Hi. I just bought a 1980's bb series yamaha bass and it has the P-J setup. It has the cavities lined with a black shielding paint and it "hums" when I switch both pickups to "on". When I just put it on the P bass pickup, it is dead quiet. Is star grounding simply getting off all grounds off the pots and putting them all to one ground? I don't want to mess with the stock wiring all that much but if all it takes is putting all ground wires to single place i can do this. Do i need a new switch? Please help me.