The majority of electronic repairs will be on the WPC Power Driver board. To do any repairs to the driver board, it must be removed from the game. Yes, there are seemingly an endless array of connectors that will have to be dealt with. Fear not, all are keyed so they can't be plugged into the wrong place (in most cases!). For confidence and simplification, always label the connectors as they are removed. Sure, this is probably unnecessary. But if there are any problems, the idea that I might have incorrectly plugged the connectors can be eliminated. It only takes a minute, and there is never any doubt about what goes where.

Using a mark-all "Sharpie" pen, label the sides of all the connectors as they are removed.

Use a mark-all "Sharpie" pen to label the connectors. The side of each connector has room for writing. After the connectors are all marked and removed, loosen the phillips head screws that hold the driver board in place. The screws don't have to removed all the way! Only loosen them. The board has slots for all the screws, so the board will lift up and out of the backbox.

Note: some connectors are "parallel". That is, they have the same keyed pin configuration so as many as three plugs, can be switched around. To minimize this confusion, again just mark the plugs with a Sharpie as they are removed.

If a bad component has been found, now comes the hard part; replacing it! Transistors, bridge rectifiers, and most chips are not socketed. They are soldered directly into the driver board. Care must be taken when replacing a bad component.

Please see http://marvin3m.com/begin for details on the basic electronics skills and tools needed when replacing circuit board components.

When replacing components, the object is to subject the board to the least amount of heat as possible. Too much heat can lift or crack the board's traces. Too little heat and the plated-through holes can be ripped out when removing the part. New circuit boards are too expensive to replace. So be careful when doing this.

To remove a bad component, just CUT it off of the board, leaving as much of its original lead(s) as possible. Then using needle nose pliers, grab the lead in the board while heating it with the soldering iron, and pull it out. Clean up the solder left behind with a desoldering tool.

When replacing chips, alway install a socket. Buy good quality sockets. Avoid "Scanbe" sockets at all costs! A good machine pin socket is desirable.

If a coil is "stuck on" when the game is turned on, a shorted driver transistor could be cause. If a coil does not work (and the fuses are good!), an open driver transistor could be the cause. This section will help diagnose this.

What do the Driver Transistors Do?
Basically, a driver transistors completes each coil's path to ground. There is power at each coil, all the time. The driving transitor is "turned on" by the game's software, through a TTL (Transistor to Transistor Logic) chip. When the transistor is turned on, this completes the coil's power path to ground, energizing the coil. Driver transistors also work the CPU controlled lamps and flash lamps, causing a lamp to "lock on".

Sometimes these driver transistors short "on" internally. This completes a coil or flash lamp's power path to ground permanently, making it "stuck on", as soon as the game is turned on. Also a shorted pre-driver transistor, or shorted TTL chip (which controls the transistors) could be the problem (though a shorted driver transistor is the most common cause). To fix this, the defective component (and perhaps some other not defective, but over stressed componets) will need to be replaced.

TIP36 and TIP102 transistors on the driver board.

TIP102 transistors, the small 2N5401 pre-driver transistors, and the coil diodes on the driver board.

There are basically four types of driver and pre-driver transistors used on a WPC driver board:

• TIP36c (PNP, NTE393): used for solenoid numbers 1 to 8. High power transistors used for more powerful solenoids (and the flipper, on their initial "flip" on the Fliptronics board).
• TIP102 (NPN, NTE2343): used for solenoid numbers 9 to 28. Low power solenoid and flash lamp drivers, used for most devices (and for the flippers on their "hold" circuit on the Fliptronics board). Numbers 9 to 16 are used for low power solenoids, number 17-20 for flash lamps, and number 21 to 28 for general purpose solenoids or flash lamps. TIP102's are also used to switch GND on for any particular lamp row.
• TIP107 (PNP, NTE2344): used to drive the CPU controlled lamp (columns) on the playfield. The TIP107 switches the +18 volts on for any particular lamp column.
• 2N5401 & MPSD52 (PNP): used as a pre-driver for the TIP102 transistors. 2N5401, MPSD52 and NTE288 are all equivalent transistors.
• 2N4403 (PNP, NTE159): used as a pre-driver for the Fliptronics board.

Driver Transistor Operation.
As described above, the main driver transistor (a TIP102 or TIP36) completes the coil or flash lamp's power path the ground, energizing it. But there are other components involved too!

Each driver transistor has a "pre-driver" transistor. In the case of a TIP102 (the most common WPC driver transistor), this is a smaller 2N5401/MPSD52 or 2N4403 transistor.

If the main driver transistor is a TIP36c, this is pre-driven by both a TIP102 and a smaller 2N5401/MPSD52 or 2N4403 transistor. The bigger TIP36c transistor is an even more robust than the TIP102, and controls very high powered, high use coils (like the flippers).

Then before even the smaller 2N5401/MPSD52 or 2N4403 pre-driver transistor, there is a TTL (Transistor to Transistor Logic) 74LS374 chip. This is really the first link in the chain. This is what in affect turns on the smaller 2N5401/MPSD52 or 2N4403 pre-driver transistor, which then turns on the TIP102 (which then turns on the TIP36c, if used for the coil/flash lamp in question), and energized the device.

This series of smaller to bigger transistors is done to isolate the hi-powered coil voltage (50 volts), from the low-power logic (5 volts) on the driver board. Also the 74LS374 chip (operating at +5 volts), which really controls the transistors, can not directly drive a high power TIP102 or TIP36c transistor (which is controlling 50 volts).

If ANY of these components in the chain have failed, a coil/flashlamp can be stuck on, and will energize as soon as the game is powered on!

I have a Stuck-on Coil (or Flashlamp), What should I Replace?
The following procedures will test the driver and pre-driver transistors in question. If either is bad, it will need to be replaced. When replacing either a driver or pre-driver transistor, replace them both (or in the case of a TIP36, replace the TIP102 and smaller 2N5401/MPSD52 or 2N4403 transistor)! A shorted transistor will cause the other transistors in the link to be stressed, and they should all be replaced.

