Jackbot has been plagued by an annoyance, where it seems to be confused about whenever all the balls are in the ball trough. When not on the playfield, the balls sit in there waiting for their turn and their presence is detected by mechanical, or in this case optical switches. This is a 4 ball game so there's 5 optos, the extra is for detecting a ball jam situation where two balls are on top of each other and the game software needs to deal with that.

I observed that jam detection opto blink in the switch matrix test, so I pulled the whole ball trough out to have a looksie. I did however not expect to find a 3mm LED in there! It has been like that the whole 23 years I've owned the game and has now gone bad.

I replaced both boards by an updated version that has activity LEDs for all the optos to quickly diagnose if we're having a ball trough or other issues should the game hallucinate about what's going on in the the trough.