[R-390] 17 MHz osc pull from AGC

[Jim M.]

I have noted the same thing.  I put a counter on each oscillator and ran 
some tests looking at the frequencies as AGC voltage changed.  It is indeed 
the 17 osc.  Trying different tubes in the mixer that it drives seemed to 
improve things.  My guess is that as the AGC to the first mixer grid (V202) 
changes, it changes some operating condition or impedance that the 17 osc 
looks into through T207, and that pulls the osc. slightly.  There is also a 
variation in B+ to the osc. as the AGC changes current draw by the other 
tubes.  The B+ to the 17 osc. is not regulated.  That could be causing some 
pulling.  I thnk both the osc. and 1st mixer tube operate from the same B+ 
line, maybe there is some interaction there.  It does this more on my 
Collins than on my SW radio.  Could be some aged resistors or caps, ... 
don't know.  I have been thinking of going in to check values but too lazy.
----- Original Message ----- 
From: "Conard Murray" <cmurray at tntech.edu>
To: <R-390 at mailman.qth.net>
Sent: Friday, December 09, 2005 11:17 AM
Subject: [R-390] 17 MHz osc pull from AGC


> Hey gang,
> I'm working on a '62 Amelco and I have just noticed the 17 Mhz xtal
> oscillator in the first mixer is pulling just under a hundred cps or so
> in step with the AGC voltage. The stronger the signal, the further it
> shifts. CW on the bottom 8 bands is chirpy when using the AGC. MVC is
> fine. Signals above 9 MHz are fine.
> Replacing the oscillator tube and mixer tube didn't make things better
> or worse. The 150 volt line is dropping about a volt between AVC and MVC
> mode with a strong signal applied. I don't think that is enough to cause
> this problem .... is it?
> I have had other lesser receivers with combined mixer/oscillator stages
> chirp, but I refuse to believe the '390A is supposed to act in this
> manner as some of the posts in the archives suggest. I have owned three
> other 390As in the past 25 years and I don't remember any of them doing
> this.
> Has anyone else been able to cure this problem?
> Thanks and 73,
> Conard, WS4S
>
>
>
>
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