Let's go over shielding once again

[rdhbass]

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.

[gapupten]

Your post leads me to think that you may just be experiencing a single coil hum in your J pickup. The P bass pickups are split coils and as a result their seperate magnetic fields cancel each others hum. If the J pickup is a single coil, it is more likely to hum. Does the hum get worse if you are near other electronic components, lights, etc? If so better cavity shielding may help.
Is there any way you can tell what type of pickups it has?

[rdhbass]

i dont know, it was humming when i switched it onto the J bass pickup and not the P, they are stock yamaha pickups from 1980's. It's wired like a stock BB1000MA bass except mine is 22 years older and neck through. I opened up the cavity and took the pot partially out to spray it with contact cleaner, put it back in, and it seems like it got worse. I have read things on star grounding and really all i have is a J pickup problem i guess Yes the J is Single Coil. There is black stuff sprayed in the cavity which i assume is stock shielding paint, but old.