I have this exact problem with my Bad Cats machine. Background sound doesn't come back until the start of the next ball. When it drops out I don't hear a bong though, but I've suspected that it may be losing power or something. I'm going to replace all connectors in and out of the power supply here real soon too. I will check the +/12 voltages as well.
This is what I've done so far (for various reasons): I've replaced the MPU with Rottendog (until DumbAss board is ready) and put in new 6802 processors, replaced original sound board with Kohout sound board, replaced power supply with Kohout one, replaced audio ribbon cable between MPU and sound board, replaced all connectors in and out of the sound board, has new speakers, has all new ROMs in MPU and sound board. I programmed the ones for the MPU; the new sound board from Kohout came with the ROMs.
Quoted from slochar:
Had same problem on pinbot board.... it was the eproms. They verified good, but I burned a set anyway after doing much of what you've been doing, and that fixed it. As a test a couple weeks later swapped the suspect set back in (to make sure it wasn't something goofy with the IC sockets) and sure enough, issue came back.
Which ROMs did you replace?
Edit: Getting +12 and ~ -13.3 or -13.4. Varies a bit. Cause for concern? Could sound board reset if voltage is too low, I guess in this case since it’s a negative voltage?
Edit 2: Burned a whole new set of ROMs (I didn't mess with the ones that came with the sound board, yet anyways) and switched the processors around and still am getting the same results; background sounds drop out randomly and then come back at the start of the next ball, or in the case of the last time, it was on Ball 3 and came back at the start of the next game. I did check the ROMs on the sound board and they did verify Ok. Any other ideas?