User:Chibler

Chris Hibler has been a "pinhead" for a very long time, starting at an early age.
First game I remember playing: 1962 Gottlieb Rack-A-Ball
First game purchased: 1977 Gottlieb Centigrade 37
Resident of St. Louis, MO
Typically attends Pinball Expo in Chicago
Webpage: http://www.ChrisHiblerPinball.com/Contact

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Stern Pinball SB-100 Sound Board Author: Wayne EggertStern SB-100 Sound Board Date: 2/28/2011

12/4/2011 - fixed a few errors, mentioned c47 is a 220uf bipolar capacitor. Added some more tips on troubleshooting.

Introduction This is a project page for the Stern SB-100 Sound Board. I created this page because I was unable to find much information on the SB-100 after searching on Google & the rec.games.pinball newsgroup. I have several early Stern pinball machines that use the SB-100 and the board is not functioning in several of the machines. This project is my attempt to understand the SB-100 board in more detail, figure out how to diagnose it and fix the current boards I have, and document my research.

Brief History of Stern Stern began producing electronic pinballs in August 1977. Gary Stern with financial backing from his father (a prior president of Williams) bought out Chicago Coin in the mid-70s. The first pinball machine that was mass-produced by Stern in 1977 was called "Pinball" (I know, how imaginative). Many of the electronic boards Stern used early-on were actually copies of the Bally pinball boards. As the saying goes "Imitation is the sincerest form of flattery" and well, a lot of technology start-ups during the 70s and 80s copied successful designs to get a jump into the market. Bally of course didn't like this and sued Stern for infringement -- resulting in Stern paying a royalty to Bally for each game sold.

Stern's First Pinball Machines - Chime Sound Sterns first 4 pinball machines had no electronic sound board. They instead used actual chimes. A solenoid with a plunger was located below these chimes and when the solenoid/coil was activated it would drive the plunger into the chime and cause the chime to ring. The MPU controls the solenoids like any other coil and the dip switches on the MPU board could be used to turn the chimes on or off.

"Chime" Sound

1. 101 - Pinball (Aug 1977)
2. 102 - Stingray (Dec 1977)
3. 103 - Stars (Mar 1978)
4. 104 - Memory Lane (Jun 1978)

Here's a picture of the chimes inside one of Stern's early pinball machines:

Chimes Picture: Chimes in a "Memory Lane" Image Courtesy of Ken Kemp and Memory Lane page at IPDB.org

Stern's First Sound Card - The SB-100 After "Memory Lane", Stern used sound boards in most of their games. As far as I'm aware, the SB-100 was Stern's very first sound card used in a pinball machine and was not a copy of any of Bally's existing sound modules. Stern references this board as sound module part B-521.

SB-100 Sound Board

1. 105 - Lectronamo (Aug 1978)
2. 106 - Wild Fyre (Oct 1978)
3. 108 - Nugent (Nov 1978)
4. 109 - Dracula (Jan 1979)
5. 110 - Trident (Mar 1979)
6. 112 - Hot Hand (Jun 1979)
7. 115 - Magic (Aug 1979)

Multiple Board Versions It should be noted that there are at three revisions/versions of the SB-100 used on these games. The first revision has circuitry for chimes + electronic sounds. The 2nd board lacks the circuitry for chimes, but still has the silkscreen drawings for all of the missing comonents. The 3rd board "Rev C-1" also lacks the chime circuitry and does not have the silkscreen drawings for the missing components. If using electronic tones, any version of these boards is compatible with the games listed above.. however only the boards that include the chimes circuitry can be used if setting the sounds on the game to "chime" mode. It's likely the boards with chime circuitry were only used in the first few electronic games produced to give operators the option to have the familiar chimes sounds or the new electronic sounds.

Also worth noting is the frequency of the sounds on these boards is adjustable via potentiometers r2, r6 and r13. These pots are used in the multivibrator circuits along with LM324 and 4013 ICs. It's unknown at this time if the frequencies were adjusted per game title or if all games were dialed-in to the same frequency tones. If tones sound really bad, it's possible someone has played around with these potentiometers and made the sound less appealing in the process.

