I finally solved a problem with sheilding my Fender Jazz bass and I wanted to share....
A couple of years ago, I did what I thought was a first rate job at shielding the cavities of one of my Jazzes. I used copper foil and soldered it together and tested continuity and grounded it. Just like your supposed to.
Well, it just hummed from any interference in a room. In fact it was almost unplayable in certain rooms. I couldn't figure it out.
Long story short, I had shielded the inside of the Neck pickup cavity with copper foil and connected it to ground and the rest of the foil in the control cavity.
Today I ripped the shielding out of the pickup cavity and left the control cavity as it was and bingo! No hum. I still have the single coil problem when both p/us are not full on, but THAT I can live with.
As near as I can figure, the p/u cavity shielding was somehow collecting and 'focusing' the noise and directing it to the p/u. Does this make any sense?????
All I know is that it worked and it's like I have a new bass!
Shielding Revelations 
