« Reply #2 on: November 07, 2019, 04:15:05 AM »
Those 75 customs have a elaborate shielding system in order to eliminate the bridge ground. Normally the strings are grounded through the bridge (there is a wire going from underneath the bridge post to a ground connection in the wiring cavity). The pots are screwed to a metal plate, and a can covers the electronics. This system is like a Faraday's cage and very effective BUT it also requires that the pickups have metal covers (they are grounded by being soldered to the pickup base plate which is also connected to a ground connection).
Do you use uncovered pickups? If yes, then the Faraday cage is open, and you get hum that does not reduce when you touch the strings.
How do I know? I had one of these beasts in the 80s and faced this exact problem. I removed the covers to lessen the microphonic feedback - only to get more hum in the system.
So if that and microphonic feedback are the problems the steps to solve them would be
1) pot the pickups
2a) put the covers back on or
2b) install a bridge ground wire
Good luck
Stephan
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