Williams WPC

Note: This page is a work in progress. Please help get it to a completed state by adding any useful information to it.

1 Introduction

This guide covers Williams WPC, WPC-S, and WPC-95 games.

2 Game List

2.1 WPC (Alphanumeric)

• Funhouse
• Harley-Davidson
• The Machine: Bride of Pin*Bot

2.2 WPC (Dot Matrix)

• Gilligan's Island
• Terminator 2: Judgement Day
• Hurricane
• Party Zone

2.3 WPC Fliptronics I & II

• The Addams Family / Addams Family Gold
• The Getaway: High Speed II
• Black Rose
• Fish Tales
• Doctor Who
• Creature from the Black Lagoon
• White Water
• Bram Stoker's Dracula
• Twilight Zone

2.4 WPC DCS Sound

• Indiana Jones: The Pinball Adventure
• Judge Dredd
• Star Trek: The Next Generation
• Popeye Saves The Earth
• Demolition Man

2.5 WPC-S CPU

• World Cup Soccer
• The Flintstones
• Corvette
• Red & Ted Road Show
• The Shadow
• Dirty Harry
• Theatre of Magic
• No Fear: Dangerous Sports
• Indianapolis 500
• Johnny Mnemonic
• Jack*Bot
• WHO Dunnit

2.6 WPC-95 CPU

• Congo
• Attack from Mars
• Safecracker
• Tales of the Arabian Nights
• Scared Stiff
• Junk Yard
• NBA Fastbreak
• Medieval Madness
• Cirqus Voltaire
• No Good Gofers
• The Championship Pub
• Monster Bash
• Cactus Canyon

3 Technical Info

Motorola 68B09E, running at 2Mhz. It is an 8-bit/16-bit CPU with a 64KB address space. Bank switching is required to address more than 64KB. The game ROM size varies from 128KB to 1MB, depending on the game. 8KB of battery backed RAM is available.

For more information, see The FreeWPC Manual

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3.8 Miscellaneous WPC Boards

Note: Some of these might best be located in the "Game Specific Problems & Fixes" section. Perhaps the pervasiveness of their use within the WPC game list would drive the decision.

3.8.3 Auxiliary 8-Driver Board

as used in TZ, DM, IJ, etc...

3.8.7 Coin Door Interface Board

If your coin door interface board gets dusty from flipper parts wearing, clean it. That dust conducts electricity and will give you some interesting coinage issues.

4 Problems and Solutions

4.1 MPU boot issues

4.1.1 Relocating the battery from the MPU board

Relocating the 3xAA battery pack from the MPU board is always a good idea. Leaky alkaline batteries are the #1 killer of MPU boards. Simply removing the batteries is not an option with WPC games as you will always receive a "Factory Settings Restored" message when the game boots.

Options:

1. Remotely locate the battery holder somewhere below all other boards. This ensures that even if the remotely located batteries leak, they won't leak onto (or even drip onto) any circuit board. Replace the batteries annually, dating them with a Sharpie! as you do.
2. Replace the 6264 static RAM with a SIMTEK non-volatile RAM (STK12C68). These SIMTEK RAM chips are increasingly hard to find but offer a nice alternative to changing batteries annually. This method requires desoldering/soldering on the MPU and also has the down-side of not maintaining the Real Time Clock (meaningless in some games...nice in games like Twilight Zone that moves the playfield "toy" clock to the current time during attract mode, and Who?Dunnit which has a "Midnight Madness" feature.

4.3 Check fuses F114 and F115 message

This message is sometimes displayed when the game boots and a game cannot be started. Most of the time, F114/F115 will be found to be just fine.

The game issues this message because it cannot read the switch matrix normally. The crafty designers at Williams inserted an “always closed“ switch into the matrix as switch 24. Switch 24 isn’t a switch at all, but instead provides the game software with a “known closed” switch matrix return. Switch 24 is actually a diode across column 2, row 4 of the switch matrix and is located on the coin door interface board.