Inside the front cover of the game manual is a list of each coil used in the game. Also listed are the driving transistor(s) for each coil. Use this chart to determine which transistors could potentially be bad. Also use the schematics.

If after replacing the driver transistors the coil/flashlamp is still stuck on, then replace the TTL 74LS374 logic chip. The TTL 74LS374 can also go bad (though it is not real common).

A Coil just Does Not Work - What is Wrong?
Driver transistors can go "open" too. This means all the logic prior to the open transistor could be working fine, but the coil will not energize. If there is power at the coil, this is something to consider (but first see the test procedures below to make sure the coil itself is actually OK).

Do the Transistor Test Procedures work 100%?
In short, no. But they do work about 98% of the time, and are an excellent starting point. But yes, a transistor can test as "good", but still be bad. The DMM test procedures test the transistors with no load. Under load, a transistor could not work.

Testing a transistor on the driver board. Note the DMM is set to the diode position, and one lead is connected to the metal tab on the TIP transistor. The two outside leads are then tested.

Transistor Testing procedures using a DMM.
If the driver board is out of the game for some reason (like to fix the burnt GI connector pins), test all the transistors. It only takes a moment, and will ultimately save time. To test a transistor, a digital multi-meter (DMM) is needed, set to the "diode" position. NOTE: testing transistors with a DMM is not 100% fool-proof. A transistor can test as "good" and still be bad (rare, but it does happen!).

Testing Transistors INSTALLED in the WPC driver board.


• TIP36c: Put the red lead of the DMM on the metal tab of the transistor. Put the black lead of the DMM on each of the two outside legs of the transistor. A reading of .4 to .6 volts should be seen. Put the black lead on the center transistor leg (collector) and the red lead on the metal tab, and a zero reading should be seen. Put the black lead of the DMM on the left/top (base) leg of the transistor. The red lead on the center transistor leg should show .4 to .6 volts. The red lead on the right/bottom leg should be .2 volts. Any other value, and the transistor is bad and will need to be replaced.
• TIP102: Put the black lead of the DMM on the metal tab of the transistor. Put the red lead of the DMM on each of the two outside legs of the transistor. A reading of .4 to .6 volts should be seen. Put the red lead on the center transistor leg (collector), and a zero reading should be seen. Any other value, and the transistor is bad and will need to be replaced.
• TIP107: Put the red lead of the DMM on the center leg or on the metal tab of the transistor. Put the black lead of the DMM on each of the two outside legs of the transistor. A reading of .4 to .6 volts should be seen. Put the black lead on the center transistor leg (collector) and the red lead on the metal tab, and a zero reading should be seen. Any other value, and the transistor is bad and will need to be replaced.
• 2N5401, MPSD52, 2N4403 (pre-drivers): Put the black lead of the DMM on the center leg of the transistor (note this transistor doesn't have a metal tab). Put the red lead of the DMM on each of the two outside legs of the transistor. A reading of .4 to .6 volts should be seen. Any other value, and the transistor is bad and will need to be replaced.

Testing Transistors NOT INSTALLED.
Only the TIP36c will test slightly different out of circuit. The other transistors will test the same as described above. All transistors are laying on the workbench with their "face" (side with the markings) up, and metal tab away from you. Orientation is BCE (base collector emitter), from left to right for the TIP transistors. Orientation for the small plastic transistors is EBC (emitter base collector) with the flat side up.

• TIP36c: Put the black lead of the DMM on the left (base) leg of the transistor. Put the red lead of the DMM on each of the two other legs (center and right legs) of the transistor. A reading of .4 to .6 volts should be seen. Put the DMM leads on the metal tab and the center transistor leg (collector), and a zero reading should be seen. Any other value, and the transistor is bad.

Most often transistors short when they go bad. This will usually give a reading of zero or near zero, instead of .4 or .6 volts.

Testing Coils and Transistors; a Systematic Approach.

If a coil is not working, the following approach is a good one to take. It starts with the easiest test first; using the internal WPC diagnostics. Then the tests moves to the coil itself, and goes back towards the driver board. This makes the chain smaller, and gives a very systematic approach to finding the problem.

Pressing the "start game" button on the outside of the cabinet during the Solenoid Test gives important information. In this example (the Auto Plunger coil), it shows the coil's wire colors, the board connectors/pins used, the fuse rating and position, and the transistors that drive this coil. Note Q72 is a TIP36 transistor with Q60 (a TIP102) as a pre-driver, and Q56 (a MPSD52) as a pre-driver to the TIP102.

Testing Transistors/Coils, Driver board installed in a (near) WORKING game, using the Diagnostics Test.
If the game powers on, the WPC diagnostics can be used to test most devices.
• Press the "Begin Test" button inside the coin door.
• Select "MAIN MENU: TESTS".
• Select "TEST MENU: SOLENOID TEST".
• Use the "+" and "-" buttons to move the test from coil to coil. Each coil should fire. (Note the coin door interlock switch must be held in on 1993 and later games. Otherwise the coil 50 volts will be turned off, and the coils won't fire. Also make sure the "REPEAT" portion of the test is used. This can be changed using the "Begin Test" button.)
• Press the "help" button. The game's start button during the coil test wil give more coil information including coil wire colors, Driver board connector and pin numbers; related fuse number; Driver board transistor and pre-driver transistor numbers.

Solenoid Doesn't Work during WPC Diagnostic Tests.
If a solenoid doesn't work from the diagnostic tests, here's what to check. Turn the game off before doing this.
• Check all the fuses on the driver board. A non-working solenoid could be as easy to fix as just replacing a fuse.
• Find the solenoid in question under the playfield. Make sure the wire hasn't fallen off or become cut from the coil (a very common problem).
• If the above is correct, make sure the winding of the coil haven't broken off from the solder lugs. If one has broken, it can be re-soldered. Make sure the painted enamel insulation is sanded from the wire before re-soldering.
• Check the coil diode (for any other pinball game, this would be the next step). The coil diode for all games (except WPC) are attached right to the coil, with the banded side of the diode connecting to the power side of the coil. On WPC games however, Williams moved this diode to the power driver board for all coils but the flipper coils. This increases reliability as the diode is not subject to the jarring and heat a coil can produce. It also eliminates the need for the operator to know which coil wire goes to the banded side of the diode when replacing a coil! On a WPC game, these coil diodes are mounted on the driver board next to the transistor that drives each particular coil.

