • Replace all 3 of the 5k potentiometers at R2,R6, & R13 with 25k ohm pots. Suggested part # is 15mm Piher PT15LH06-253A2020
  • Replace the 2.2k ohm resistor at R12 with an 8.2k ohm 1/4 watt resistor
  • Upon completion of component replacement, adjust the trim pots according to the instructions below
The 3rd generation SB-100 with pots and complimentary test points notated



Adjust the frequencies by putting the BLACK lead on ground (TP4) on the sound board and the RED lead of the multimeter set to the frequency setting (Hz) on the appropriate test point (see below and the pic to the left). Then, turn the pots one way or the other in small increments to the following specs.

TP2 Adjust R6 138 Hz +/- 3Hz
TP3 Adjust R2 104 Hz +/- 3 Hz
TP5 Adjust R13 172 Hz +/- 3 Hz

4.12.9 SB-300 Sound Board

Technical write up regarding the SB-300 sound board is located here.


4.12.9.1 Stern SB-300 Sound Board Modification for VSU-100 Usage

Games with a VSU-100 speech card require a modified SB-300 to utilize the speech. First, see if your SB-300 already has a trace for this connection by checking continuity between pin 5 of the rightmost connector at the bottom and the top of capacitor C14. If you read continuity, you do not need to do this mod.


  • Stern SB-300 Added Jumper Modification for Speech
  • First solder point for jumper
    (pin 5 of rightmost connector)
  • Second solder point for jumper
    (top of capacitor C14)
  • Overall picture of jumper


4.12.10 VSU-100 Speech Board

Work in progress.

Pray the S14001A speech chip isn't bad on this board!! Really hard to find.


4.13 Flipper problems

4.13.1 Rebuilding Flippers

Nothing makes more of a difference on a game than having taut, crisp, flippers. Even if every other mechanical component on a game is rebuilt, if the flippers are sluggish and weak, players walk away with a sour taste of the games' experience. At a minimum, flippers should be taken apart, inspected for wear, have their coil sleeve replaced with new, and reassembled while dressing any worn plungers/stops with a file. Additionally, the end of stroke switch should be filed and adjusted, as well as the cabinet switches that actuate the flippers.

It is best to replace the plungers and links, the coil stop, and the end of stroke switch(es) with brand new components. Quality varies in replacement parts for this era game, with some components being completely unavailable in suitable replacements. One example is the proper Stern coil stop for flippers - while there is a modern replacement, it is very expensive and does not seem to work as well as the original copper colored stops. You can swap the copper stops from stern pop bumpers which show very little wear into the flipper positions. The worn stop from the flipper will usually work on the pop bumper fine as the pulse rarely pulls the pop plunger down to the stop (hence why they are perfect to swap).

Another part that seems to be available are replacement flipper bats for Stern games - while they will fit and work, they are slightly fatter in profile at the inlane end and will cause flipper hop.

Recently a supplier (pinballlife.com) has started remanufacturing classic Stern style flipper parts in the generation 2 style. The flipper plates are a welcome addition to the repair arsenal, as the holes are usually stripped out from hacky repairs over the years.

When reassembling the flipper, check the bushing for cracks or wear - the bushing's job is to hold the flipper bat square to the playfield at a high enough height to prevent the flipper from dragging across the playfield surface, causing gouging and wear. Be sure the flipper has a slight up-down leeway built in at both rest and at extension. The inlane guide should be a smooth transition from guide to flipper; while you can't adjust the flipper to tweak this, you can and should adjust the lane guide.

4.14 Setting Free Play

If you read the manual that came with your game, you'll notice there are no factory provisions for setting the games on non-coin play. There are several options for homeowners to set their machines up so quarters or opening the coin door aren't required.

Free play via low replay scores

The easiest way to keep a quantity of credits on the machine is to set the first replay score to a very low value (10,000 on most machines). This way you can put several credits on the machine initially, and then each game played will probably win a credit. Press the self-test button inside the coin door until you see "01" in the ball-in-play display. Press the button on the mpu board itself (S33) to clear this value to zero, then use the credit button (on the coin door) to step the setting one click forward to 10,000. (Some games let you 'flick' a coin door switch to zero the value, S33 works for all games)

In most cases you will score 10,000 very quickly enabling the machine to self-replenish its credits.

