[R-390] AGC problem

[James Miller]

I read your original post again, where you said "this is not the way others
work..."  I have one 390a (a SW mfgr) that handles SSB pretty well at high
gain.  I have a Collins 390a that doesn't do well at high gain, on SSB, but
works well otherwise.  But I think the 390a that handles SSB without
distortion is not the norm.  Your receiver may be just fine and working
normally, since you say it is OK when the RF gain is reduced.  There are
some AGC mods on the web that can help, one simple one uses a couple of
diodes in the IF deck.  But in general I think that a 390a that distorts on
SSB at high gain is normal.  Just my opinion..


----- Original Message ----- 
From: "Bill Abate" <wabate at dandy.net>
To: <r-390 at mailman.qth.net>
Sent: Friday, December 10, 2004 12:31 AM
Subject: [R-390] AGC problem


> My 390A is almost finished its restoration.  Looks pretty good and works
> well.  Well, except for SSB reception.  If I try to tune in a ssb signal
> I get a lot of distortion in the audio and the signal can't be tuned in
> properly.  If I try the old method of max audio gain and reduced RF
> gain, all is well.  Of course this is not the way other R-390A's work.
> BTW, AM reception is pretty good but has some distortion in the audio.
>
> I figured that the AGC was not working properly so I ran the AGC test in
> the manual.  I adjusted the ,local audio for 1.7V of audio with 5
> microvolts of RF.  I then increased the RF sig gen to 1000 microvolts
> and the audio output went to 20 volts.  Its supposed to increase to 3
> volts, so I found the problem.
>
> The problem is I can't seem to find the cause.  I have AGC voltage on
> pin 2 of v506 (AGC line).  I got these AGC voltages with my sig gen:
>
> 5 mv    ->     .04VDC
> 50mv    ->     -.2 VDC
> 500mv   ->    -3.8 VDC
> 5000mv  ->    -7.1 VDC
> 50000mv ->    -9.6 VDC
>
> I also found the AGC voltage on all the tube grids it was supposed to
> control.  The only thing I can conclude is that the AGC voltage is
> insufficient but the manual does not say what the voltage should be with
> varying inputs.  At least I could not find it.  I also tried slow, med
> and fast AGC and the results were the same.
>
> Anybody have data on the AGC line?  Any ideas as to the cause??
>
> Thanks,
>
> Bill, K3PGB
>
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