Quick and Dirty TIP102 Transistor Testing.
There is an easy way to test TIP102 (only) transistors. This procedure takes about 20 seconds to test all the TIP102 transistors:

• Make sure the game is off.
• Put the DMM (digital multi meter) on ohms (buzz tone).
• Put one lead on the ground strap in the backbox.
• Touch the other lead to the metal tab on the TIP102 transistors.
• If zero ohms (buzz) is indicated, the transistor is bad! (shorted on)

The Coin Door Interlock switch.
In the middle of Twilight Zone's production in 1993, Williams added a coin door interlock switch. This turned off the power to all the coils when the coil door was opened (for safety reasons). On 1993 and later games with this interlock switch, make sure the coin door is closed when testing coils!

Failed Coin Door Interlock switch.
Yes it does happen. The coin door interlock switch can fail, or does not get pushed in enough when the coin door is closed. This will prevent voltage from getting to the solenoids. If none of the solenoids work, and the fuses are good, check the coin door interlock switch for problems. A sure sign of this is the Driver board solenoid power LED's will NOT be lit if the coin door interlock switch is not closed! The interlock switch opens the coil power coming from the transformer, which is way before the power gets to the Driver board's fuses and power circuits.

Testing for Power at the Coil.
Most pinball games (including WPC) have power at each and every coil at all times. To activate a coil, GROUND is turned on momentarily by the driving transistor to complete the power path. Since only ground (and not power) is turned on and off, the driving transistors have less stress on them. With this in mind, if we artificially attach a coil to ground, it will fire (assuming the game is turned on).

• Turn the game on and leave it in "attract" mode.
• Lift the playfield.
• Put the DMM on DC voltage (100 volts).
• Attach the black lead of the DMM to the metal side rail.
• Touch the red lead of the DMM on either lug of the coil in question.
• A reading of 20 to 80 volts DC should be indicated. Switch the red test lead to the other lug of the coil, and the same voltage should be seen again. On flipper coils, test the two outside lugs of the coil. If no voltage reading is shown, no power is getting to the coil. On a two lug coil, if there is only voltage at one lug, the coil winding is broken. On 1993 and newer WPC games, make sure the coin door is closed!
• If no power is getting to the coil, a wire is probably broken somewhere. Trace the power wire.

No Coil Power, Fuse is Good and No Broken Wires.
I recently had a problem on a Safe Cracker (WPC-95) where none of the low power (20 volt) coils worked. It was very frustrating; the fuse was good, and power was getting to the Driver board, but not out of the driver board and to the coils.

It turned out that the capacitor that filters the DC voltage after the bridge rectifier on the Driver board had a cracked solder pad. This prevented the voltage from getting any further than it's associated bridge rectifier (I should have known; the +20 volt LED on the Driver board was not lit!). To fix this, I soldered jumper wires from the bridge to the capacitor, as outlined in the below Game Resets (Bridge Rectifiers and Diodes) section.

Testing the Coil and the Power Together.
This test will show if the power and the coil are indeed working together:

• Game is on and in "attract" mode, and the playfield lifted. On 1993 and newer WPC games, coin door is closed.
• Connect an alligator clip to the metal side rail of the game.
• Momentarily touch the other end of the alligator clip to the GROUND lead of the coil in question. This will be the coil lug with the single wire attached (usually brown). On flipper coils, this is the middle lug (the power wire on most coils is usually the thicker violet or red wire). This works on both Fliptronics and non-Fliptronics WPC games.
• The coil should fire (if the alligator clip is accidentally touched to the power side of the coil, the game will reset and/or blow a fuse, as the solenoid high voltage is being shorted directly to ground).
• If the coil does not fire, either the coil itself is bad, or the coil's fuse is blown and power to the coil is not present.

Testing the TIP102 Transistor and Wiring to the Coil.
If the coil fires in the above test, there may be a transistor problem. The TIP102 transistors can be tested this way. Only do this for the TIP102 transistors! Damage can occur if this test is done on other transistors (like TIP107 or TIP36).

• Game is on, and the "test mode" button is pressed once. On 1993 and newer WPC games, coin door is closed.
• Remove the backglass.
• Find the transistor that controls the coil in question (refer to the manual).
• Attach an alligator clip to the grounding strap in the bottom of the backbox.
• Momentarily touch the other lead of the alligator clip to the metal tab on any TIP102 transistor (only works on these).
• The coil should fire.
• If the coil does not fire, and the coil did fire in the previous test, there probably is a wiring problem. A broken wire or bad connection at the connector would be most common. It is also possible there is a bad transistor. Use the DMM meter and test the transistor on the board (see Transistors Testing Procedures for details).

The Above Tests Worked, but the Coil Still doesn't Work.
If all the above tests worked, there is probably a driver board problem. Everything has been tested from the TIP102 back to the coil itself. That only leaves the TIP102 itself, its pre-driver transistor, and the logic chip that controls the transistors. It has to be one (or more!) of these devices that are causing the problem.

Installing a New Transistor.
If it has been determined a coil's driver board transistor is bad, there are a few things to keep in mind. Most TIP102 transistors also have a "pre-driver" transistor (2N5401 for WPC-S and prior, or MPSD52 for WPC-95). Both 2N5401 and MPSD52 transistors are basically the same (use either). They both cross to NTE288.

If a coil's TIP102 transistor is replaced, it's a good idea to also replace its corresponding pre-driver. It will be located near the TIP102 transistor. See the schematics or the internal solenoid test "help" to determine the specific pre-driver transistor(s).

