[R-390] Strange modulation on carriers

[Jim M.]

More theorizing:
I notice the pulling of the BFO frequency (causing the "chirp") tends to be 
more noticable when the BFO is tuned very close to the signal frequency in 
the IF (near zero beat).  I have read of a phenomenon in older radios where 
a very strong signal in the IF, coupling back into the BFO, can cause the 
BFO to pull in toward the frequency of the strong signal (sort of a phase 
lock effect).  During the AGC attack time, a few milliseconds, the IF stages 
are still at max gain resulting in a very strong IF signal coupling back 
into the BFO, hence pulling its oscillation frequency slightly toward the 
strong signal.  I notice this only on one of the two 390a's I have here - in 
a Motorola IF module.  If I tune the BFO way off the signal (for a high 
pitch tone, or a BFO frequency further removed from the strong signal), the 
chirp is less noticable.  So if this is happening I am not sure what the 
cure is, other than reducing the BFO coupling capacitor (which is now 12 pf 
between the BFO tube and the detector tube) - which would further reduce BFO 
injection and hurt product detector performance.  Some older "valve" 
receivers even ran the BFO at half the IF frequency, using the 2nd harmonic 
for injection, to reduce pulling by strong signals.  Just some random 
thoughts.


----- Original Message ----- 
From: "Jim M." <jmiller1706 at cfl.rr.com>
To: <r-390 at mailman.qth.net>
Sent: Sunday, February 18, 2007 2:32 PM
Subject: Re: [R-390] Strange modulation on carriers


>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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