[Hammarlund] HQ-170A drift reduction progress

[Kenneth G. Gordon]

On 15 Apr 2011 at 11:32, Mitch wrote:

> Input to receiver coax antenna connection from signal generator for
> all tests -- 3 microvolts
>
> 14.2mhz            No discernable drift until the house heating system
> kicks on. Then it shifts in frequency about 100-200 hz.

How quickly does it shift frequency? Instantly (as soon as the heating system 
kicks on) or after a brief interval when the warmer air starts circulating? What 
kind of heathing system does your home have? Hot air? Hot water? What?

The reason I ask is that the FIRST thing that leapt to my mind when I read 
this was that there was some VOLTAGE change that induced this shift, not 
temperature.

If, as I suppose, the B+ to the oscillator (HFO) is regulated, and there is no 
discernible change in THAT voltage, perhaps the problem is a change in 
filament voltage.

Many military receivers used some means to regulate the filament voltage to 
the HFO: the R-390/390A, the SRR-11/12/13, and others.

There is a fairly simple circuit you can add to the filament circuit that will 
regulate the AC voltage. This circuit, using two Zener diodes, was developed 
by RCA for their SRR-11/12/13 receivers to eliminate the ballast tube which 
was previously used to regulate the filament voltage.

Anyway, before proceeding further, please answer my questions above.

Ken Gordon W7EKB