Free play via adding a switch

If you like having the 'coin-up' sound most machines make, adding a free play switch to a game is relatively simple. Any type of normally open switch can be soldered to one of the coin switches and mounted in an unobtrusive way (through the coin door return is a popular example, or mounted to the coin return instead). While it is also possible to drill a hole in the cabinet or coin door to mount this type of switch, it is discouraged to do this as it is difficult or impossible to reverse cleanly.

This type of switch can be piggybacked onto the credit button switch itself, the initial full press will add a credit, and the pullback stroke will start a game.

Free play via ROM replacement

As of this writing, free play replacement ROMS are available for all Bally/Stern games. For many years only Bally freeplay roms of various flavors were available, and these romsets are incorporated into some of the aftermarket replacement MPU boards. In 2009 freeplay roms became available for all the stern mpu-100 and mpu-200 games as well, but as of yet (5/2011) are not incorporated into any aftermarket boards.

Some varieties of free play roms never let the credits go below one, some require a DIP switch setting, and some are free play only. Regardless of the type of free play rom used, they all work similarly. When the credits fall below a certain amount, the software code either increments a credit automatically, simply allows a game start with credits showing 00, or never lets the credits decrement below 01.

For many people free play roms are the preferred method of handling free play as no hardware modifications are needed to incorporate free play, and you can still have replay levels set to challenging levels. It also allows the machines to be placed in clubs, businesses, shows, etc. without the owner having to worry about people not knowing how to add credits via some other method. Press the credit button and go.

4.15 Game Trips Power Line Circuit Breaker When Plugged into Outlet

Incoming power cord, line filter, and blown / shorted varistor (Stern Seawitch)



Had a thunderstorm roll through or a power surge from the electric company recently? If so, and the game is now tripping the circuit breaker of the outlet it's plugged into, without the game's power switch turned on, the varistor may have shorted. The varistor is used to protect the rest of the game's circuitry from power surges.

Both Bally and Stern use varistors, which are typically located on the left cabinet wall. The varistor leads are soldered directly to the junction of the two power cord leads (hot and neutral) and the line side of the line filter.

The obvious sign of a failed varistor is typically a black, charred discoloration throughout the "button" portion of the varistor. Varistors can blow rather violently, as seen in the adjacent pic, and there may be charred or burnt remains in the area surrounding the varistor.

5 Game Specific Problems and Fixes

5.1 Stern Seawitch

5.1.1 Ball Trapped on Upper Left Flipper Plastic

Problem:The ball gets trapped between the glass and the left side plastic above the 3 drop targets, from a hard shot to the center 3 drop targets.

Solution: Cut a 11" (28cm) long by 5/8" to 7/8" (16mm to 22mm) strip of acrylic commonly used for home window glass replacement. Acrylic sheet can be scored and cut with a knife, but a bandsaw works best. Remove the plastic protector sheet, and plan the areas and angle you wish to bend.

With a propane torch (the instant on Benzomatic is great for this), heat the bend area for 2 seconds, and form the bend freehand or with a tabletop. The acrylic will stay soft and bendable for about 20 seconds, so if the angle is not right you can change the bend with or without additional heat from the torch. Clean up the cut edges and corners with a file and/or sandpaper. Brackets can be made out of thin stainless or mild steel. Keystone makes a bracket that is small and has 1 hole tapped for a 6-32 screw. (Manufacturer P/N#614 or Mouser P/N 534-614) A 6-32 nut & 3/8" computer case screw can be used to provide the needed clearance.

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Ball deflector installed on a Seawitch





5.1.2 Upper Flippers Power Resistor

Power Resistor Installed in Series on Upper Left Flipper (Seawitch)



On Seawitch, both upper flippers have a single 1 ohm 5 watt power resistor installed in series with the power winding of each flipper coil. Neither resistor is listed in the game's manual or schematics. It is presumed that the resistors were installed due to the upper flippers close proximity of nearby drop targets and plastics. The resistors are not necessary for proper function, and could be by-passed, but it is best to keep the resistors installed. The lack of this resistor will increase the flipper power, and more frequent breakage of drop targets or plastics may result.

Similar resistors were installed on other Stern games at this time typically on upper flippers that were close to targets.