Lets look at the board differences further..

Boards With CHIME Circuits (Original SB-100) The first version of the SB-100 we'll look at is the fully populated board with the U5 chip and CHIMES circuitry.

Original SB-100 Picture: Stern SB-100 with CHIMES circuitry. Board is fully populated (and a little dusty)

Schematic Original SB-100 schematic from Hot Hand manual

sb100_schematic.gif

Component List Here is a component list that I gathered from the SB-100 schematics in a Stern Hot Hand manual. Again please note that these components are for the SB-100 that has a U5 IC and whose board is fully populated. If your board is missing U5 and instead looks like the Rev C-1 board (see below), then its component list is slightly different.

Resistors Capacitors 12 100k ohm r3, r4, r5, r7, r8, r9, r10, r11, r14, r17, r81, r86 1 0.003uf c38 9 10k ohm r24, r27, r30, r33, r36, r67, r71, r83, r84 1 0.022uf (50v) c45 5 10m ohm r40, r41, r52, r53, r90 1 0.033uf c39 1 15k ohm r21 32 0.1uf c2, c3, c4, c5, c6, c7, c8, c10, c12, c13, c14, c16, c17, c23, c24, c25, c26, c27, c28, c29, c30, c31, c32, c33, c34, c35, c37, c40, c43, c46, c49, c54 5 1k ohm r19, r20, r65, r70, r88 1 1000uf (25v) elec. c51 5 1m ohm r22, r25, r28, r31, r34 1 1000uf (35v) elec. c53 2 2.2 ohm r64, r66 1 100pf c44 11 2.2k ohm r1, r12, r15, r16, r23, r26, r29, r32, r35, r68, r89 2 10uf c42, c52 5 2.2m ohm r69, r77, r78, r79, r80 1 220uf (25v) bi-polar elec. c47 (could also use 100uf in parallel at c47, c48) 2 220 ohm r91, r92 1 22uf (10v) tant. c9 1 22k ohm r63 1 3.3uf c18 4 390k ohm r73, r74, r75, r76 6 4.7uf (10v) tant. c1, c19, c20, c21, c22, c50 1 39k ohm r62 1 47uf (35v) elec. c55 8 47k ohm r39, r42, r51, r54, r61, r82, r85, r87 2 56pf c36, c41 1 56k ohm r72 2 680pf c11, c15 3 5k ohm (pot) r2, r6, r13

1 680k ohm r18 ICs 16 820k ohm r37, r38, r43, r44, r45, r46, r47, r48, r49, r50, r55, r56, r57, r58, r59, r60 1 7402 u1

1 7411 u2 Transistors / Semiconductors 3 7475 u3, u4, u5 5 2n4403 q1, q2, q3, q4, q5 1 74107 u6 2 2n3904 q6, q7 1 7408 u7 1 1n763a cr1 2 4013 u8, u9 1 1n758a cr2 5 mc3340p u10, u11, u12, u13, u14

4 lm324n u15, u16, u17, u18

1 556 u19

2 lm380 u20, u21

1 78L05 u22

Boards Without CHIME Circuits (Revision C-1) The next board version we'll look at is a Rev C-1. This board does not have the U5 IC or CHIMES circuitry, nor does it have the silkscreen drawings/labels for the missing components. There is another [assumed earlier] revision of this board that has the silkscreen labels on the omitted components.

Stern SB-100 Rev C-1 Picture: Stern SB-100 Rev C-1. No CHIME circuitry.

Schematic The schematic below is the original SB-100 with the components that are not on the Rev C-1 board grayed out.

sb100_revc_schematic.gif

Component List For boards with a U5 IC and chimes circuits, the component list should be something like below. **This may be incorrect as I've eliminated components from the fully populated board schematics and have not verified every single component values for the Rev C-1 board, but should be pretty close. The on-board potentiometer values are definitely different on my Rev C-1 boards.

