This same fix applies to Bally -32/-50 sound boards. Cracked header pins cause ground isolation / looping issues. The cheater fix sometimes is to insulate the grounds from backbox mounting or. Best fix is to figure out the ground issue. I think this happens on the early bally sound boards because the solenoid voltage (for the amps) and the logic voltage have isolated grounds... somewhat.

Since the noise is often coming from the feature lamps switching, i wonder if the 18v get cleaned up more on the mpu would help. You got the big honking 30000uf cap in the head but not much filtering on the mpu, maybe 100uf, inductor, and ceramic cap.

Not to pump my bg sound board, but I added more -12v filtering to the replacement i make and it cleaned up more general all the time hum my high speed had.