Wisconsin Pinball

So I'm having a switch issue. When I have a ball in the ball lock and its waiting for me to hit the upper saucer 3 times to release the the multi ball, if the ball hits the lower pop bumper, the ball is released from the ball lock and the pinball machine thinks there is only one ball in play and not 2, which really messes up the game when it drains, thinking I've lost a ball when I still have a ball in play. I have replaced all diodes in the line that I have suspected of being the issue, and when I do a switch test, everything checks out fine. I also swapped out U11 and U10 chips to see if the issue changed. After that, the issue was still there. Could it be the MPU? How can I begin to figure that out?

I'd suggesting looking at the ceramic capacitors that are soldered across some of the switches. For fast acting switches, like pop bumpers, the capacitor makes the switch better at reading quick switch hits and sometimes they start to fail and cause erratic behavior. Start by just clipping one lead off one switch and start a game to see if it made an impact. If no change, re-solder and move to the next switch.

On old bally games. I have found the flipper caps to also be a culprit.

Rebuild the flips if the switch trick Eric mentions does not work

ok, I will give these a try. Thank You

I tried what you guys said and It still did the same thing. I'm thinking there might be something wrong with the MPU.

It is an odd problem, so definitely could be an MPU issue. If memory serves me correctly, I don't think there is a lot of circuitry on the MPU for the switch matrix. Check for any acid damage around the resistors/capacitors. The PIA chips are known to get flakey as they age - I think U10 or U11 handles the switch matrix. Also, flakey IC sockets could be a culprit too.

Found the issue. Found a weak spot on one of the wires that was still insulated but felt like it was hanging on one strand. Repaired the wire and everything was good. It's just to bad I bought a Alltek board thinking it was the board issue, but with out it I probably wouldn't of found the issue.