Resistors Capacitors 12 100k ohm r3, r4, r5, r7, r8, r9, r10, r11, r14, r17, r81, r86 1 0.003uf c38 5 10k ohm r24, r67, r71, r83, r84 1 0.022uf (50v) c45 1 10m ohm r90 1 0.033uf c39 1 15k ohm r21 20 0.1uf c2, c3, c4, c5, c6, c7, c8, c10, c17, c23, c24, c28, c31, c35, c37, c40, c43, c46, c49, c54 5 1k ohm r19, r20, r65, r70, r88 1 1000uf (25v) elec. c51 1 1m ohm r22 1 1000uf (35v) elec. c53 2 2.2 ohm r64, r66 1 100pf c44 7 2.2k ohm r1, r12, r15, r16, r23, r68, r89 1 10uf c42 5 2.2m ohm r69, r77, r78, r79, r80 1 220uf (25v) bi-polar elec. c47 (could also use 100uf in parallel at c47, c48) 1 220 ohm r92 1 22uf (10v) tant. c9 8 47k ohm r39, r42, r51, r54, r61, r82, r85, r87 1 3.3uf c18 1 56k ohm r72 2 4.7uf (10v) tant. c1, c50 3 5k ohm (pot) r2, r6, r13 1 47uf (35v) elec. c55 1 680k ohm r18 2 56pf c36, c41

1 680pf c11 Transistors / Semiconductors

1 2n4403 q1 ICs 2 2n3904 q6, q7 1 7402 u1 1 1n758a cr2 1 7411 u2

2 7475 u3, u4

1 74107 u6

1 7408 u7

2 4013 u8, u9

1 mc3340p u14

2 lm324n u17, u18

1 556 u19

2 lm380 u20, u21

1 78L05 u22

Continue to How The Boards Work..

In Theory - How The Board Works In order to diagnose these boards better, it's best to know how the circuit(s) are actually working. I wasn't finding much information available on these boards anywhere until I stumbled into a rec.games.pinball post from 2000 where someone took the time to explain their thoughts on how the components on the board are interacting. This significantly helped me fill in some blanks in my knowledge of how the board is working. And so I will attempt to bring all of this information together along with my own theories. If I mistate anything, just contact me and I'll evaluate and fix if necessary.

Theory of Operation

1. The 6800 CPU connects directly to the sound board via a wire harness between the 32-pin connector A4J5 on the MPU board to A6J1 on the sound board. The sound board is mapped into the MPU board's 6800 CPU, which means there are no independent CPU or PIA chips driving the sound card. The bad news here is a faulty sound board then has the potential to lock up the MPU board (seems to be pretty common occurrence).

2. Power is supplied to ICs U1 thru U5 through the 32-pin harness between the MPU board and sound board (pin 30 on J1). The rest of the ICs on the board are powered from the 11.5v that enters the sound board at connector J2 (pins 5 & 6). The 11.5v is broken into 11.5v, 10v, 6.2v and 5v power circuits (though on the Rev-3 boards without the CHIME circuit there would be no 6.2v power circuit.

Power

3. During boot-up sound is disabled until the CPU's /RESET goes HIGH. There is a reset circuit consisting of the /RESET line from the 6800 CPU on the MPU board, a 74107 (U6) "Dual J-K Flip Flop", 2n3904 NPN transistor (Q6) and a few resistors. When the /RESET line goes low, the chip is cleared and /Q (U6 pin 6) is HIGH which activates the Q6 transistor. When Q6 is activated, it would pull the bypass pin 1 on both LM380 amplifier chips (U20, U21) to ground. Normally pin 1 on both LM380 chips is "bypassed" via a 10uf capacitor (c42) and this would reduce power supply noise on the LM380's output. Instead, when Q6 activates, pin 1 on the LM380 is pulled to ground and the LM380 is muted (*note: other sources seem to indicate muting would occur when pin 1 is pulled to vcc, so more research here may be needed).

