[R-390] AGC problems

[Michael Melland]

Your problem sounds very much like the one that plagued my 
R-390A.  The cause of my problem was eventually found to 
be an intermitant short in the 4 Kc filter.  

My 390A would start out with good audio and agc but when 
it warmed up the audio would distort until I turned down 
the rf gain... obviously no agc action was present.  I 
also found the agc voltage to swing way low when this 
occurred.  Sometimes after some minutes audio would 
improve and agc would return to normal.  It was discovered 
that when the filter heated up as the radio was on it's 
internal components would expand/contract and short to the 
case of the filter grounding the agc voltage.

Since all the filteres are, I believe in this circuit, it 
could be any filter that is bad.  The usual suspect seems 
to be the 4 Kc filter as I'm told it was generally the 
most fatigued due to use.

Mike, W9WIS



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Michael Melland, W9WIS
Winneconne, WI USA