Since the WPC switch matrix circuitry on the MPU uses 12V to determine switch state, the game assumes that the 12VDC power has been interrupted. F114/F115 fuse the 12V generation circuit; F114 on the AC side of BR1; F115 on the DC side of BR1. Hence, the game assumes that one of these fuses is blown and the message is displayed.

Diagnosing the problem

Start by checking DC voltage at TP3. You should see about 12VDC. If not, follow these steps.

1. Actually check fuses F114 and F115 using the procedure here. If a blown fuse is found, replace it. If F114 immediately blows again, then BR1 is probably shorted. Skip to the paragraph below to test the bridge.
2. Check LED6. If not lit (indicating the absence of 18V at TP8), then suspect BR1 or the filter caps for BR1, C6 and C7. If BR1 had failed shorted or if C6 or C7 where shorted (rare) then you would expect to see F114 blown. BR1 can be tested using the procedure below.
3. Check LED1. If not lit (indicating the absence of “digital” 12V at TP3, then test D1 and D2 (both 1N4004 diodes). If D1/D2 test good, then check continuity from pin 2 of the 7812 voltage regulator at Q2 to J114, pin 1. This verifies the path through F115.
4. If this all checks out, yet 12VDC is still not at TP3, then suspect the 7812 voltage regulator at Q2. Q2 is in a TO-220 package (like a TIP-102) and has a small heat sink attached.

If TP3 does, in fact, have 12VDC present, then we need to dig deeper.

1. First check for 12VDC on the CPU board to ensure that the 12V is getting to the board via the 2 (and possibly 4 if your game has a “Z-Connector”) connectors that carry the 12VDC. An easy place to measure 12VDC on the CPU is at pin 10 of the ULN2803 (U20). Pin 10 is on the bottom row of pins, furthest left. Note that this connection (an also the 12VDC connection to pin 3 of the LM339s) is not shown on the schematic.
2. If the CPU is receiving 12VDC, and you still have the “Check fuses F114 and F115” message, then the problem lies within the CPUs switch matrix logic circuitry.
3. Prime candidates for CPU switch matrix failure are U20 (a ULN2803), the two LM339s at U18 and U19, the 74LS374 at U14, and the 74LS240 at U13. All of these ICs are in the “corrosion zone” caused by leaky alkaline batteries. If your CPU exhibits the “blue/green fuzzies”, address the alkaline damage first.
4. Testing the 74LSXXX ICs is simple using the procedure here.
5. If coil power, flasher power, or even lamp power was somehow shorted to the switch matrix, there is a near 100% probability that the ULN2803 at U20 has been damaged. There is no good way to test the ULN2803 other than using a logic probe with the game powered on. Both the input side and the output side of the ULN2803 should be toggling between logic 0 and logic 1. If not, socket and replace the ULN2803.

File:Test-br.jpg

Checking an uninstalled bridge rectifier

Testing a bridge rectifier is simple.

1. Place your DMM into "diode test".
2. Put the black lead of your DMM on the "oddball" lead of the bridge rectifier. This will be the lead that isn't oriented the same as the others (as with the "spade" type of bridge) or the lead that prevents the four legs from forming a square (as with the "wire lead" type of bridge). This will also be the DC positive lead of the bridge.
3. Place the red lead of the DMM on each of the adjacent legs, one at a time.
4. A reading nominally between .5 and .7 should be seen (this represents the voltage drop across the bridge's internal diodes).
5. Now place the red lead of the DMM on the lead opposite of the "oddball" lead, or the DC negative lead of the bridge.
6. Place the black lead of the DMM on each of the adjacent legs, one at a time.
7. Again, a nominal reading between .5 and .7 should be seen.

Readings outside of these ranges indicate a failed or failing bridge. Note that these readings are not "hard and fast". For instance, a reading of .462 is probably acceptable. We are looking for an "open" or a "short". Note also that this test is not conducted "under load" and it is possible for the bridge to test "good" when it will in fact fail under load (this is also true when testing diodes, transistors, etc).

