[Milsurplus] BC-221 temperature compensating capacitor question -

[John Hutchins]

Richard,
BC-221-AK mounted in the wood case.  Supply is the RA-??? 120V regulated 
supply designed for the unit to replace the batteries.
Fluke 1953A frequency counter.
The BC-221-AK internal 1000KHz calibrator/oscillator.
Adjust the BC-221-AK Master Oscillator to 4000.000 KHz with the 1000KHz 
reference oscillator on, Mode switch in Check position.
On the BC-221-AK Listen in the head phones for and Adjust the M. O. for 
the lowest beat note possible or Silence which is easy to do.
Tracking the readings over time.  The 1000KHz internal reference 
oscillator drifts down over time as the BC-221-AK warms up.
After 2 hours or so reads steady at Master Osc setting of 3999.080 KHZ 
with a Zero Beat.
I can adjust, tweak, the BC-221-AK Internal reference Osc which I will do. 

The Internal 1000 Khz reference Osc. drift down wards over time as the 
BC-221-AK warms up is about 240Hz.as measured by the Fluke Frequency 
Counter.  Reading the Master Osc. output while adjusted to Zero Beat on 
the Internal Reference Osc. 

Do I need to "re-stabilize" the internal reference circuit or is this 
the Norm for this old equipment?

Thanks
Hutch


C Whitaker wrote:
> de WB2CPN
> How are you coupling the signal out
> of the 221?  Are you in any way getting
> near the oscillator components?
> 73  Clete
>
>
>   
RICHARD SOLOMON wrote:
> Before digging into the oscillator, first I would ask how you are 
> measuring
> the frequency and how good is the reference oscillator of the measuring
> device.
> If your frequency counter has an external reference input (hopefully 
> 10 MHz),
> then you can easily make a GPS Disciplined Oscillator that will suppky an
> external reference that is far more accurate than almost anything else.
> (It is derived from the Cesium Beam Standard on the GPS satellite).
> With such a device one can accurately measure to within 10 Hz at
> 10 GHz, sometimes even better.
>  
> 73, Dick, W1KSZ
>
> > Date: Thu, 22 Jan 2009 19:07:27 -0600
> > From: olegerityincj at austin.rr.com
> > To: milsurplus at mailman.qth.net
> > Subject: [Milsurplus] BC-221 temperature compensating capacitor 
> question -
> >
> > Oh grand wizards -
> > temperature compensating capacitor question
> > The BC-221-AK 1000Khz reference Oscillator has been observed starting
> > about 220 HZ high, 4000.220KHz on cold start 4000.220 KHz then after
> > about 2 hours of warm-up drifts down to about 20 Hz below 4000 KHZ at
> > measured 3999.080 KHZ end of high band. This found by finding the zero
> > beat taking a frequency reading on the Freq counter then doing the same
> > process periodically over a 2 hour period. 240Hz swing so far.
> >
> > The temperature compensating coils are clean and can move freely, so
> > that is not the problem.
> >
> > Is this the norm expected considering the period in which the unit 
> is made?
> >
> > If not what considerations and procedures would I use to re-compensate
> > for the temperature environment.
> >
> > Thanks
> > Hutch
> >
> >
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