[Hammarlund] HQ-170A drift reduction progress - filament voltage regulator

[Kenneth G. Gordon]

On 15 Apr 2011 at 12:37, Mitch wrote:

> The frequency drift I have been experiencing has been greatly
> reduced. If you will look at the drift data I posted, you will see
> that once the shield was in place, the receiver was very stable. This
> testing happened over 4-5 hrs. The heater came on severla times in
> that period of time.
> 
> Also, the house furnace is a propane forced air. The blower is  1/3hp.
> Not enough hp to affect the line voltage. It is also on a different
> circuit.

OK.That eliminates voltage flucutation as the root cause then.

> blowing air through the tuning slug hole. The biggest frequency change
> was caused by directng the air at the tuning capacitor.

Boy! That is very interesting to me. I had never thought of the tuning cap 
being so sensitive. Many of the early Hammarlund and National tuning caps 
were made of "Invar" which was automatically temperature compensating.

However, it was rather expensive material, and was not used in the less 
expensive receivers. I am surprised it was not used in the HQ-170A.

I know the National NC-98 tuning capacitor is particularly bad for drift due to 
temperature.

> I have been considering installing a filament voltage regulation
> circuit, since we do have some voltage fluctuations. Your circuit
> sound interesting. I'll have to do a little research on it.

That circuit I mentioned is at this URL:

http://www.roveroresearch.org/srr13/ballast_tubes.html

Ken W7EKB