Reset Circuit Picture: Reset Circuit

4. U1 (7402 NOR-Gate) and U2 (7411 AND-Gate) outputs are used to enable the 7475 "4-bit Bistable Latches" ICs (U3 thru U5) at the CLK pins. The inputs to these latches are the data lines from the 6800 CPU. Depending on the sound being played, any one of the data lines D0 thru D7 could be pulsed. The 7475 would read the input and set its output to a 0 or 1. The latch will remain in that state until the input is pulsed again by the CPU.

If the SB-100 has U5 (for chimes) then the DIP switches on the MPU board that are read during MPU bootup (and stored in RAM) would select between chime or electronic sounds. U2 would either enable both U3 and U4 for electronic sounds, or ONLY enable U5 if chime sound was selected.

U1 thru U5 Picture: ICs U1 thru U5

5. The outputs on U3 and U4 7575 ICs are connected to one of the input pins on U7 (a 7408 "Quad 2-input AND Gates"). As the "Quad" in the name suggests, the U7 IC has a total of four "2-input AND Gates". Each of these AND gates has two inputs. The first input as mentioned previously is coming from the 7475 bi-stable latch. The second input is the frequency being created by a multivibrator circuit. There are test points TP2, TP3, TP5 which can be used to test the frequency output of the multivibrators.

U7 7408 Picture: U7 pin 5 enables the frequency on pin 4 to pass to the output pin 6

6. Multivibrator circuits are used to create 3 distinct sound frequencies. These are shaded in red below. They each consist of an LM324, potentiometer, a capacitor, and a few resistors. They are always running and creating a frequency at their outputs. The frequency can be adjusted via the potentiometer in each of these circuits and according to an RGP post are adjustable between 1-3.3kHz, although after they are fed to the 4013 ICs it would be halved (and once again at TP3 since the frequency goes through yet another 4013). These frequencies are used for both the electronic sounds and chime sounds.

Multivibrators Picture: Three separate multivibrators using LM324 (U17), potentiometers, resistors, capacitor

6. For electronic sounds, the frequency output of the multivibrators are piped into the data input (D) on a 4013 "CMOS Dual D Flip Flop". Here the logic state at the data input (D) on the 4013 is transferred to the (Q) output on the rising edge of the clock signal / frequency. In doing this, the 4013 acts as a frequency divider in which the output (Q) pin 1 is half the input frequency (CLK) pin 3. An oscilloscope hooked up to TP2, TP3 or TP5 will show a square wave.

U7 7408 Picture: Output at 4013 pin 1 would be half the frequency of the multivibrator

Square Wave at TP2 Picture: Square Wave at TP2 on a Stern Hot Hand SB-100 Sound Board (Left); Frequency Reading on VMM (Right)

The 4013's creates the second input on the 7408 IC. The frequency is always at the 7408 IC's input pin, waiting to be enabled by the other input to the 7408 Gate (again coming from the 7475 IC). When both inputs are HIGH, the output is also HIGH and the frequency is allowed to output on the 7408's output pin for that Gate. This signal would then feed to the final output stage.

7. For chime sounds (on boards with a U5 IC), the frequency follows a slightly different path. The raw frequency / pulse generated by the multivibrators appear to be overlayed on the square waveforms. In the picture below, the blue arrows show where the multivibrator output is tapped directly & the red arrows show where the 4013 half-frequency output is tapped.

Multivibrators 2 Picture: Blue arrows are frequency from multivibrator; Red arrows are half-frequency from 4013

Both of these frequencies for each tone are used as an input to the LM324 ICs on the bottom-right of the schematic. The output of the LM324 is used as the input to the MC3340P electronic attenuator ICs. I'm not 100% sure but I would guess that the resulting sound waveform at MC3340P's pin 1 would be triangular or sawtooth instead of square.

Overlayed Picture: Two waveforms being used for the non-inverting input pin 3 (+) to the LM324

According to its Motorola Datasheet, the MC3340P is primarily used in volume control or compression/expansion amplifier applications. Each of the chime circuits has its own MC3340P. I found this interesting at first until I thought about it more. There is only one other MC3340P used on the SB-100 and that's at U14 that has the cabinet volume control potentiometer hooked into it on pin 2.

