I've scanned the forum for threads on System 6 sound problems and particularly Blackout which is the machine I'm having trouble with, but I haven't quite found a similar scenario: the background sound is intermittent or mostly not there. It might start a game with it, then it drops out on the second ball, or it starts a game with no background then it comes in when hitting a spinner. Other sounds are fine. Very weird.

Pushing the sound self-test results in what appears to be a success (a bunch of sound effects and all the words cycling). The solenoid test runs through some sounds too. Grounding the sound selection pins also results in sounds at each pin.

I understand that bulletproofing these boards should always be step number one, but I have an extra set of boards that have indeed been cleaned up that I have been swapping with to do testing. I have to swap chips of course, and even have a new set of sound ROMs. But the problem persists. I do not have a new game ROM though, would that be the culprit here?

Hoping someone has had a similar experience and can advise, thanks!

The power supply is something I have not yet tried swapping (I have a reconditioned one to try), will get to that next. The MPU/driver interconnect seems sound to me, on both my original and reconditioned boards, but will keep an eye on that, thanks.

Cool, looks like great info. I'm still stuck and will try this out, thanks.