[R-390] Why is it so ?

[Larry H]

Hi Pete,  Sure looks like you have an agc problem.  As you might guess, there are numerous problems that can cause these symptoms, but 2 would be more common:
1. Tubes: v508, v509, and v506.2. C551 can get resistance to gnd, c547 and c548 can become leaky.
I suggest replacing these 3 tubes, first.  If that's no help, measure resistance to gnd on v506 pin 2 with large connector disconnected or agc link on term 3 and 4 removed and power off.  It should be 500k.  If less, 1 of those 3 caps in 2 above may be bad.  If =, they may be ok.  Put it back together.  If you have a tube socket test adapter, plug it into v509 and power on and look for -15 v on pin 1 or 2 on a strong station.  If you have it or close, 1 of the 3 caps or ??? is loading the agc down.  If you have low - v on pin 1 or 2, you will need to find the loss of signal after the 3rd IF.
Regards, Larry

      From: Pete Williams <jupete at internode.on.net>
 To: r-390 at mailman.qth.net 
 Sent: Wednesday, August 3, 2016 12:33 AM
 Subject: [R-390] Why is it so ?
   
​IF  Module  R-390A  prob.
 Preliminary  observations  herewith.
A  supposedly  reconditioned  Module .
Very  distorted  audio;
1.. Diode load  measures -60 (sixty) V  on local  BC  station
2..Tune off- no  signal  diode  load shows -16  V.
3..AGC  terminal measures -3  V  on signal  as  above .
4.. AGC  terminal measures -0.4V no  signal.
Selecting  all  AGC  switch. slow/med/fast  positions  no  change.
RF  gain  at  maximum  and  in  AGC  mode.
Limiter  switch on-  minor  variation  in any of  above conditions.
 No  in depth  forensic  examination as  yet, but  someone  may have been
this  route  before  and  have  heads up  diagnosis for me !
Radio operates  OK on a  different  module. It all seems  rather  odd
 Cheers
Pete VK3IZ
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