Taking it back to basics lets think how physical chime units work in older pinball machines. A solenoid would be activated causing a plunger to wack a chime. The initial impact would sound the chime VERY loud and resonate at its correct frequency and slowly fade in volume. So a separate MC3340P used in each of the chime circuits makes a lot of sense if it's used to create that "fade sound into the distance" effect. So that's my theory until someone tells me otherwise!

Let's look at the other half of the chime circuit feeds into pin 2 on one of the MC3340P ICs. Going all the way back to U5 (7475) which again would be switched on at the DIP switches on the MPU board, we see the following circuit for the output on 1Q.

U5 Chimes Picture: Other half of chime circuit

There's a common 2n4403 transistor, some resistors and capacitors and an LM324. The output of the LM324 pin 1 then feeds to MC3340P pin 2 (the pin that could have a potentiometer output sent to it to control the volume). So, the circuit above seems to be entirely chime volume related and what creates that "fade" effect. Picture manually throwing a volume knob HIGH and then slowly turning it LOWER until the sound is no longer heard.

How do I think this happens? The CPU would tell the 7475 to play the chime sound through pin 2 (1D) and this would cause the output (1Q) to latch at 5v. This would then cause the 2n4403 (Q5) transistor to activate and the capacitor C21 would dump its charge to the path of least resistance (r47 in circuit above) to pin 2 on the LM324. The output of the LM324 would be a voltage frequency akin to adjusting a potentiometer HIGH quickly, then LOWER as the capacitor is drained further and further. During all of this, the CPU would need to latch the 1Q input back to 0v to stop the chime from sounding. This is just my theory on what's in the schematic. If I'm wrong please contact me and I will update this info if necessary.

So with that, we have the sound waveform input on pin 1 of the MC3340P and the volume input on pin 2. These then go to the final output stage.

8. The last thing to discuss as far as sound generation are the outputs on U4. These outputs appear to be used only when electronic sounds are selected for the game.

Noise Generator / Triangle Wave Picture: Noise Generator / Triangle Wave Generator

The 3Q output looks like it would generate a triangle waveform based on the two LM324s circuits used. And interestingly enough, the first part of the circuit with the 2n4403, capacitor and resistors resembles the chime circuit. So a varying voltage is being created using Q1, the 3.3uf cap and resistors and that's fed into the LM324. This is then used to create a frequency for the LM324 inputs.

The 4Q output looks like a noise generator and uses a 556 dual timer IC and 74107 "Dual J-K Flip Flop". When 4Q is latched to 5v it would reset the 556 timer and start producing noise based on the configuration of the trigger/threshold/discharge pins. The output is sent to the 74107 (U6) pin 12 as the CLK. This would create a frequency at 74107 pin 13.. I would again guess here that it would be a square wave.

That's it for all the sounds on the board, lets look at the final output stages.

9. All of these sounds, regardless of where they're created end up at the same output stages consisting of yet another LM324 (U18), an MC3340P (u14) used with the volume pot, and two LM380 ICs (U20, U21).

First the sound frequencies enter an LM324 (U18) at pin10. This LM324 acts as a voltage follower and gives effective isolation of the output from the source signal. Very little power is drawn from the signal source so you avoid "loading" effects. In any case, voltage at pin 10 on U18 should be the same as on pin 8.

Output 1 Picture: LM324 Voltage Follower & MC3340P Electronic Attenuator

The output of the LM324 is sent to the MC3340P (Electronic Attenuator) input pin 1. Here the MC3340P acts at the volume control for the entire machine. The potentiometer seen in the schematic above is the volume pot inside the cabinet. As you adjust the volume, the MC3340P does basically the opposite of an amplifier and reduces the volume.

Output 2 Picture: LM380 Power Amplifiers used in "bridged" configuration

The output of the MC3340P on pin 7 then feeds to a voltage divider consisting of r19, r72 and finally makes its way to the LM380 power amps. The power amps are used in a "bridged configuration" which doubles their output from around 2 watts to 4 watts. The output of these power amps is then sent through to the speakers.