An alternate to leaving the resistors in place (used in this application as a crude voltage reduction device; i.e. runs very hot and causes burn marks on the bottom of the playfield) is to open up the end of stroke switches instead. The flipper will flip with less power which is the intended goal.

5.2 Bally/Stern LED stays on after boot

7th flash produced but no led dimming afterward: The problem was one of the capacitors around the u12 (clock signal) were bad. I replaced c12,c16?, and c17. After that I got the 7th flash and the dimmed led afterward.

5.3 Solenoid Expander Board

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Bally Solenoid Expander board- Arrow indicates necessary bulb


There is a 555 bulb near (or sometimes not so near) that lights when the solenoid expander is activated. This bulb is essential for proper operation of the expander as the main opto isolator chip on the expander board needs a load to work. If you're having entire sets of solenoids not working, check this bulb.

The header pins on this board like to crack, also, so standard operating procedure should be to replace or at minimum resolder the headers, and re-pin the connector with Trifurcon connectors.

Also, it's very important that all solenoids on the expanded circuits have *TWO* diodes on them! If a coil was replaced with one that one had only a single diode, multiple solenoids could potentially activate on any solenoid expanded solenoid. Some side effects of a single diode coil installed could be a "memory" drop target activating when it shouldn't, or a target bank reset engaging when a saucer is supposed to eject.

5.4 Using a Bally 7 digit display in a Stern game or using a Stern 7 digit display in a Bally game

Bally and Stern 7 digit displays are interchangeable. However, Stern displays do not support commas and a small modification to the printed circuit board is necessary to make each board display all 7 digits in a "foreign" platform.

First, use a small jumper to connect J1, pins 11 and 12 together. Bally and Stern chose different pins to strobe the 7th digit. Without this mod, placing a Stern 7 digit display into a Bally game will result in the 7th digit not being displayed (and vice versa). Making this modification creates a "universal" Bally/Stern 7 digit display.

Comparison between the PCB size of a Bally and Stern 7-digit display
Stern display trays - 7-digit player displays are mounted to back of insert, while shallower 6-digit status display is mounted to front



Bally and Stern also chose a different depth dimension for their display PCBs. Fortunately, the difference can be accommodated merely by mounting the display tray on the opposite side of the lamp insert panel. Note that this is also necessary when using "PinScore" displays in a Stern game. Alternatively, if using a Bally (or PinScore) display in a Stern game, since the Bally circuit board itself is not as deep, insert the display only as far as necessary for it to "flush up" to the backglass when installed.

Bally 7-Digit display used as a Stern status display before modification




Bally displays automatically show commas whenever the thousands and millions digits are lit. To make a Bally display "match" the Stern display's non-comma capable display, clip the base lead from Q22 and commas will no longer light up. You can alternatively clip one end of diodes CR1 and CR2 and lift the diodes off the board. This modification would be easier to reverse.

Since Stern 7 digit displays do not support commas, using a Stern display in a Bally game will look a little "off" versus the other displays in the game, but this will get your displays running if you have no other choice.

5.5 Bally Game running slow(delayed score, controlled lamp flicker) or if game crashes before attract mode starts

If you bally game is running very slow(solenoid pulses are delayed and long, controlled lamps flicker, scoring takes a few seconds to register) or if your games boots and flashes 7 times but crashes(dead game with junk on the displays) before attract mode starts you may have a bad C16 cap. C16 cap controls the display interrupt speed. If it is out of spec it could cause display interrupts to be too quick and bog down the processor. C16 cap is in the corrosion area along with the 555 timer.

See youtube video of slow game caused by c16 cap. http://www.youtube.com/watch?v=TlGMGLHrDD8&feature=channel_video_title

Game running slow can also be caused by a bad U14.

5.6 Converting a Bally -17, -35 or Stern -100 CPU Board to a Stern -200

Stern MPU-200 CPU boards are probably the least common of the four CPU boards. The -200 boards are compatible in -17, -35, and -100 games. Although considering that -17, -35, and -100 boards are much more prevalent, it would be somewhat of a waste to use a -200 in any of these games. What if the more common boards could be converted to a -200? Check out WarpZoneArcade's blog entry to find out how to modify a -17, -35, or -100 to work as a -200.

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