Heavier duty coils use a bigger TIP36c driver transistor. These transistors have TWO pre-drivers: a TIP102 and a 2N5401 (or MPSD52) transistor. Again, if the TIP36c has failed, it's a good idea to replace both corresponding pre-driver transistors.

Replacing the pre-driver transistors is optional (if they test Ok). Test these pre-drivers instead of just replacing them. But if the driver transistor has failed, the pre-driver was probably over-stressed too. It is a good idea to replace the pre-driver transistor(s) too.

Don't Forget the 74LS374 TTL Chip!
If a coil locked on really hard and for a period of time (and without blowing the coil fuse, over fused?), the controlling 74LS374 chip may have also died. If after replacing the TIP driver transistor(s) and the smaller pre-driver transistor, the coil is still locked on, now is the time to replace the 74LS374 TTL chip. Use the schematics and trace the transistors in question back to the 74LS374 chip. This will be chip U2, U3, U4, or U5 on WPC-S and prior driver boards, or chip U4, U5, U6, or U7 on WPC-95 driver boards.

WPC Coil Diodes.
On all electronic pinball games, each and every CPU controlled coil must have a coil diode. This diode is VERY important. When a coil is energized, it produces a magnetic field. As the coil's magnetic field collapses (when the power shuts off to the coil), a surge of power as much as twice the energizing voltage spikes backwards through the coil. The coil diode prevents this surge from going back to the driver board and damaging components.

If the coil diode is bad or missing, it can cause several problems. If the diode is shorted on, coil fuse(s) will blow. If the diode is open or missing, strange game play will result (because the driver board is trying to absorb the return voltage from the coil's magnetic field collapsing). At worse a missing or open diode can cause the driver transistor or other components to fail.

On non-WPC games, sometimes a diode lead breaks on the coil from vibration. Also, when replacing a coil, the operator can install the coil wires incorrectly (the power wire should always be attached to the coil's lug with the banded side of the diode). To prevent this, Williams moved the coil diode to the Driver board. This isolates the coil diode from vibration and eliminates the possibility of installing the coil's wires in reverse. This was done on all coils except the flipper coils.

The coil diodes on a Fliptronics flipper coil. The red (bottom) wire is the "hot" wire. The yellow (middle) wire handles the initial hi-power "flip", and the orange (top) wire handles the flipper's "hold".

Flipper Wire Colors.
From game to game, Williams often used a consistent set of wire colors for flipper wiring (unfortunately, this is not always the case, as seen in the picture above). In the picture below, the flipper coil lugs are labeled "lug1" to "lug3". Here are the wire color break down for most games:
Lug 1 (outside banded diode lug, two winding wires, 50 volts):
• Lower Left flipper: Grey/Yellow
• Lower Right flipper: Blue/Yellow
• Upper Left flipper: Grey/Yellow
• Uppper Right flipper: Blue/Yellow

Lug 3 (outside non-banded diode lug, one winding wire):

• Lower Left flipper: Orange/Blue
• Lower Right flipper: Orange/Green
• Upper Left flipper: Orange/Grey
• Uppper Right flipper: Orange/Purple

Lug 2 (middle lug):

• Lower Left flipper: Blue/Grey
• Lower Right flipper: Blue/Purple
• Upper Left flipper: Black/Blue
• Uppper Right flipper: Black/Yellow

Flipper coil wiring. Note the wire color rules specified below are the "usual" wire colors (but can't be 100% guarenteed).

The coil diodes on a Non-fliptronics flipper coil. Note the solo center wire and the all blue wire on the top lug goes to the EOS switch and the 2.2 mfd 250 volt spark arresting capacitor (the EOS switch and capacitor are wired in parallel). The blue/yellow (lower) wire (or gray/yellow) is the "hot" wire. The blue/violet (upper) wire continues to the cabinet switch, the driver board relay, and ultimately ground.

Even on WPC games, the coil diode can fail. The coil diode can be tested. It is mounted on the driver board, near it's corresponding driver transistor (refer to the schematics; it's the diode that is tied to one of the legs of the driver transistor).

Use a DMM set to "diode" setting, and test the board mounted coil diode. With the black lead on the banded side of the diode and the red lead on the non-banded side, a reading of .4 and .6 volts should be seen. Reverse the leads (red lead to banded side of diode), and a null reading should be seen. If this reading is not indicated, cut one lead of the diode from the driver board, and repeat the test. If these results are still not seen, replace the diode with a new 1N4004 diode.

Installing a New Coil.
Many replacement coils will come with a diode soldered across its solder lugs. On WPC games, all coils except the flipper coils have the diode mounted on the Driver board. For all coils except flipper coils, cut the diode off the coil before installing. Then solder the coil wires to either coil lug. The diode can also be left in place, but the coil wires must be installed correctly. The green (ground) wire MUST go to the lug of the coil with the non-banded side of the diode. The power wire solders to the lug with the banded side of the diode. If the wires are reversed, this essentially causes a shorted diode. Though the Driver board mounted diode is still present as protection, damage can occur to the coil's driver board transistor.

Coil Doesn't Work Check List.
If a coil doesn't work in a game, here's a check list to help determine the problem.

Before starting, is the coil stuck on? (Hint: is there heat, smoke and a bad smell?). If so, the coil's driving transistor has probably failed. Turn the game off and check the driving transistor, and replace if needed. See Transistors Testing Procedures for more info.

If the coil just doesn't work, here's a list of things to check:

• Have the power wires fallen off the coil's solder lugs?
• Is the coil damaged? Has the internal winding broken off the coil's solder lug?
• Is there power at the coil? See Testing for Power at the Coil for more details.
• If there is no power at the coil, check its fuse. Use the internal diagnostics and the "help" button to determine which fuse controls the coil. See Testing Transistors/Coils using the Diagnostics for details.
• Check the other coils that share one of the same wire colors. Are they working too? If not, suspect the fuse that handles these coils.
• Power to coils are often ganged together. If the power wire for this coil has fallen off a previous coil in the link, power may not get to this coil.
• Using the DMM and its continuity test, make sure the coil connects to the correct connector/pins on the driver board. This information can be seen from the Diagnostics solenoid test.
• Check the driving transistor. Usually this transistor will short on when it fails, but not always.