That's it! That's the basic theory of how the sound board works from everything I have put together so far.

So how do you diagnose these boards? Continue onward..

Diagnosing - How Dead Is Your SB-100? The very first thing to check if you're having sound problems are the solder points where the wire harnesses/connectors plug into the sound board. So turn off the machine, remove the harnesses & remove the sound card and look at the back of the sound board at the solder points at these connectors. If any of them look cracked or if in doubt, reflowing the solder points may fix any lockups/sound issues occuring. If not, then choose your path below..

Is you MPU board locking up when you connect the SB-100? Skip to the "Diagnosing - Sound Board Locking Up MPU?" below.

Is there a loud THUD when you turn the machine on, followed by what sounds like MANY sound effects playing at once? Skip to the "Diagnosing - Miscellaneous Troubleshooting" section.

If you don't have any sounds, when you turn the potentiometer to both extremes do you hear a buzz from the speaker? That means the LM380 power amplifiers should be working and you can eliminate them as the problem.

Do you have some sounds, but are missing other sounds? Check voltages at the multivibrator test points in the "Diagnosing - Voltage Test Points" section below.

Is there constant white noise that increases/decreases in intensity when you turn the volume potentiometer? Skip to the "Diagnosing - Miscellaneous Troubleshooting" section.

Diagnosing - Sound Board Locking Up MPU? If your MPU board is locking up it may be the SB-100 causing the issues. Think of the SB-100 as another peripheral IC that the CPU is talking to, except instead of being a single chip the "sound chip" is composed of various logic ICs, amplifier ICs, etc. There are data and address lines going from the CPU into the sound card that are shared by other stuff like RAM, ROM, PIA chips, etc. If there is a short on the board, a bad IC, cold/broken solder joints that allow an address or data line to float, etc... the SB-100 can cause the CPU to lock up or do weird things.

So your MPU is locking up.. first step is to turn off the machine, try disconnecting the wire harness at the sound board's J1 connector and turn the machine back on and off several times. If the MPU boots, once again connect the sound board harnesses to verify the MPU locks up. If the sound card is indeed causing the MPU to lock up.. make sure you've checked all voltages as above! If the the board is missing 5v at TP1 or TP6 then some of the logic chips aren't being powered & can cause logic levels to "float" and cause lock-ups. If voltages look good, double-check to make sure the harness isn't connected backwards (ie pin1 on sound board goes to pin1 on mpu board). Try wiggling the connectors while powering on/off the machine to see if that makes a difference. If not, then you'll need to turn the machine off and pull the SB-100 sound board to examine for cracked/cold solder joints or obvious issues. It's usually a good idea to reflow any header pins for the 32-pin header on the SB-100 that look suspect.

Diagnosing - Voltage Test Points The following test points should be checked with a multimeter first before trying anything else. Some of the ICs on this board are hard to come by or expensive, so why start cutting and hacking when there may be a more systematic approach to finding where the fault is, especially if its in the initial voltages to the boards.

If any or all of the voltages at TP1, TP7, TP8, TP9 are not present you will likely not have any sound.

TP1 - 5v (for ICs u1 thru u5)
TP2 - around 2.6v (if multivibrator is working)
TP3 - around 2.6v (if multivibrator is working)
TP4 - GND
TP5 - around 2.6v (if multivibrator is working)
TP6 - 5v (for all other 5v logic chips, ICs)
TP7 - 11.5v (for ICs u20, u21)
TP8 - 6.2v (for ICs u10 thru u13)
TP9 - 10v (for ICs u14)

• All voltages will be D/C

Missing Voltages? If any of these voltages measure low or are non-existent, check for 12v and GND directly at the wires connected to J2 on the SB-100. There should be around 12v at J2 pins 5 and 6. The 12v is used to supply power to everything except u1 thru u5 (which are powered via pin 30 on the ribbon cable from the MPU board that hooks to J1 on the SB-100).