The heat sink covering BR1 and BR2 on a WPC-board

Take special care when replacing a bridge rectifier. It is easy to lift the traces of the plated through holes when removing these. After removing the heat sink, cut the old bridge rectifier off of the board close to the top leaving as much of the lug as possible. Then add a small amount of solder to the connection on the solder side to improve heat transfer. Finally apply the iron to the solder side to heat the joint and remove the remaining lug with small pliers.

When replacing the new bridge rectifier you should remove the old thermal compound and apply a new thin layer. The old compound can be removed with a little rubbing alcohol. Apply the thermal compound to the top of the bridge rectifiers and reattach the heat sink with the screws. It is much easier to replace the heat sink before resoldering the new bridge rectifier. Make sure that the heatsink is sitting flush with the tops of the bridge rectifiers and that you leave about a 5/8" gap between the bridge rectifier and the board to improve air flow.

Splitting the heat sink between BR1 and BR2 was at one time thought to improve reliability. This is no longer considered a best practice and is not recommended.

More info about replacing bridge rectifiers can be found in the section regarding system resets.

Diodes replaced BR1 and BR2 on WPC-95 boards

In WPC-95 games, BR1 and BR2 were replaced by a series of diodes. D11-D14 replaced BR1. D7-D10 replaced BR2. These are much less prone to failure. Still, they do fail on occasion. If they fail, it's best to replace all four in the "gang".

4.4 Voltage Problems - LED2 & LED3

LED2 and LED3 on the Power Driver Board are indicators for the line voltage detection circuit. On on-board comparator circuit uses +5V and +18V to check the line voltage. The comparator consists of a resistor voltage divider and an LM339 comparator.

The following table shows how to interpret the LED states. Note that other problems with the +18V or +5V supply could cause a high or low line voltage indication.

Line Voltage Indicator Chart

LED2	LED3	Voltage
On	Off	OK
Off	Off	High
On	On	Low

4.5 Solenoid & Flasher problems

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

4.6 Lamp problems

Lamps (or globes for those of you in the UK) fall into two categories. "General Illumination" and "Controlled Lamps". Your game probably has other lamps which are actually "flashers" that require an 89 or 906 bulb. Flashers are covered elsewhere in this Wiki.

General illumination lamps (GI) provide the ambient lighting for the playfield, backbox, and coin door. These lamps are list most of the time. The only "controllable" aspect of these lamps is their brightness.

Controlled lamps are under complete CPU control via the lamp matrix. These lamps illuminate the various "features" of the game such as mode inserts, pop bumper lamps, inlane/outlane insert lamps, etc.

4.6.1 General Illumination Problems

Background

The WPC power/driver board provides 5 GI circuits under CPU "brightness control". The WPC-95 power/driver board provides 3 GI circuits under CPU "brightness" control and 2 circuits that are always powered. Those two circuits are always connected to the backbox lamps.

GI power is provided by the transformer in the form of 6.3VAC at power/driver board connector J115. Each of the GI circuits is "metered" by a Triac on one side of AC power. AC power is then supplied to the backbox, playfield, and coin door via power driver board connectors J120, J121, and J119 respectively. J119 is a 3-pin header that is always connected to the coin door lamps. J120 and J121 are electrically and physically identical (keyed at pin 4) and therefore can be connected to either the female playfield GI connector or the female backbox GI connector without care.

The other side of the AC GI circuit on the power/driver board is fused by F106 through F110 which are all 5ASB fuses.

Each Triac is switched on/off by a 2N5401 transistor which is controlled by U1, a 74LS374, an octal D-Type Flip-Flop with three state outputs (a fancy way of saying that the device is an 8-bit data buffer).

Wire colors to J115 during the early WPC games were always yellow (AC) and yellow-white (AC return). Pin 1 of J115 provides the "ground reference". Later WPC games used different wire colors at J115 and instead of "looping" power through the connector at J115, connected two wires to the same pin at the 9-pin connector between the transformer secondary and J115.

