Quoted from songofsixpence:A little disappointed that it didn't boot up. I've checked the power output voltages, and the circuit that powers the MPU seems a bit low. It's supposed to read 12V, but I'm getting a reading of around 9V. I don't know if this is problem or not, but I figure rebuilding the PS board is where to start.
Any Classic Bally or Stern needs ALL the connectors rebuilt if you want bulletproof operation.
Measure your 12v at the PS, then at your board - see how the voltage has dropped?
Male pins can be tarnished, or have cracked solder joints. Female connectors can be broken, corroded, or fatigued.
https://pinside.com/pinball/forum/topic/vids-guide-quick-bally-driver-board-repair-bulletproofing