Issue: Game resets when freeplay knocker fires. This does not happen always, but most of the time.

Solution: First, voltages for +5v and +12v where checked, and they are both rocksolid, even at reset time. Power Driver was swapped with a known good board, issue remained. CPU was swapped with a known good board, issue disappeared - so we know the issue is on the CPU board or with some intermittent connection. Swapped original board back, issue returned.

It seems the problem is vibration based - the knocker causes the components to vibrate a bit. Using a small piece of plastic, i started knocking slightly on components - it turns out that by slightly knocking U4 (the 6821 CPU) the reset would occur.

Taking the board to the workbench and removing U4 reveals corrosion that seems to be a remainder of an improperly cleaned battery leakage.

I desoldered the socket, and you can clearly see the corrosion on pins 4,5,6,7. The other pins dont look to good either


Other side of the same socket, no corrosion visible


Soldered in a new socket for U4, cleaned the pins of the 6821 and reinstalled in machine. Issue no longer appears. Another one done!