J119 is always keyed at pin2, with a white-Violet wire at pin 1 and a Violet wire at pin 3

J120 and J121 are 11 pin connectors. Wire color/positions (when used) are identical for all WPC games and are shown in the table.

Pin	Wire color at J120/J121
1	Brown
2	Orange
3	Yellow (yellow)
4	Key
5	Green
6	Violet
7	White-Brown
8	White-Orange
9	White-Yellow (yellow)
10	White-Green
11	White-Violet

Common GI Problem Causes (listed by probability/ease of testing and correction)

Let's start by determining if the problem is "off board", i.e. not on your power/driver board, or "on board".

1. Take note of the wire color of the GI string that is not working.
2. Set your DMM to AC voltage. If your DMM is not an "auto-ranging" model, expect to measure about 7 volts AC.
3. Turn the game on.
4. Insert one probe of your DMM (either one) into the rear of the female connector position with the same wire color as noted earlier. Make sure you make contact with the conductor.
5. Insert the other probe into the rear of the female connector with the white wire that has the same color "tracer" as the color noted earlier.
6. You should be measuring 6 to 7 VAC.
7. If there is voltage at the connector, then the problem is "off board".
8. If there is no voltage at the connector, then the problem is "on board".

Off board GI problems

1. All lamps blown. Don't laugh. Just for giggles, put a known working lamp into one of the lamp sockets of the suspected GI string. I've seen it more than once. Usually the result of operators never changing a bulb, or somehow connecting higher voltage to the circuit.
2. "Open" in GI wiring. The GI wires leading from J120/J121 may be cut/broken between the connector and the GI lamp sockets. Remove both of the female connectors at J120/J121. Use your DMM to check continuity between the lamp socket and the appropriate wire/pin at J120/J121.

On board GI problems

1. Blown fuses. This is also the easiest problem to fix. Check each GI fuse following this procedure. Don't trust your eyes.
2. Burned connectors. <need a picture> Over long periods of time, both the male and female GI connectors at J115, J120, and J121 often burn. This was especially prevalent in early WPC games like Terminator 2. Numerous "hacks" have been employed over the years by well meaning repairmen but the only real way to fix burned connectors is to replace both the male header pins and the female housings/pins. "Trifurcon" crimp-on pins are recommended to provide a better connection at the female connector.
3. Burned AC power input connector. There is also a 9-pin (3x3) connector between the transformer secondary and J115. This connector/pins sometimes burn and may require replacement. Check to ensure that AC power is being delivered to the power/driver board at J115.
   1. Set your DMM to AC voltage. If your DMM is not an "auto-ranging" model, expect to measure about 7 volts AC.
   2. Turn the game on.
   3. Insert one probe of your DMM (either one) into the rear of the female connector at J115, pin 3.
   4. Insert the other probe into the rear of the female connector at J115, pin 11.
   5. You should be measuring 6 to 7 VAC.
   6. If no voltage is present, examine the 9-pin connector between the transformer secondary and J115.
4. Burned traces. Usually, this is a secondary problem created after the connectors burn. Visual inspection of the traces might uncover the problem, but "buzzing" them for continuity is the best practice.
5. Failed 2N5401. These don't fail very often, but are easy to test following this procedure.
6. Failed resistors. Again, these don't fail very often but are easy to test with your DMM. Keep in mind that "in-circuit" measurements may not be reliable.
7. Failed 74LS374. This is rare too but easy to test following this procedure. You can also test the outputs of the LS374 by probing pins 2, 5, 6, 16 and 19 with your logic probe. At full GI brightness, these pins should test as "high".
8. Failed Triac. These rarely fail. But, if you've gotten to this step in the procedure, it's time to replace the Triac.

As these boards age it is easy to lose through board continuity. If you put in new connector pins. Be sure and check continuity from the pin onto the board somewhere. Only takes a couple minutes and can save you a lot of work.

4.6.2 Controlled Lamp Problems

Under construction...

4.7 Switch problems

"Switch Matrix" switches section under construction...
Switches in WPC games fall into two categories, those within the switch matrix, and "direct" switches.

