Mr. Mrs. Pacman pinball S&T quiet voices

shardian

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A friend of mine has a pacman pinball with an odd sound issue. The general sound works great, but the voices are super quiet with the pot turned all the way up. All 3 pots have been checked with a multimeter, and a cap kit has been done.

The only thing that seems out of the ordinary, is that the 3 identical U chip PIA's, one of them is a different part number. He swapped them around, and the voices went away.

Sorry I don't have technical details at the moment - just going off of memory while I was over at his place troubleshooting last night.

Personally, I think it seems like a sound amp is bad, but I don't know where the suspected sound amp is one the board. Is it U16? Is it one of those PIA chips?
 
Here's the schematic:

http://www.pinrepair.com/bally/squalkandtalk.pdf

The only thing that seems out of the ordinary, is that the 3 identical U chip PIA's, one of them is a different part number. He swapped them around, and the voices went away.

There are only 2 PIAs on that board. The third 40 pin chip is the processor (6802 or 6808 with 6810 RAM at U6).

I'm not really sure what would happen swapping a PIA with a processor. I wouldn't be surprised to see both chips survive it or both chips be destroyed. Best to look at the schematic before swapping stuff.

Personally, I think it seems like a sound amp is bad, but I don't know where the suspected sound amp is one the board. Is it U16? Is it one of those PIA chips?

PIAs are basically just a bunch of I/O pins that interface with the CPU. No amplification there. I would suspect U14 (dual op-amp for speech and sound) first but there are a bunch of discrete components (like caps) in the circuit that you could look at as well.

There's some good info here:
http://www.pinrepair.com/bally/index3.htm#sound

EDIT: U16 is a bunch of inverters that invert the data lines and sound interrupt coming from the MPU. There may be another inverter in that IC used somewhere else in the circuit but I didn't really look too closely at it. It can be ruled out as the cause of this problem. I would try U14 first and go from there but keep in mind it could just as easily be a cap.

U7 is the speech PIA (6820 or 6821). You could try swapping it with U11 but I wouldn't expect the PIA to cause a volume problem.
 
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