[ARC5] OT: Hally Instability - Hammarlund drift.

[Kenneth G. Gordon]

> 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