Direct switch operation

Direct switches include:

• Left coin chute
• Center coin chute
• Right coin chute
• 4th coin chute
• Service Credits/Escape (referred to as "escape" here)
• Volume down/Down
• Volume up/Up
• Begin Test/Enter

Direct switches are not part of the WPC switch matrix. All of the direct switches are located on the coin door, and connect to the MPU at J205. The MPU senses these switches individually, and apart from the switch matrix. Therefore, isolation diodes are not used with direct switches.

Normally, with the switch open, the LM339s at U16 and U17 compare 12V (supplied on the MPU to both the switch and to the LM339) with 5V (as a comparison level) and signals the 74LS240 at U15 that the switch is open. When the switch closes, it shorts the 12V to ground and the comparison at the LM339 then indicates to U15 that the switch is closed. U15 is "clocked" by pin 48 of the ASIC (SW DIR), causing U15 to present its data to the data bus. The 6809/ASIC "debounces" the switch. Debouncing is not a factor to be considered here and doesn't factor in switch testing at all.

Direct Switch Pinout at J205 for both WPC and WPC-S MPUs
Notes:

J205, pin 5 is the "key" pin.

J205, pin 10 provides ground (black wire)

J205, pin 11 is unused

J205, pin 12 is an "enable" back to the coin door interface board

Signal	Wire color at J205	MPU pin	Diode	LM339 & input pin	U15 input pin
Left chute	orange/brown	J205-1	D15	U17-5	11
Center chute	orange/red	J205-2	D16	U17-7	13
Right chute	orange/black	J205-3	D17	U17-11	15
4th chute	orange/yellow	J205-4	D18	U17-9	17
Escape	orange/green	J205-6	D11	U16-9	2
Down	orange/blue	J205-7	D12	U16-11	4
Up	orange/violet	J205-8	D13	U16-7	6
Enter	orange/gray	J205-9	D14	U16-5	8

Debugging direct switch problems

To discover the problem as quickly as possible, we'll divide the problem into smaller pieces. We must first determine if the problem is on the MPU board or the problem lies with the wiring to the coin door and/or the specific switch. If your MPU has obvious alkaline damage at U15, U16, or U17, address the alkaline damage first.

Begin testing with the game OFF.

Isolate the problem to the MPU or to the game wiring/switch

1. Remove connector J205 from the MPU.
2. Build a jumper wire with alligator clips on both ends.
3. Clip one end of the jumper wire to the game's ground braid in the game head.
4. Turn the game on
5. Carefully touch the other end of your jumper wire to the appropriate pin on J205 as shown in the table.
6. If the MPU carries out the function of the switch, the problem is not on the MPU.
7. If the MPU does nothing, the problem is on the MPU.
8. Skip to the appropriate section below.

Identifying problems not on the MPU
Turn the game OFF.

Test the direct switch's path to ground

1. DMM set to continuity
2. Red lead on the solder joint between the switch and the black wire that provides ground.
3. Black lead on any game ground, like the lockdown bar.
4. You should hear "tone". If not, further diagnose the break in the black wire between the solder joint and game ground.

Test the direct switch itself

1. Clip the black lead of your DMM to game ground
2. Red lead on the solder joint opposite the black wire (or bare wire jumper) on the switch under test
3. Depress the switch. You should hear "tone". If not, the switch is defective and not "making". See the section below that describes cleaning these switches.

Test the signal path to the MPU

1. Clip the black lead of your DMM on the solder joint opposite the black wire (or bare wire jumper) for the switch under test.
2. Remove the connector plug at J205 from the MPU.
3. Red lead on the appropriate pin of J205 for the switch under test. See the table above. It's easiest to access the pin through the rectangular hole in the back of the connector where the pin's "tang" snaps in.
4. You should hear "tone". If not, there is a discontinuity in the wire between the direct switch and J205. Note that the coin door interface board is between these two points. The coin door interface board is a "pass-through" for these signals and rarely causes a problem. Still, reseating connectors J1, J3, and J4 on the coin door interface board might uncover the problem. More likely, the wire between J1 on the coin door interface board and J205 has a break in it.