Game resets are probably the biggest problem with WPC to WPC-S games (and to a lesser extent, WPC-95 games). The pinball will seemingly shut off, then power back on (like the game was turned off and back on quickly). Typically, this will happen during game play, when the flippers are used. If the +5 volts (which powers all the logic circuits) dips momentarily below 5 volts (from heavy voltage draw when the 50 volt flippers are used), the "watchdog" circuit shuts the game down. The high current draw flippers stresses the other power components in the system. If these power components are starting to fail, the +5 volts dips, and the watchdog circuit resets the game. When the game shuts down, the power components under stress are relieved. Then the voltage returns to +5 volts, and the game powers back up. This reset process can happen anytime, but usually happens during game play.

Check the Easy Stuff First.

Proper AC Wall Voltage?
Important: Before starting to dig in and try to diagnose the bridge rectifiers, set the DMM to AC Voltage and test the wall socket voltage. Make sure there is 110 to 120 volts AC present! If there is only 100 volts, this can cause the game to reset. If in Europe, and the game is set to 240 volts AC and yet there is only 210 volts at the wall plug, this can also cause the game to reset too. This problem happens mostly in the summer, when household power consumption is at a high, or if the game is plugged into the same circuit as another high power device. WPC pinball games draw a maximum of 8 amps of power. Most home circuits are 15 amps, so two pinballs on one circuit should be the maximum. Don't have the game plugged into the same circuit as another power sucking device (like an air conditioner, refrigerator, etc.).

Re-seat the Transformer connectors.
Though this doesn't tend to be a big problem, try re-seating the connectors on the large transformer in the bottom of the cabinet. Unplugging then plugging these connectors effectively cleans them and makes sure they are snuggly attached. If there is any resistance in the transformer plugs, that can reduce the voltages going to the rest of the game. This only takes a moment to do, so it's not a bad thing to try.

Flipper Coil Diodes.
Though not a big problem on WPC games, if the flipper coil diodes (there are two per coil) are damaged or missing, this too can cause game resets. This is a lot more common on games prior to WPC, but it can happen here too, and the diodes are needed. If missing or broken, resets can happen on and WPC or WPC-95 game.

Failing Bridge Rectifiers, WPC-95 Diodes and Filter Caps.

Bridge rectifiers or diodes (and their corresponding filter capacitor) convert AC voltage to smooth DC voltage. This is very important, as all the circuit boards run on DC voltage. If a game plays fine, but randomly resets, often the bridge rectifiers (or diodes) and their capacitor are over stressed and need replacement. On WPC-S and prior games, a bad bridge rectifier is probably the single most commonly failed component relating to game resets.

A bad bridge rectifier (or diodes on WPC-95), or cracked solder pads around a bridge can also give game boot-up error messages saying fuse F114/F115 (or F106/F101 on WPC-95) have failed, when the fuses are actually good. See the Check the Fuses section (and below) for a list of fuses and what bridges they connect to.

WPC bridge rectifiers and diodes reside on the driver board (although there is also a bridge on the Fliptronics board prior to WPC-95). A bridge rectifier is mearly four diodes strung together in a square. There are two AC input voltages, and two DC (positive and negative) output voltages. These diodes are encased in epoxy, and covered with a square metal casing.

Failed bridges/diodes can often short or "go open". BOTH of these problems are quite common! A shorted bridge/diode will immediately blow a fuse when powered on. An open bridge/diode will cause lower or no voltage to get past the bridge. If the fuses are good, but power driver board LEDs are not lit, this could be an indicator of a bridge/diode that has "gone open".

Bridge rectifiers on a WPC-S and earlier generation driver boards. From the left to right: BR3, BR4, BR2 (top), BR1 (bottom). BR2 and BR1 have a large silver heat sink over them.

The BR5 bridge used on WPC-S and earlier generation driver boards. Note the "+" lead of the bridge is offset slightly, from an otherwise perfect square shape. Notice the bridge is installed about 1/4" above the board. This aids air flow and keeps the bridge cool.

WPC-95 "Bridges".
When WPC-95 was released, Williams decided to stop using bridge rectifiers. Instead they just installed four diodes right on the driver board for each replaced bridge. By using four discrete diodes instead of a single bridge, the heat generated by the components is spread out and reliability is greatly improved. Bridge rectifier failure is very common in WPC-S and prior systems. Replacement of even a single diode in the WPC-95 system games is very rare (but these games are not as old as WPC-S and prior systems). Certainly all the problems associated with the bridge rectifiers can still be exhibited in a WPC-95 system too.

The diodes used in WPC-95 are called P600D (or 6A4 or 6A400). These are 6 amp at 400 volt rectifiers. A substitute device is NTE5814.

WPC-95 P600D diodes D7 to D22 which replaced MB3502W/MB352W bridge rectifiers. Also note the smaller "T" fuses (on the right) used in WPC-95.

The Electrolytic Capacitor: the Bridge Rectifier and Diode's Partner.
Each bridge rectifier or diode set must also have an associated electrolytic capacitor. These are needed to polish the converted rough DC voltage to smooth DC voltage.

Electrolytic caps are largely mechanical devices. With time, they can fail. Expect about 10 years maximum life from an electrolytic filter capacitor. It is fairly common for these caps to fail. A failing electrolytic capacitor can cause the game to reset, as the DC voltage won't be "smooth". Because of this, when replacing the BR2 bridge on pre-WPC95 games, it is a good idea to also replace the associated filter capacitor C5 (15,000 mfd 25 volts).