CONSIDER SHORTS! If no voltage is present at TP8, but voltages are present at TP7 and TP9.. then CR2 may be shorted. This was the case with one of my SB-100 cards and after removing CR2 (a 1n763a diode) it read 0 ohms between both pins. After replacing CR2 the voltage re-appeared at TP8.

Voltages Present If voltages are present at TP1, TP6, TP7, TP8 (if board has chimes circuitry) and TP9 then you can now check for voltages at the multivibrators. Test points TP2, TP3 and TP5 should be roughly 2.6v DC.. this is because the multivibrator output at these test points is a 50% duty cycle. If your multimeter can measure frequency (Hz) you could also check the frequency here, but you would need to have an idea of what frequency to expect and they will vary depending on the game so easier to measure for 2.6v. If the test points measure very low (say 100mv or something far less than 2.6v) then it's likely the LM324 IC or 4013 for that particular multivibrator circuit. These test points should always read around 2.6v as long as the sound board is receiving power, regardless of whether you've started a game or not.

Diagnosing - Checking Frequencies at the 7408 IC (U7) As long as the sound board's not locking up the MPU, you can continue to pinpoint the issue. If you have a logic probe and have electronic sounds enabled on the pinball machine (ie. not using the CHIMES circuits) you can quickly eliminate 90% of the components on the board from being the problem if you check for pulses at the U7 IC. The frequencies from all of the multivibrator circuits feed to the U7 IC.

Using the logic probe, test the output of the 7408 (U7) at pin 3, 6, 8, 11 while the game is in sound test mode or during game mode and hitting switches on the machine. Again you will most likely need someone else to help press these switches while you hold the logic probe on the pins. You should see pulses (HIGH/LOW) at these pins depending on what sound was being generated for the switch pressed.

You can also use a multimeter -- it won't show pulsing but you should see the output from the U7 go from 0v to 1-2v for a split second. Try hitting switches a few times and seeing if the voltage jumps from near 0v to 1-2v.

Pulses / Voltage at U7 Output If you are seeing voltage or pulses at the U7 output pins corresponding to the tone being played when you press a switch, but still don't have sound, then you have just eliminated a large amount of components as the issue. Check that U6 pin 6 is LOW since HIGH here would mute the LM380 power amps. If it is LOW, then check for frequency (if you have Hz on your multimeter) at LM324 Voltage Follower (U18) pin 10 which is the sound input and also pin 8 which is the output. When a sound is played, the frequency should jump to something like 90-400 Hz for a split second. If you're hitting switches or the game is in sound test mode and you don't see the frequency changing on pin 8 or pin 10 of u18, but you had pulsing at the U7's output, then the LM324 at U18 may be bad.

No Pulses / Voltage at the U7 Output If you're not seeing any pulses, then check the input pins for U7. There are two inputs, one that enables the sound and the other with the frequency. Check that one of the input pins measures around 2.6v (input from multivibrator) and that there is pulsing on the other input pin when a switch is pressed (or if using the multimeter you should see voltage go from 0v to 1-2v for a split second). You will need to find the correct pins for the tone being played for the switch you are pressing if you're manually hitting a switch and not in sound test mode. If both input pins are working as expected but the output pin does not pulse or jump from 0 to 1-2v when a tone is played, then suspect the 7408 and check the U7 pin with the machine off using the diode setting on your multimeter.

Test Point 2 Picture: Measuring Voltage at TP2 shows 2.5v DC (multivibrator shoud be functioning)

Diagnosing - Miscellaneous Troubleshooting / Some Other Helpful Tips If your SB-100 makes a *very loud* HUM / static noise constantly, but the game sounds are there, the 5 electrolytic capacitors on the board may need to be replaced. Take note that C47 is a bi-polar 220uf electrolytic capacitor. The main ones seem to be the 1000uf capacitors but if you'll be replacing those you might as well replace all of the electrolytic caps.

