[R-390] Strange modulation on carriers

[Jim M.]

I have been chasing a whoop-whoop "chirp" on CW (with BFO turned on) heard 
when received on this 390a, but not on the actual signals.  The CW note 
would tend to chirp on strong signals with AGC on.  I measured the 
frequencies of all oscillators with a calibration signal into the receiver, 
keying it on and off so the AGC would jump.  This caused the chirp, but it 
appeard to be the BFO frequency that was being pulled with AGC, not the 
other oscillators.  I noticed the B+ to the IF stage varying a few volts 
with AGC action. (Changing AGC causes other tubes to draw more or less 
current, hence causing slight fluctuations in the B+).   Changing out the 
6BA6 BFO tube V505 reduced the chirp considerably.  I believe that the older 
6BA6 tube may have been more susceptible to instability caused by B+ 
variations.  Jim N4BE


----- Original Message ----- 
From: "Tim Shoppa" <tshoppa at wmata.com>
To: <DJED1 at aol.com>; <r-390 at mailman.qth.net>
Sent: Wednesday, February 07, 2007 11:34 AM
Subject: Re: [R-390] Strange modulation on carriers


> WB2LHI writes:
>> I finished tweaking all the adjustments on my newly
>> recapped IF stage and started checking on the PTO
>> runout.  When I would tune through the  calibrator
>> signal with BFO off I get a modulation, which varies
>> somewhat with  where in the passband I tune.
>> It's there for all bandwidths, and I'm not  sure if it was
>> there before I took the IF module out.  Strangely, it
>> occurs  on all the bands below 8 MHz, but
>> not above.  I assume it's modulation of  the 17 MHz
>> oscillator, and will check it out when I remove the
>> RF module for  recap.  Has anyone seen a symptom
>> like this?  Any suggestions for  troubleshooting
>> it?
>
> FM'ing of the 17 Mc oscillator, with the deviation
> strongly correlated to the AGC action (thus you noticing
> it as you tune through the calibrator), is commonly
> noted. In my experience it is a few hundred Hz
> in the "bad" cases but maybe it's more or less for you.
>
> It is partly a design flaw but for example a flaky
> 17Mc crystal will be more pulled than a good strong
> 17Mc crystal. This is probably at least one of the
> reasons why surplus 17Mc crystals are hard to find!
>
> Some blame this on B+ regulation (or lack thereof) and
> this is possible too. You can determine whether it's
> B+ regulation or AGC action by turning off AGC.
>
> In one of my particular 390As, replacing R211 (which was
> high by several hundred percent) and R209 (which was
> charred black) made the problem
> much less evident. Yeah, I still hear some whoop-whoop
> on strong CW with AGC on, but it's a reminder that I don't like
> AGC on CW anyway!
>
> Unfortunately I did not independently replace R211 and
> R209 to see which one was responsible for the fix. Yeah,
> guilty of shotgunning, but resistors that are high by
> several hundred percent or already burnt to a crisp
> deserve a lot more than a shotgun!
>
> Tim.
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