Smaller Filter Caps Used with WPC-95. Why?
Interestingly, Williams changed from 15,000 mfd (at C5) on WPC-S and prior, to a lower value of 10,000 mfd on WPC-95 (at C9). With time, WPC-95 games may be more sensitive to bad filter caps, because of this lower value. Right now, since these games are fairly new (1996 and later), this isn't a huge problem.

Higher filter cap values are generally good; they provide a better level of AC filtering as the capacitor gets older. As electrolytic capacitors wear (they really are a mechanical device), they are less efficient at AC filtering, and their MFD value drops. However, the higher the MFD value of a capacitor, the more strain it puts on the rectifying bridge or diodes. When a game is turned on, the filter cap draws significant current during the first half AC cycle (since this power is used to "charge" the capacitor). This can subject the bridge rectifier (or diodes) to an excessive in-rush of current. This in-rush current can be up to ten times the current needed after the filtering capacitor has charged. This can cause a connection inside a bridge to instantly go open (this is not the same as over-current, which can cause the bridge to short). In-rush current is a factor of both voltage and the capacitor. A larger cap will force more in-rush current to the bridge, potentially causing damage. Also capacitors with higher MFD values cost more (the change from 15,000 to 10,000 mfd could have been in fact a cost/availability issue; the 10,000 mfd capacitors may have had a shorter lead time, and were cheaper for Williams to buy).

Bridge Rectifier, Diode, and Filter Capacitor Device List.
Here's a list of what bridge rectifiers and diodes control which functions, and their associated capacitors. All are located on the driver board, unless otherwise stated.

WPC-S and Earlier Driver Board:
• BR1 to C6 & C7 (15,000 mfd @ 25v) to F114: +18 volts used for lamp driver columns. Then the 18 volts goes through voltage regulator Q2 (LM7812) and F115, and is converted to 12 volts (regulated) for the switch matrix.
• BR2 to C5 (15,000 mfd @ 25v) to F113: +5 volts. The bridge and cap that fail the most.
• BR3 to C8 (100 mfd @ 100v) to F112: +50 volts, used for solenoids.
• BR4 to C11 (15,000 mfd @ 25v) to F111: +20 volts, used for flash lamps.
• BR5 to C30 (15,000 mfd @ 25v) to F116: +12 volts unregulated for playfield devices, dot matrix display, and the coin door.
• BR1 (on Fliptronics II board) to C2 (100 mfd @ 100v) to F901-F904: +50 volts used for the flippers. Located on the Fliptronics II board.

WPC-95 Driver Board:

• D3, D4, D5, D6 to C8 (10,000 mfd @ 35v) to F109: +12 volts unregulated for playfield devices, dot matrix display, and the coin door.
• D7, D8, D9, D10 to C9 (10,000 mfd @ 35v) to F105: +5 volts for all board logic circuits. The diodes and cap that fail the most.
• D11, D12, D13, D14 to C12 (10,000 mfd @ 35v) to F106/F101: +18 volts used for lamp driver columns. Then the 18 volts goes through voltage regulator Q2 (LM7812) and F101, and is converted to 12 volts (regulated) for the switch matrix.
• D15, D16, D17, D18 to C10 (10,000 mfd @ 35v) to F109: +20 volts for flash lamps.
• D19, D20, D21, D22 to C22 (100 mfd @ 100v) to F102/F103/F104: +50 volts for solenoids.
• D25 to D32: +6.3 volts for general illumination. These were replaced with jumpers starting with Scared Stiff. See the Burnt Connector section (WPC-95 GI diodes D25-D32 remove and jumper) for a description of this.

Testing a Bridge (WPC-S and prior), Board Removed.
Note testing a bridge with the game off is NOT conclusive to whether the bridge is bad! The bridge is being tested under NO load. Only a bridge which is shorted (and hence is blowing fuses) or open will test as "bad". A bridge could test as "good", and still cause the game to reset. Also testing a bridge "in circuit" (while still soldered in the board) can often not give proper results.

A bridge has four terminals: two AC terminals, and two DC terminals (postive and negative). On the side of each bridge, printed on the metal casing, there will be two labels: "AC" and "+". From the solder side of the driver board, mark with a Sharpie pen these two terminals. Figuring out the other two terminals is easy: the other AC terminal is diagonal to the labeled AC lead. The negative DC lead is diagonal to the labeled positive DC lead. Mark these right on the board with the Sharpie pen. To double check, the two DC leads (positive and negative) connect to that bridge's respective electrolytic capacitor, and it's positive and negative leads. Testing a bridge while soldered in the board (in curcuit) may not give the following results. For example, testing BR2 in curcuit will not give these results (but most of the other bridges will). To test the bridge:

1. Put the DMM on diode setting.
2. Put the red lead of the DMM on one of the AC terminals.
3. Put the black lead of the DMM on the positive DC lead. A reading of between .4 and .6 volts should be indicated.
4. Put the red lead of the DMM on the other AC terminal. Repeat step #3.
5. Put the black lead of the DMM on one of the AC terminals.
6. Put the red lead of the DMM on the negative DC lead. A reading between .4 and .6 volts should be indicated.
7. Put the black lead of the DMM on the other AC terminal. Repeat step #6.

If values outside of .4 to .6 volts are shown for any of the above tests, the bridge is bad. Typically you will get a zero value (a short) for at least one of the above tests in a bad bridge.

Testing a Bridge (WPC-S and prior), Under Load, In the Game.
This tip is from John Robertson. This test is a more conclusive way to test a bridge. This procedure requires a DMM, two alligator jumper wires, and a 6 amp rectifying diode (6A50 or 6A2 or 6A4, or whatever is available; Radio Shack sells 6A50 diodes, part number 276-1661). Here is the procedure:

1. With the game off, clip one end of an alligator test wire on the "+" lead of bridge BR2 (top most bridge) on the driver board. The "+" lead is the top left most lead (see picture below). Often the side of the bridge is labeled too. One lead is "AC", and the other is "+" (connect the alligator clip to the "+" lead, which is the left lead as facing the board).
2. Connect the other end of the alligator test wire on the RED lead of the DMM.
3. Put the BLACK lead of the DMM on the braided metal grounding strap at the bottom of the backbox.
4. Turn the DMM on, and set it to DC Volts (20 volt range).
5. Turn the game on. A value of 12 to 13 volts should be shown. Any less than 12 volts, and the bridge (or the connection to the bridge) is bad.

