[ARC5] BC-453 drift

[Bob kb8tq]

Hi

One “home compatible” test is to grab a hot air gun or a hair dryer. Funnel it down to a narrow stream.
Hit each suspected area and listen to the result. If you find one area much more sensitive than the rest,
dig deeper.

Don’t rule out the air variable. Time may not kind to them. Hard to spot cracks do happen. Wipers that
once grounded plates stop doing so reliably …..

Bob


> On Nov 5, 2017, at 9:47 AM, J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:
> 
> Thanks to all who have responded. It's not that I don't have access to good test equipment, it's just that I don't have that much at home where the radio is at the moment. It will go to my shop on Monday for further analysis.
> 
> However, I am a bit puzzled as to the "why" of the drift. The BFO is an "air variable" tuned LC . There is a 1000pf mica inside the can, but that is only a used as a bypass cap, not tuning. It checks out OK on ESR as well.
> There isn't much to it to be defective!
> An LC oscillator can be reasonably stable, because I have an ancient URM-25D that once warmed up, only moves a hertz or two, according to my freq counter.
> My HP 8935 only goes down to 400KHZ , so for some MF and VLF , I actually use the URM-25D below 400HKZ..
> 
> I'll loosely Couple the freq counter into the oscillator and set it aside , checking for drift periodically.
> One or the other or both are drifting, so that is what has to be determined first.
> 
> 
> 
> 
> 
> 
> 
> On Saturday, November 4, 2017, 11:48:46 AM EDT, J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:
> 
> 
> Still trying to get a handle on the drift problem.
> I disconnected the lower bleeder/bias resistor (R23) and installed a Zener stack that added up to 109V.
> 
> The radio sound OK on CW, but has an almost imperceptible slow drift to higher audio frequency on the WJST-x 
> waterfall. It's more pronounced on WJST-x because the waterfall is very slow, so it shows the drift over time better.
> Doesn't look bad on FL digi, but if you notice where you start and where you are in 10 minutes, there is quite a spread.
> 
> In 15 minutes it went from 1050 Hz to almost 1400 HZ and doesn't seem to be slowing down.
> 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?
> If the BW is say, 1000 HZ, a 400 hz shift in the LO should start degrading the signal-right? The signal sounds steady and when I fiddled with the tuning , It could not make it batter than it already  was. I think the bigger issue may be the BFO.
> I'm trying figure out if it's the BFO or the LO- or both...but it ain't easy!
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