[ARC5] OT: Hally Instability

[Kenneth G. Gordon]

On 15 Nov 2015 at 23:00, David Stinson wrote:

> Lots of folks are addressing thermal drift, but IMHO
> that's not the big issue with Hally instability.

I agree.

> The big issue is mechanical- the "wibble wobbles" 
> above 10 or 12 MC when you just tap the table.

Yes. My experiences with them exactly.
   
> With the last SX-101 I had, a window air conditioner
> across the room coming on would FM the 
> LO in the receiver.

Not good.
  
> 
> Has anyone ever tried to determine the source
> of this mechanical instability?  Cheap tuning cap?
> LO coils mounted by their wires on the band switch?
> Anyone tried to find out?

I have not, but what always puzzled me was that the VFOs in their 
transmitters were quite acceptable and did not suffer from that mechanical 
instability that their receivers did. I never understood that. Even the VFO in 
the BC-610, which is as basic as one can get, was far more stable than the 
HFO in the general run of their receivers.

Without testing for it, I put it down to insufficent support of the coils in the 
HFO. Neither Hammarlund nor National had that problem, but their coils 
were very solidly mounted.

ALL receivers of the time, except perhaps Collins, had thermal drift, but that 
was slow enough that once stabilized after being left on for a while, it was a 
"non-issue"...at least for me.

BTW, the only "issue" I ever had with the BC-610's VFO was that Halli used 
a beam-tube for the oscillator, the 6V6. They should have used something 
like a 6AG7 or a 6SK7 or even an 837 like the Navy did, instead, but even 
so, the 6V6 worked well enough. 

Beam-tubes do not make good ECOs, since the beam-forming plates, 
essentially, couple the RF around the screen-grid "plate" of the oscillator 
section directly to the output, thereby defeating any "ECO" advantage.

Still, a lot of circuits of the time used the 6V6 or the 6L6 (which was even 
worse in my opinion) as an oscillator.

Ken W7EKB