Identifying problems on the MPU
Turn the game OFF.

Test the signal path through J205 and onto the MPU
J205 is right below the battery holder and as such, sometimes receives the unwanted gift of dripping alkaline from depleted batteries. Carefully examine both the male and female connections of J205 for alkaline "greenies". Assuming no alkaline damage...

1. Connect J205 to the MPU.
2. Clip the black lead of your DMM to the solder joint opposite the black wire (or bare wire jumper) for the switch under test.
3. Red lead on the banded end of the appropriate diode shown in the table above.
4. You should hear "tone". If not, there is either a problem with the physical connection at J205 or the alkaline "greenies" are sneakier than you gave them credit for. Re-examine J205 and the surrounding area of the board for alkaline damage. The traces from the switch connectors are very small and it takes very little alkaline damage to compromise them. You may also re-pin the female side of J205.

At this point, a logic probe would be the best tool to use. You can pick up 5V power for your probe across the electrolytic cap at C31 which is immediately to the right of the battery holder. Black lead on the negative side (top of the cap). Red lead on positive side (bottom). The board is silkscreened with polarity markings. Set the logic probe to "CMOS" test mode, as you will be measuring 12VDC.

If you don't already have a logic probe, you should. Although for this test, you can still get by with your trusty DMM. Clip the black lead of your DMM to game ground. The ground braid in the head is a good place to pickup ground at this point. Set your DMM to DC volts.

Test the LM339 inputs

1. Start with the game turned off.
2. Again, set your logic probe to "CMOS"
3. Clip one end of your jumper wire to the appopriate pin of J205. Leave the other end of your jumper wire unconnected, but handy as you'll touch it to ground later.
4. You now need to turn the game ON.
5. Measure the signal at the appropriate LM339's appropriate pin shown in the table above. Either place your logic probe on the pin or place the Red lead of your DMM on the pin. The signal should measure high (or about 12VDC with your DMM).
6. Now, with your other hand, touch the free end of your jumper wire to the ground braid in the backbox as you observe the results.
7. You should see the signal transition to low. If you still measure high, then the pin isn't being grounded correctly. Candidates are a failed diode/resistor in the circuit, the board trace between the diode and the LM339 is compromised, or a badly failed LM339.

Test the LM339 outputs/74LS240 (U15) inputs

1. Set your logic probe back to TTL as you will be measuring 0 - 5V signals
2. Measure the signal at the appropriate pin (see table) of U15. The signal should measure high (or about 5VDC with your DMM).
3. Touch the free end of your jumper wire to the ground braid in the backbox as you observe the results.
4. You should see the signal transition to low. If you still measure high with the jumper wire grounded, then either the LM339 outputs have failed, the board trace between the LM339 and U15 is compromised, or the 74LS240 has failed and is corrupting the signal. You can test U15 using the procedure in the "How to..." section of PinWiki, here.

The output side of U15 can't be tested effectively since that is the processor/ASIC data bus and should be constantly and irregularly changing states.

If you've followed this process step-by-step, you should have identified the problem with the signal and will be able to effectively perform the appropriate repair.

Debugging direct switch problems

Some techs will start at the end of the signal path and work back to the switch. This article works from the switch signal source to U15 on the MPU. This allows you to test the easiest, yet high probability failure points, first. If your MPU has obvious alkaline damage at U15, U16, or U17, address the alkaline damage first.

Begin testing with the game OFF.

Test the direct switch's path to ground

1. DMM set to continuity
2. Red lead on the solder joint between the switch and the black wire that provides ground.
3. Black lead on any game ground, like the lockdown bar.
4. You should hear "tone". If not, further diagnose the break in the black wire between the solder joint and game ground.

Test the direct switch itself

1. Clip the black lead of your DMM to game ground
2. Red lead on the solder joint opposite the black wire (or bare wire jumper) on the switch under test
3. Depress the switch. You should hear "tone". If not, the switch is defective and not "making". See the section below that describes cleaning these switches.

