I just completed a playfield transfer for a Bally S&S. I noticed that many of the switches had the cap clipped, so I resoldered them.

When I fired it up, I had four stuck closed switches: 17, 21, 30 & 39.
I clipped those caps and the problem went away. I put in new 0.047 disc caps and the problem reoccurred in 17, 21 & 30. When I removed the new caps, the switches opened. Switch 39 is no longer an issue.

I know that ops were told to clip those caps, but I have never had a problem with phantom switch closures during game play in other early Bally SS pins. But I have had issues with targets not responding to ball hits during play without these caps, so I would like to install them.

SW 17 Row 1 Col 3 (roll over pin 1)
SW 21 Row 5 Col 3 (roll over pin 5)
SW 30 Row 2 Col 1 ("A" of SPARE target)

I do not see any commonality here.

I do not understand why there is an issue with these switches only when the cap is in place? That cap should not close a switch, unless it is shorted.

I put the pin into the switch test, then unplugged J3, which feeds the cabinet and coin door switches. Hooking up a cap across SW 17 still causes it to register. So it is not a short on the coin door.

I turned the power off, disconnected J2 (playfield switch matrix) and connected a cap from pin 8 to 3 (SW 17). Turned it on and set it to switch test and SW 17 shows as closed.

I unplugged J3 (cabinet switches) and SW 17 stayed closed.

So it has to be in the MPU board? What the heck am I looking for? This problem is on on these three switches, 17, 21, 30.

Yes, I checked the DC resistance of the cap. What the heck is going on?

I believe that the only reason a diode is needed is because there is a switch matrix. With J2, J3 removed and all the dip switches turned off, there is no switch matrix. A cap, which is not supposed to let DC flow, should not cause the switch to trip.

But to test your theory, I set the pin to switch test, removed J2 and J3, turned all dip switches off, then put the diode in series with the cap, and SW 17 then activated. So the presence of the diode did not make any difference.

And it cannot be an issue with the diodes in the switch matrix since they are all disconnected when J2 and J3 are removed from the MPU.

The only time a shorted diode would be an issue is when a switch is closed. It would then cause false readings - other switches would appear as closed. A cap across a switch should not activate it, diode or not.

"Do those switches show as active when you disconnect J2 off the MPU board (with no caps on J2)?"

I do not understand this question. The only time SW17, 21 & 30 show active is when the caps are in place. When I remove the caps, SW17, 21 & 30 are not active. And if I disconnect J2, the playfield portion of the switch matrix is disconnected, so no, none of the switches are active then either, whether the caps are in place or not.

Thanks for the note about the diode. I was wondering if the cap charging and discharging would set off the switch matrix. That is why I tried it with the diode, and had the same result as without it.

Yes, the diode was in the right direction. To confirm, I tried the diode alone.
If the diode was in the wrong direction, I expect that the cap would never get charged. And the switch would not register.

Everything was connected via small clip leads. No fingers were involved.

17, 21 & 30 work just fine without the caps - except that they are not as responsive to a quick hit - as one would expect. They close when pushed and open when released.

All other switches have the original caps and have been resoldered and have caps in place. Not all were cut in the first place.

My new caps work just fine on the other switches, but they close SW 17, 21 & 30 when placed across the switch. Both the new caps and the removed caps set off those switches. I have used those caps on other pins, such as EB and EBD to replace defective or missing caps.

All switches register as they should when manually pressed during the switch test.

I am still digging.

I am close to saying 'what the heck'. It plays fine. Pin #1 is tougher to get. I can live with that and move onto my next project. Not sure I want to go to the problem of inserting another board.