Attaching the red alligator test lead to the "+" leg of bridge BR2. The other end of the alligator lead is attached to the DMM's red probe.

6. Turn the game off. Take the second alligator jumper wire, and connect the clip to the BANDED end of the 6 amp diode.
7. Connect the other loose end of the alligator jumper wire to where the first alligator clip connects to the red lead of the DMM (see picture below). This is essentially the same as connecting the second alligator clip to the "+" lead of bridge BR2 (but there is not enough room at the bridge to do this, since the first alligator clip is in the way).
8. Turn the game on.
9. Touch the non-banded end of the diode to connector J101 in either pin 1 or 2 (two top most pins). Note the IDC connector will have some exposed metal at the top of the connector to touch, and plug should not be removed.
10. While doing the above step, examine the DMM voltage reading. If the voltage rises when the diode lead is touched to Connector J101 pin 1 or 2, the bridge BR2 is bad (bad internal positive diode).

A second alligator clip is connected to where the first alligator clip connects to the red lead of the DMM. Now touch the second alligator clip with a 6 amp diode, NON-BANDED end, to connector J101 pins 1 or 2. The voltage on the DMM should NOT drop when the diode is touched to connector J101 pins 1 or 2.

12. Turn the game off. Reverse the diode in the alligator clip so the NON-BANDED end of the 6 amp diode is connected to the alligator clip.
13. Connect the other end of the alligator clip to TP5 (ground).
14. Turn the game on.
15. Touch the banded end of the diode to connector J101 in either pin 1 or 2 (two top most pins). Note the IDC connector will have some exposed metal at the top of the connector to touch, and the plug should not be removed.
16. While doing the above step, examine the DMM voltage reading. If the voltage rises when the diode lead is touched to Connector J101 pin 1 or 2, the bridge BR2 is bad.

If the above tests all work as described (no voltage drops or readings below 12 volts), the problem is mostly likely a bad C5 (15,000 mfd 25 volt) filter cap (or a cracked solder joint to the bridge and/or capacitor, which can be solved by installing the jumper wires described below).

The second alligator clip is now connected to TP5 (ground), and the diode is reversed in the alligator clip. Touch the other end of the second alligator clip with the 6 amp diode, BANDED end, to connector J101 pins 1 or 2. The voltage should not drop when the diode is touched to connector J101 pins 1 or 2.

Testing a Diode (WPC-95)
The diodes that replaced the bridge rectifiers in WPC-95 are even easier to test. Again, testing diodes in curcuit may not give the following results.

Also, testing a diode is NOT conclusive to whether the dioide is bad! The diode is being tested under NO load. Only a diode which is shorted (and hence is blowing fuses) will test as "bad". A diode could test as "good", and still cause the game to reset.

1. Put the DMM on diode setting.
2. Put the black lead of the DMM on the banded lead of the diode.
3. Put the red lead of the DMM on the non-banded lead of the diode.
4. A reading between .4 and .6 volts should be indicated.

The Above Bridge/Diode Tests Don't Always Work!
Yes, you heard right. The above outlined bridge and diode tests above don't always find a faulty component. These devices can just start to fail, and this will cause the game to reset. But a bridge or diode can become "leaky", which will cause the game to reset, and may not show as bad in the above tests (though the bridge test "under load" as explained above is the most accurate of the tests).

So what do you do now? How can you be sure the resetting game has a bad bridge or diode? Well you really can't! First make sure the wall voltage is at the proper level. Then re-solder the bridge/diodes and their associated capacitor's solder pads. Then just go ahead and replace the suspected bad bridge/diode (BR2 or D7, D8, D9, D10 on WPC-95). If the game is still resetting, replace the associated filter capacitor (C5 or C9 on WPC-95). If the game is still resetting, replace the LM339 voltage comparitor at U6 (U1 on WPC-95) as a last resort.

Replacing a Bridge or Diode.
Replacement is as simple as cutting out the old component and soldering in a new one. When installing the new bridge, mount it 1/4" or even 1/2" above the board. This allows for air to flow underneath the bridge for better cooling.

Replacing BR1 and/or BR2 on WPC-S and Prior.
When replacing either (or both) bridges BR1 and BR2 on WPC-S and prior, both bridges will have to be dealt with. These two bridges share a single large silver heat sink. Since they both share the same heat sink (and one failed due to heat), the other probably will need replacement shortly. If either BR1 or BR2 is bad, replace them both. To remove them, both will need to be unsoldered from the Driver board, and the heat sink un-screwed from the bottom of each bridge. The new bridge is then screwed to the heat sink, and both bridges re-installed. There should be some white heat sink compound on the top of the bridges too. Make sure to add some heat sink compound when replacing the bridges. Heat sink compound can be purchased at Radio Shack.

Replacement Bridges and Diodes.
The stock bridge installed in WPC games is 35 amps at 200 volts. The original part number will be something like "MB3502W" or "MB352W". The "MB" signifies a metal cased bridge. The "35" signifies 35 amps. The "02" or "2" signifies 200 volts peak. The "W" at the end means the bridge has wire leads. Higher amps or voltage ratings work fine. I generally use 35 amps at 400 volts for example.

Replacement wire lead bridges are available from Competive Products Corp (800-562-7283), or from Williams, part number 5100-09690. Mouser also sells them, part number 625-GBPC3502W ($3.48). And so does Digikey, part number MB352WMS-ND. Radio Shack even sells 35 amp bridges at 50 volts (which isn't enough voltage). But look at the bridge inside the Radio Shack package, as often they are labeled 3502W or 352W (35 amps 200 volts), and not 50 volts. Always buy only the labeled bridges from Radio Shack. Sometimes these "35 amp" bridges are labeled 1001W (10 amp 100 volts!). Obviously put that one back and grab another!

