[ARC5] BC-453 drift

[Kenneth G. Gordon]

On 4 Nov 2017 at 15:43, J Mcvey via ARC5 wrote:

> In 15 minutes it went from 1050 Hz to almost 1400 HZ and doesn't seem to be 
> slowing down.

What?!? In my experience, that is MOST unusual ! For my restored BC-453s or 
R-23/ARC-5s, I have never, ever seen such drift with the BFO on.

> I used a beacon with a reasonably strong signal to do the tracking.
> I zero-beated the signal at 1050 then watched the drift. When I turned the BFO 
> off, the audio was still at 1050.
> The beacon is AM , so I guess the IF's would still be broad enough to 
> compensate for the front end drift?

Well....yes...the IF selectivity with the rods pulled up is on the order of 2.8 kHz at the 6 dB points.

> I'm trying figure out if it's the BFO or the LO- or both...but it ain't easy!

The LO is supposed to be temperature compensated a bit. There is a temp-co capacitor on 
the bottom of the mixer/osc socket. It IS possible that yours has degraded. After all, they are 
around 70 years old by now. As I remember it (bad idea at my age) that temp-co cap is 3 
pfd.

I suggest you follow the suggestion of another fellow here who suggested you use an 
EXTERNAL high-stability oscillator in place of your internal BFO. You can then check and 
record the stability of your external oscillator alone, then using that for your BFO, can then 
determine the drift characteristics of your LO.

Eliminate as many variables as you can, concentrate on ONE variable, then see what you 
get.

Ken W7EKB



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