I just bought a used 1985 Electrum with 3 single coils and it seems to tune in a radio signal through the amp (I hear music playing through the amp when I am not playing, it decreases when I turn down the tone knob)? I have tried it several times, and with different amps. If I grab another guitar and plug into the same chord, no music. Any ideas? Thanks,
Scott

Check and see if there is a large ferrite bead the pickup wires pass through. It should look something like if not exactly like this:

Both pickup wires should pass through at least once. If you don't have this, email me (click on mica in the left column) with your name and address and I will send one to you.