Fender Rumble 500 Power Supply Issue

[g1]

First I would try disconnecting the jfet Q3. Either of the legs connected to R42.

[nevetslab]

I don’t recall their pot details. On larger dia pots that have the mechanical terminals riveted to the resistance wafer, I have had to generally tap the rivet between a suitable center punch pin mounted in a vice and further flatten the other side of the terminal revit to restore solid mounting and restoring conductivity.