Fixing a dead Williams System 11 logic board

A customer bought a William’s Alley Cats shuffle alley about 3 years ago and one day it wouldn’t power up. The only thing that happened was the GI and the pin section lights came on. At least we know the game is getting power so its not a dead outlet, power switch or primary fuse issue. The GI and pin lights aren’t controlled by any game logic, they will always come on when the game is powered up. Later games such as Williams WPC won’t have any lights come on until the logic board is running. All lights on those games are controlled by the logic board and will not come on by default.

Whenever you have a dead logic board the first thing to check is the RESET circuit. The RESET circuit is a time delay that holds the processor in RESET for a few hundred milliseconds or so to give the +5 volts time to stabilize. If the processor is stuck in RESET then you won’t get any signs of life at all from the board.

Some games, depending on the system, you may hear some tones or power up music. That’s because the sound board is running independently of the rest of the game. It will do its power up routine then wait for instructions from the game’s main logic board. On this particular system the sound is part of the main board and its processor is tied to the same RESET line. Even though it doesn’t execute any power up music or tones, you still wouldn’t be able to test the sound section by pressing the sound test button on the board.

Williams system 11 logic board reset circuit
Notice how ‘RESET’ has a bar over it.

In the photo above you see how RESET has a bar over it. This means that RESET is active low. The status of RESET can be checked by the test point noted on the board or you can probe pin 40 of the 6802 processor. A running board will show HIGH with a logic probe or +/- 5.00 volts with a meter. A processor held in RESET will show a low signal with your logic probe or +/- 1.00 volt. Since my logic probe died and I haven’t gotten a new one yet I noticed I was only seeing about 1 volt on the RESET line.

One thing to mention is that the RESET circuit is next to the batteries and usually a victim of battery corrosion. This was the case here and someone had already replaced a bunch of the RESET transistors. I rechecked them and they all checked out fine. The next culprits to check are the two Zener diodes. ZR-1 is a 6.8v and ZR-2 is a 3.9v Zener.

A diode is a device that will only allow current to flow in one direction, and block it in the other. I include a diode on my battery board in case the one on the game board fails. It allows the battery to power the memory when the game is turned off but prevents the game from trying to charge the battery when it is turned on. The photo below shows a schematic of the RESET circuit.

Williams pinball ball and shuffle alley System 11 RESET circuit schematic.
Williams System 11 RESET circuit schematic.

A Zener diode will allow current to flow in one direction and block it in the other until its Zener breakdown voltage is exceeded. All this means is that for example a 5 volt Zener diode will block a reverse voltage until it exceeds 5 volts. So if you apply 12 volts in reverse to a Zener diode you will get 7 volts out of the other side. So from this example you can see that Zener diodes can be used as voltage regulators. If you look at the schematic you can see that the RESET circuit is looking for the +12v input, if its missing the game will not come out of RESET. ZR2 is a 6.8v Zener so it steps the 12v down to approximately 5.2v since the circuit doesn’t need 12v to operate, this is simply how the game detects is presence.

In this case both Zeners were fine. Sometimes you have to remove them from the circuit for testing to get accurate results. I have a tester that will test and identify them and they both tested within spec. I have had faulty Zeners in the past so make this the next thing you check.

In this case the problem ended up being two 1/4 watt resistors were reading about 6 meg ohms each, which is rare for 1/4 watt resistors to fail in this way, especially two of them. Unless they get hot during normal operation, which in this case these do not. Techs sometimes overlook them. I’ve found many boards where repairs had been attempted, every chip or transistor replaced and the board still not working. The whole time the problem being a bad Zener diode or 1/4 watt resistor. R53 and R57 were both going open, which is what resistors do when they fail, they never go short.

Williams System 11 logic board reset and battery section
With the two faulty 1/4 watt resistors replace the board boots up fine.

When ever you have a problem that has you scratching your head, or you are trying to fix it after someone else and all suspected components have been replaced, start looking at your Zeners and your resistors. Remove them from the board if you have to to get an accurate reading. Frequently they can be the cause of failure.

Readers Comments (2)

  1. Thank you for taking the time to share!

  2. Charlie Shelden March 2, 2020 @ 12:18 am

    Great post up Frank, appreciate the blog!

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