[RodB]

Silver migration disease didn't occur until the late 40s and only in one brand of transformer, the miniature cans. That's not to say it won't be anywhere else, but it's not worth chasing a problem that isn't likely there. Motor boating is a feedback problem, SMD is high voltage leaking through the mica caps causing continuous crackling noise.

Quite often motorboating is caused by something in the AVC circuit.