[Hammarlund] HQ-129-X BFO drift

[Richard Knoppow]

----- Original Message ----- 
From: "Bill Cromwell" <wrcromwell at gmail.com>
To: <Hammarlund at mailman.qth.net>
Sent: Thursday, September 06, 2012 4:22 AM
Subject: Re: [Hammarlund] HQ-129-X BFO drift


> On Wed, 2012-09-05 at 11:29 -0700, Richard Knoppow wrote:
>> The BFO in my HQ-129-X drifts a lot. This is thermal
>> drift and is more than a khz from a cold start but it can
>> drift that much after a half-hour warm up. I think this 
>> is
>> too much and wonder if its typical.  If not something is
>> wrong, maybe one of the mica caps in the circuit has gone
>> bad. The values are not stated in the handbook but are in
>> the RBG book. Since the circuits are almost identical I
>> expect the cap values are the same or very close, does
>> anyone know for certain?  If anyone else has experienced
>> this drift and found a cure I would be glad to know it.
>>
>>
>> --
>> Richard Knoppow
>> Los Angeles
>> WB6KBL
>> dickburk at ix.netcom.com
>>
>
>
> Hi Richard,
>
> Assuming that you monitored the BFO frequency by an 
> independent means
> that doesn't drift that much (frequency counter, crystal 
> oscillator,
> etc)....
>
> One source of drift like yours is the tube. In your your 
> case the BFO
> tube - V-9, 6SJ7 on the schematic I have. In the HQ-145 I 
> had it was the
> VFO that drifted so badly. The VFO tube was great in a 
> tube tester but
> an oscillator is NOT a tube tester. I had about a dozen 
> replacement
> tubes. One was even worse. Another was just as bad but 
> some stopped the
> drift. I could receive an SSB Navy Mars traffic net for an 
> hour without
> having to fiddle with it. I did warm it up for a half hour 
> or more
> before use.  Tube substitution is going to be MUCH easier 
> than those
> tank caps. If it is one of those just cut n try...starting 
> with the
> values you found for the RGB.
>
> I looked at the schematic and my next suspect after the 
> tube would be
> that voltage divider chain that supplies voltage to the 
> BFO tube R47,
> R48, R49. Old resistors can be flaky and way out of spec.
>
> Many of the old radios have drift specified much higher 
> than is
> acceptable in newer radios but 1 kc in a 455 kc oscillator 
> is just too
> much. Good luck with your receiver.
>
> 73,
>
> Bill  KU8H
>

    Thanks Bill, I check the frequency simply by seeing it 
is zero beat with the narrowest crystal passband.  I can 
check the BFO with a counter to make sure that it and not 
the crystal filter is what is drifting.  Its possible its 
the tube but I think a bad resistor or cap is more likely. 
Usually caps jump rather than drift so I think the resistors 
are the likely culprits.  I have worked with several 
Hammarlund receivers and the BFO's generally are pretty 
stable.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com