My guess would be if I'm using the correct term is ( Electrolysis), I had the same problem with two 311's. The first time I noticed this I thought someone had put a piece of cork in the rf connection on the of the receiver, what it was is the copper wire had like melted to a flat wafer. I unplugged the receiver than with a small pointed tool took the copper out,cut the wire back about an inch and put a new fitting on,this happened about every six months until I had a service call and he finally grounded the antenna wire. I think the two metals reacted and or moisture plus current.
It's been almost two years now and no problem. I hope!!