[ARC5] BC-453 drift

[J Mcvey]

well, I have this thing home at the moment, so all of the fancy equipment is at the shop 20 miles away.I'm trying to do this by the seat of my pants here and it's not working out too well...
 

    On Saturday, November 4, 2017, 12:55:45 PM EDT, Dave Jackson <cjack93907 at razzolink.com> wrote:  
 
 Or you could directly measure the frequency of the BFO and LO to determine
the source of the drift. It shouldn't be too difficult to lightly couple
into either one of them.

Sincerely,

Dave WA4OBJ

-----Original Message-----
From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On
Behalf Of Kenneth G. Gordon
Sent: Saturday, November 04, 2017 9:38 AM
To: J Mcvey via ARC5
Cc: Arc5 at mailman.qth.net
Subject: Re: [ARC5] BC-453 drift

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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