If you wiggle the 32-pin harness and sounds fire, stop working, or it locks the MPU, then most likely you have cold or broken solder joints on the sound board or MPU board. Pull the boards to examine this.

If you hear a loud *THUD* when you turn the pinball machine on and then what sounds like a whole bunch of sounds all at once, it could be cold/cracked solder joints at the 32-pin connector. Try wiggling the connector or putting a little tension up or down on it while turning the machine off/on to see if that makes a difference. If so, pull the board & reflow any cracked header pins.

At power-up U6 pin 6 should very quickly blip HIGH then LOW. If it stays high then it would cause muting on the LM380 power amps. You may need someone else to turn the machine on/off while you hold the logic probe on the U6 pin.

If you've selected electronic sounds on the MPU's DIP switches then U2 pin 8 should be HIGH to enable the 7475 ICs that correspond to electronic sounds (U3, U4). If you've selected chime sounds then U2 pin 6 should be HIGH. If these don't check out depending on the sound you've selected, then suspect the U2 or U1 ICs and trace the signal back.

You may want to pull the board and with no power going into the board, check all of the U1 through U6 ICs with a multimeter in diode test. Put the red lead on the GND pin for that IC & check the other pins with the black lead. Most of the pins should read 0.5-0.6v if the internal circuits are working in the IC. This isn't always 100% but you may be able to diagnose the issue quicker this way. This is especially necessary if the MPU board is locking up and not booting at all when the sound board is connected.

To check whether the CPU is even sending any sound data through the wire harness at J1 (or to see if you have a wire harness issue) you can test the input pins on U3, U4, U5. You could also check inputs to U1, U2 and U6 in this manner. It may just be a bad connector pin or wire causing an issue.

If you hear speaker "white noise" when the machine is on, then the LM380 power amps are probably fine. You may hear a click if you try testing voltage at r65 (resistor directly before the LM380 power amps) which would also tell you the LM380 amps should be fine.

If sound is *very* faint and the volume knob does not make a difference when turned in either direction, check the wiring for the potentiomer. With the machine off use the continuity test between the connector's pins and where the wire connects to the potentiometer. Also make sure the two wires from the pot are hooked to pins 1 and 3 on the J3 connector.

If all voltages test points on the board check out, you're getting pulsing at the 7808 output pins, the LM324 (U18) has frequency at its output pin 8 when a tone is played, but there are no sounds or "white noise" coming from the speaker and no clicking noise if you test voltage at r65, then the problem should be between U18 and the speaker itself (the very right-hand side of the circuit).

Potentiometer / Frequency Values

As mentioned previously, the potentiometers at r2, r6 & r13 adjust the frequency of the tones. These appear to have been set at the factory.

Potentiometers Picture: Back of SB-100 Board (r2, r6 and r13 potentiometers)

GAME LIST & POTENTIOMETER VALUES While I'm not very sure if Stern had set any of these potentiometer values different per game, or if some may just be different because people have messed with them -- these are some values from machines that have passed through.

Nugent (unconfirmed) r2 - 87.7 ohm r6 - 2.99k ohm r13 - 1.99k ohm

Hot Hand (unconfirmed) r2 - 18.45k ohm r6 - 2.86k ohm r13 - 14.09k ohm TP2 - 307Hz TP3 - 95Hz TP5 - 206Hz

Summary I've fixed quite a few of these boards now and they are much less intimidating than they once were. I'll try and keep this page up to date as I repair the boards.

What I found very interesting is the on-board potentiometers being used to set the frequency of the tones, so even if you have two SB-100s that have the exact same components on them, the tones could be completely different if the sound board potentiometers have been played with by a previous owner. It actually threw me for a bit as I was repairing an SB-100 board and it didn't sound the same as the previous board I was using but now it makes a lot more sense. This is also my first time working with a sound board so it was a large learning curve and I really didn't want to take a shot-gun approach to replacing components. There's almost always a way to break apart a problem into chunks to help narrow down the issue.

Thanks for reading & if you have any comments or if I've mis-stated anything or completely missed the point of a circuit, just comment or let me know via email!