[ARC5] OT: Hally Instability - Hammarlund drift.

[Richard Knoppow]

     I wish I had known about the TC cap. My station receiver for years 
was a modified BC-779.  It came extensively modified using the dual 
triode amplifiers described in CQ. I am drawing a blank on the guy who 
wrote the article. Anyway, I didn't like the way it performed and 
restored it to its original circuits.  I think experimented a little. I 
wound up using 6BA6 RF amps and a 6BE6 mixer with the original LO 
changed to an electron coupled circuit copied from a General Radio 
frequency meter.  I used adaptors so that restoring it original was 
easy. I eventually did this and found the dial calibration was closer 
with the original LO.  One mod I found helped a lot was the use of a 
voltage regulator tube on the oscillator.  The B+ of this receiver 
changes with the RF gain so there is frequency pulling by the AVC or 
manual gain.  A 150V regulator fixes that. Hammarlund certainly knew 
about both temperature compensation and voltage regulation by the time 
the HQ-120-X came on the market. It is actually a quite stable 
receiver.  Whey they didn't apply this to the Super-Pro, their premium 
quality receiver is beyond me.  FWIW, my modified BC-779 was a very good 
receiver, quiet with the modified circuits and quite stable. However, I 
let run all the time.

On 11/15/2015 4:13 PM, Kenneth G. Gordon wrote:
>> another consideration. The early Hammarlund Super Pros  for
>> example(pre-war) were some of the finest receivers of their time, yet
>> suffered from the usual issues above 20mcs on the SX models. They also
>> suffered from drift until warmed up, but the manual makes it pretty clear
>> that they were meant to be turned on and left on.
> Yes, and after WWII, Hammarlund published a fairly simple fix for a good
> portion of that drift: a 3 pfd temperature compensating cap paralleled with
> the HFO section of the tuning cap. Had to be a negative 1500 PPM/degree
> temperature coeffecient, but cut the drift to a very low value.
>
>> The SPs got a bad rap
>> later for drift issues related to folks not heeding this advice, along with the
>> well-used examples that hit the surplus market and got used without any repair
>> or replacement of aged components.
> Well, I, somehow, was given an almost new BC-779 with power supply "back
> then". I modified the 6N7 noise limiter into a triode product detector, mounted
> the receiver in a rack, turned it on and never turned it off. It became, very
> shortly, my favorite receiver. I used it for RTTY. On 20 meters, it would drift,
> very slightly, back and forth. Hearing that slight back-and-forth drift was
> actually quite pleasant. I suppose it was a few 10s of cycles.
>
> Ken W7EKB
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL