I'm pretty decent at troubleshooting the switch matrix but this one has me stumped. Working on a buddy's FBC and have it down to a few switches that won't register: Middle Pop Bumper, Top Pop Bumper and On Kicker. Switch test shows 4 (Top Bumper) stuck, I tested the switch / diode / capacitor with a meter and they test fine. I pulled the J2 connector and used a diode to verify that the stuck switch goes away and that all the switches above work properly when simulated with a diode on J2 so I believe the problem has to be on the Playfield. I unsoldered both wires going to the top pop bumper and the game still shows 4 as being stuck, and so am I! The problem doesn't form your standard square or rectangle on the matrix and all the voltages at the switch (from strobes) appear to be good, I also verified wiring from switches to the connector, headers have been replaced (not by me) and look really good including the solder joints under magnification. Suggestions for next steps to take please!

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I just found a couple old posts about weird behavior from the start button and other coin-door switches. The previous owner wired up a credit switch that is adding like 14 at a time and now I'm suspecting that. Thanks for the response!

No change with J3 (coin door switches) removed from the MPU. I'm starting to wonder if the PIA isn't just a little flakey and works with only one return when right on the connector but malfunctions when the whole connector is attached along with the caps, wiring, etc. Otherwise I'm at a loss.

Yes, middle pop, sorry that's a typo on my part. I drank a bit of bourbon after working on this thing and getting frustrated!

I tested to make sure it's not shorted then just completely unsoldered both wires from the switch so it can't be the cap or the diode.

Yes, all balls out since I have the playfield up and yes the 2nd trough switch in on the same row - good question.

I believe that only the switches that are on coils (pops & slings) have the capacitors on them, pretty sure the rebounds, rollovers, etc. don't have them but I'm not near the machine now to look. Can have the owner check it later though.

Finally got back to my buddy's house to work on this and the short story is there are three shorted (though they test fine with a meter) caps on the middle pop, top pop and the on kick switches. The mistake I made was I trusted the diagram in the owners manual that shows switch 04 (middle bumper) is the top one and 05 (top) is the middle. I took the time to trace out the switch matrix and start cutting caps relative to that versus the parts diagram and the stuck 4 moved to 5 and finally to 20 (the on kicker). Cutting the caps off each one and then testing with a good cap resolved it. Thank you Quench and everyone here and in the Bally /Stern SS club that helped with input, I appreciate it. I shouldn't have trusted that switch location diagram as it really didn't make sense.