Can't really make out what's going on from the photo. Are you saying the insulator had broken somewhere and two metal parts were then touching? Unsure of your tip and would appreciate clarification as I'm sure I'll encounter it in the future
I had included the image just to show what I meant by the brown insulators. You wouldn't notice anything unusual just looking at it, but as Ballywannabe above said, it probably is a combination of subtle crap and moisture.
Whilst tearing my hair out trying to work out what was going on with the switch matrix, I narrowed it down that it had to be something about the switch assembly itself. One was the standup target, as pictured, the other was a thumper bumper switch. I was able to narrow it down by creating a dummy/separate J2 connector for the MPU, and wiring just two leads into that connector - one for the strobe and one for the return for the associated switch. I pulled out the regular J2 connector from the MPU, and plugged in the test one. I then unsoldered the switch assembly from the cabinet wiring, and connected the problematic switch to the test setup. As the problem still occurred under this situation, I was confident it came down to the switch assembly itself (and not the playfield wiring or other switch issue).
With the machine in the test setup for stuck switches (ie. after pressing the red test button inside the coin door), I noticed that as I loosened the two screws which hold the switch stack together, the switch became unstuck (as noted by the digit '0' in the credit display. Tightening the two screws caused the switch to become stuck again. I was thinking to myself, what on earth is going on? It took a little while, but I suspected something not right with the insulators between the metal in the switch stack. Sure enough, changing the single insulator between the two diode connections solved the problem. Now, tightening the two screws in the switch stack to fully tight had no issues with the switch being stuck anymore. And this was the situation in both switches.
As mentioned, maybe the cold weather/humidity has something to do with it. But it seems strange that it only affected these two switches, and not the umpteen others in the machine. I think it comes down to the properties of the insulator. Maybe over time, with crap build up etc combined with a bit of humidity causes problems. Anyway, first time I have ever seen this happen.