Replacement diodes for WPC-95 boards are P600D (6A4 or 6A400), or NTE5814. A lower voltage version can be used too, 6A2 or 6A200 (200 volts). Radio Shack sells a 6 amp 50 volt (6A50) version which can be used in a pinch, part number 276-1661.

Testing the Filter Caps.
Testing the filter capacitors on the driver board is fairly easy. With the game on, set the DMM to AC volts. Then put the leads of the DMM across the two leads of each filter capacitor (doesn't matter which DMM lead to which capacitor lead, as AC voltage is being measured). If more than 0.30 volts AC is seen, the capacitor is bad.

The problem with this test is the leads for the filter caps are nearly impossible to access, when the board is installed in the game! For this reason, I usually just replace the filter cap in question (C5 or C9 on WPC95) when in doubt.

Replacement Filter Caps.
If replacing a filter capacitor, use a 15,000 mfd 25 volt "snap" cap (on any WPC generation, even WPC-95). Higher voltage caps can be used (but are more expensive). Do not use a capacitor greater than 15,000 mfd, because the in-rush current puts more stress on the rectifying bridge/diodes. A lower value of 10,000 or 12,000 mfd could also be used (but no lower than 10,000 mfd). These are available from many sources, such as Digikey (www.digikey.com or 800-344-4539) or Mouser (www.mouser.com or 800-346-6873):

• 15,000 mfd 25 volt, Mouser part# 5985-25V15000 $4.63 each, or Digikey part# P6577-ND $5.52 each.
• 12,000 mfd 25 volt, Mouser part# 5985-25V12000 $4.95 each, or Digikey part# P6575-ND $4.74 each.
• 10,000 mfd 25 volt, Mouser part# 5985-25V10000 $3.65 each, or Digikey part# P6573-ND $4.17 each.

Reflowing Bridge or Diode Solder Joints.
Often a bridge or diode will test Ok, but the game still resets. This can be caused by cold, fatigued, or cracked solder joints on a bridge. Since bridges (especially BR2) and diodes can get hot, they will mildly heat up a solder joint, and make it go "cold" or fatigued. Reflowing these solder joints with new solder often fixes this problem. Also reflow the solder joints on the bridge or diode's associated filter capacitor. Often these solder joints crack.

The problem with reflowing the solder joints on the bridges and capacitors is this; often the traces on the top side of the board (which can not be accessed because of the components), do not get as good solder contact. This can cause an intermittent connection, which can lead to game resets. The best solution to this problem is adding some jumper wires (see below).

Insurance: Installing Bridge/Capacitor Jumpers.
Another problem with the bridge rectifiers/diodes and the filter capacitors are their solder pads. The WPC driver board is a double sided board (that is, it has "traces" running on both sides of the board, both leading to different components). Soldering of both top and bottom traces is done on the bottom (solder side) of the board. Since the components themselves are in the way on the top side of the board, it is hard to even see the component side solder pads.

The problem is this; these components (bridges/capacitors) are large, and they can get hot (softening the solder). Vibration, heat, or both, can cause the solder points to crack. This can cause an intermittent connection. This can cause game resets, or whole banks of coils or lamps to not work.

Reflowing the solder on the back of the driver board is one solution. But it really isn't the ultimate solution. Since the driver board is a double sided board, and the components on the top side of the board are large, the traces can only be soldered on the bottom side of the board. This does not guarentee a good connection to the traces on the top (component) side of the board. To fix this problem, it is recommended to add jumper wires on the solder side of the driver board. This is done to back up the bridge/capacitors' component side board traces.

The most important bridge/capacitor to jumper is BR2 and C5. Jumper two 18 guage wires on the solder side of the driver board from BR2 to C5 (positive lead of BR2 to positive lead of C5, and negative lead of BR2 to negatvie lead of C5). This will help prevent random game resets. All the other bridges/capacitors can be jumpered too.

Installing the Jumpers.
When installing the jumpers, first label the back of the driver board. Use a "sharpie" pen and label the bridge, and its "+" and "-" leads, on the back side of the driver board. The positive lead of the bridge is the one offset lead in the square. The negative lead is diagonal the postive lead. The other two diagonal legs are the AC leads. Also label the capacitor and it's positive lead with a sharpie pen (the positive lead on most of the filter caps is the "top" lead). Double check all potential connections with a DMM, and buzz out the jumper paths BEFORE you install them (installing a jumper incorrectly can cause SERIOUS problems!). This will make installing the jumpers much easier and error-free.

Here is a list of jumper wires that should be added to WPC and WPC-S driver boards. For reference, the driver board is positioned with the solder side showing, and connector J104 at the "top":

• BR2 to C5: two jumpers. Jumper the positive lead of bridge BR2 to the positive lead of C5. Repeat for the negative leads also.
• BR1: ONE jumper. Jumper the AC lead of BR1 (just below the positive lead) to connector J101 pin 7.
• C6/C7: jumper the two positive leads of capacitors C6 and C7 together (this also jumpers also helps BR1).
• BR3: three jumpers. Jumper the lower AC lead of BR3 (just below the positive lead) to connector J104 pin 1. Jumper the other upper AC lead (to the left of the positive lead) to connector J104 pin 2. Jumper the positive lead of BR3 to the large solenoid fuse trace about 2" below the bridge (see picture below).
• BR4: three jumpers: Jumper the negative lead of BR4 to the negative lead of C11. Jumper the AC lead of BR4 (just above the negative lead) to connector J102 pin 1. Jumper the other lower AC lead of BR4 (just below the positive lead) to connector J104 pin 4.
• BR5 to C30: two jumpers: Jumper the positive lead of BR5 to the positive lead of C30. Repeat for the negative leads also.