I’m chasing ghosts on a Surf n Safari that I’m working on for a friend. Upon power up, I do not get the two beep squelch from the sound board. Instead, I get a continuous hum/squeal that sounds absolutely awful. Here’s what I’ve done:

I’m getting a solid +5v to the board, as well as good +12/-12. This continuous noise still occurs whenever the main sound board is unplugged completely, but the aux sound board is still plugged in. The sound goes away when the aux sound board is unplugged (as to be expected since this board powers the main sound board). Pushing the reset buttons does nothing, and I get no audible sounds or music in game.

This is what I’ve done so far:

Replaced the TDAs on the aux sound board
Replaced the bridge rectifier on aux sound board
Replaced the caps on the aux sound board
Replaced the voltage regulators responsible for +5v, +12v, -12v

Really feel like I’m just going in circles now. Any help is greatly appreciated.