I recently got a call to look at a Williams Twilight Zone that was resetting as soon as the flippers were flipped. There is a great guide on fixing WPC reset on the Pinwiki website. My initial guess without even looking at the game was the Z-Connector; however, when I got to the game this was not the problem.
I took voltage measurements at TP2, which was 4.90V. This is too low and would cause resets. I measured the voltage on the CPU board and it was 4.86V. This proved to me that it was not a connector issue, but a board problem. I also noticed that the battery holder was starting to have some corrosion on the terminals.
Removing the driver board and CPU board out of the machine, I had to do some board work on both. I checked both BR2 and C5 and both tested fine. I went ahead and replaced C31 and the other 100uf, 10V capacitor on the driver board. The voltage regulator had some cold solder joints on it, and I reflowed the two points. The last item I did on the driver board was add some jumper wires from the BR2 to C5.
I removed the old battery block and added a remote battery pack for the game. These are nice because if the batteries leak, they are off the board and will only destroy the battery pack. A NVRAM is usually not the best for WPC games that have the Midnight Madness Mode as the battery needs to store the clock information. Even thought TZ doesn’t have a Midnight Madness Mode, the playfield clock needs the battery to keep the correct time.
I went ahead and plugged in both boards and now the voltage on TP2 was 5.0V and the voltage on the CPU board was 4.96. The voltage should be high enough to prevent resets. If the problem shows back up, my next step would be to replace the Z-connector.