While we're on the subject of hums..... on my System 11's I've noticed two distinct and different hums. One was more of a higher-pitched electronic "whine" in my High Speed, that grew louder/softer in relation to whatever was lighting up on the playfield. I tightened every screw holding the boards and that hum/whine was reduced to a very acceptable level, so problem solved.
The other, on Pinbot, is a hum that sounds something like an electric razor. It stays at the same volume during attract mode and gameplay. I've tightened all boards' screws and it didn't change the hum one bit. I had picked up a sound board cap kit but don't want to molest what looks like a brand new board unless necessary, since I don't know what the cause might be. Would I be better off capping the sound board or the power supply first? I see where owners have recapped boards and nothing changed. What's my next move here?