Quoted from ChipS:I heard a buzzing sound coming from the backbox - as if a relay was stuck and vibrating.
This would be a good place to start. The scoring (10pt, 100pt and 1000pt) relays are in the backbox. If you have a stuck playfield switch (like a sling shot or target for example) one of the scoring relays could be stuck on.
Quoted from ChipS:With just the bottom board and backbox connected (through jones plugs) I turned on the game. ... Once it finished, I walked around to the backbox but there wasn't any hum. So I'm assuming I'll need to replace the playfield and try it again in order to hear it.
That would also be consistent with a stuck playfleld switch.
Once your sort out why a back box relay is stuck on, then we can try to figure out the short.
/Mark
Forest Hill, MD