Test the signal path to the MPU

1. Clip the black lead of your DMM on the solder joint opposite the black wire (or bare wire jumper) for the switch under test.
2. Remove the connector plug at J205 from the MPU.
3. Red lead on the appropriate pin of J205 for the switch under test. See the table above. It's easiest to access the pin through the rectangular hole in the back of the connector where the pin's "tang" snaps in.
4. You should hear "tone". If not, there is a discontinuity in the wire between the direct switch and J205. Note that the coin door interface board is between these two points. The coin door interface board is a "pass-through" for these signals and rarely causes a problem. Still, reseating connectors J1, J3, and J4 on the coin door interface board might uncover the problem. More likely, the wire between J1 on the coin door interface board and J205 has a break in it.

Test the signal path through J205 and onto the MPU
J205 is right below the battery holder and as such, sometimes receives the unwanted gift of dripping alkaline from depleted batteries. Carefully examine both the male and female connections of J205 for alkaline "greenies". Assuming no alkaline damage...

1. Connect J205 to the MPU.
2. The black lead of your DMM should still be clipped to the solder joint opposite the black wire (or bare wire jumper) for the switch under test.
3. Red lead on the banded end of the appropriate diode shown in the table above.
4. You should hear "tone". If not, there is either a problem with the physical connection at J205 or the alkaline "greenies" are sneakier than you gave them credit for. Re-examine J205 and the surrounding area of the board for alkaline damage. The traces from the switch connectors are very small and it takes very little alkaline damage to compromise them. You may also re-pin the female side of J205.

At this point, a logic probe would be the best tool to use. You can pick up 5V power for your probe across the electrolytic cap at C31 which is immediately to the right of the battery holder. Black lead on the negative side (top of the cap). Red lead on positive side (bottom). The board is silkscreened with polarity markings. Set the logic probe to "CMOS" test mode, as you will be measuring 12VDC.

If you don't already have a logic probe, you should. Although for this test, you can still get by with your trusty DMM. Clip the black lead of your DMM to game ground. The ground braid in the head is a good place to pickup ground at this point. Set your DMM to DC volts.

You now need to turn the game ON.

Test the LM339 inputs

1. Again, set your logic probe to "CMOS"
2. Measure the signal at the appropriate LM339's appropriate pin. Either place your logic probe on the pin or place the Red lead of your DMM on the pin. The signal should measure high (or about 12VDC with your DMM).
3. Have someone close the switch under test and hold it closed as you observe the results.
4. You should see the signal transition to low. If you still measure high, then the pin isn't being grounded correctly. Candidates are a failed diode/resistor in the circuit, the board trace between the diode and the LM339 is compromised, or a badly failed LM339.
5. Note that if the signal transitions to low but not "crisply", then the switch is marginal and should be cleaned or replaced to ensure proper future operation. That is, the signal should transition directly from high to low and stay there without flip-flopping much at all while transitioning or when held down.

Test the LM339 outputs/74LS240 (U15) inputs

1. Set your logic probe back to TTL as you will be measuring 0 - 5V signals
2. Measure the signal at the appropriate pin (see table) of U15. The signal should measure high (or about 5VDC with your DMM).
3. Have someone close the switch under test and hold it closed as you observe the results.
4. You should see the signal transition to low. If you still measure high with the button pressed, then either the LM339 outputs have failed, the board trace between the LM339 and U15 is compromised, or the 74LS240 has failed and is corrupting the signal. You can test U15 using the procedure in the "How to..." section of PinWiki, here.

The output side of U15 can't be tested effectively since that is the processor/ASIC data bus and should be constantly and irregularly changing states.

If you've followed this process step-by-step (or even in reverse step-by-step), you should have identified the problem with the signal and will be able to effectively perform the appropriate repair.

Cleaning direct switches

Coming soon...will describe disassembly and cleaning... Sometimes, several rigorous open/close cycles will "clean" corrosion from the switch.

WPC Switch Matrix problems

Coming soon...