[Hammarlund] HQ-129-X BFO drift

[Bill Cromwell]

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