Decided to play some seldom used tube radios to listen to some Christmas music from local AM stations. Well, one radio developed a problem. Worked fine a month or two ago, but now I could hear some thunderstorming, and low sensitivity. Okay, to the shop, the AVC line seems stuck at positive half a volt, no matter the signal strength. I did see voltage variation right at the detector diode. After a few tests, I disconnected the 1st IF transformer's secondary from the IF tube and the AVC line, and measured any voltage I might find with a DVM. Saw around +30V, hummm, seems there is leakage from the primary side. Probably silver mica migration disease. Took the IF transformer out of the radio, and removed the mica wafer. Reinstalled the transformer, and added a pair of 100pF silver mica caps to complete the LC circuits again. Problem gone. And as I removed the C from the LC circuits and replaced those with new C's, I tuned up the IF transformer. Radio also more sensitive now, like it should be for an AA5.
12-25-2014, 03:40 PM
Threaded Mode