Bally Centaur playfield switch problem

Doughbroz

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Original 1981 model. Intermittent problem, sometimes works ok for hours, sometimes can not get it to stop scoring by itself. Shows as stuck switch 12, which is a daisy chain of one rollover button and three standing targets. It's not a stuck switch, because if I hold any one of the four switches closed it stops scoring, so I know the switches are ok. When I release whichever switch I am holding closed it may take anywhere from five to thirty seconds before it starts scoring again. It may score anywhere from three to twelve times, then will stop for a few seconds and show zero in the four digit display, indicating the short, wherever it is, has opened. A few seconds later it starts scoring again. I can't see a short of any kind and it's not a bad connection on the MPU board. When it's working ok, pounding on the playfield has no effect, so it's not a bare wire or tab bent out of shape and shorting out. I suspect a bad diode on one of the switches, but they are so damn hard to get at, so I am hoping someone might have an idea if there is a way to narrow down the problem to whether it's in the playfield or on the board. I do have complete schematics, the circuit in question enters the MPU board at pin 11 on connector A4J2. Any help appreciated. Thanks
 
I was going to ask if you checked the diodes on the switches. Manytimes the diode isn't bad and just came lose from the switch. Got to start somewhere. :). The only other thought is truly you have a ground short on a board. I would also check all of the grounds on the boards as well. I do not have a centaur, so I am unfamiliar with the inners. I have a Bally from 1984 which is close. I will have to look and see if it's similiar. Centaur is One of my favorite machines growing up and would love to find one.
 
Thanks for the reply. I am hoping to narrow it down to a playfield or board problem, as those four switches are very hard to get at without removing entire target assemblies or removing the playfield entirely, which is is darn near impossible to do by myself. Getting a soldering iron on the diodes is out of the question as it sits.
 
Found the problem, in case it helps anyone else. Damn .05 capacitor on one of the switches shorted when it got warm. At least it was on the only one of the four switches that was reasonably possible to get at. Also wondering why it was the only one of the four with a cap on it. Didn't have a substitute lying around and it works fine without one, so I'm good. Spent hours checking and swapping parts on the CPU because the problem usually went away when they were chilled. I guess when the cap shorted the load was too much for those CPU resistors and diodes? I dunno, just glad the damn thing is fixed.
 
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