[Milsurplus] BC-221-AK Beating a dead horse with more questions

[J. Forster]

Is the 220 Hz shift repeatable? ie: If you turn it off for a few days,
will it always be 220 Hz off at startup? Crystals can shift if left idle
for extended periods of time.

The 6 pF need not be a temperature conpensating element. It may be to pad
out the circuit capacitance so the crystal is seeing it design
capacitance. (many modern crystals expect to see 32 pF) For example,
different versions of the BC-221 use different tubes, likely with
different stray Cs. The 6 pF could well be dash number dependent, so the
same crystal can be used across all models.

Sorry, I can't be more specific. I don't have access to any documentation
at present.

Also, have you checked the BC-221 spec? In WW II, 150 Hz at 15 MHz was
very likely more than good enough for its intended purpose.

FWIW,
-John




> Group -
> I apologize for beating a dead horse BC-221-AK.  But this has been my
> re-ed-u-ma-cation project.
>
> On Page 146 paper manual TM11-300 or page 159 in PDF the 6pF part number
> 5; it is not noted as a temperature compensating capacitor maybe it is?
> It is a ceramic tube painted black marked with 6.
>
> I have been observing the crystal OSC frequency on the frequency meter
> over the past three weeks; for random periods of time.  The Crystal
> Reference Oscillator starts high 1000.220KHz. As the unit warms up after
> 1 hour + 40Hz, after  2 hours the Crystal Oscillator comes down to
> 1000KHz  +20 -6 Hz .
>
> The Crystal Osc is a critical component; Dial Readings were based on the
> Crystal Calibration points.  If  Crystal OSC was bouncing up and down
> +/- 30Hz
> that could lead to a 150Hz error or more on the high band.  It does not
> make sense that the 6pF is not a temperature compensating capacitor.
> Note: If this was SOP back in the DAY, WWII, I would have kept the freq
> meters running hot 24/7; and that is a lot of batteries.
>
> If I were to replace the 6pF temperature compensating capacitor on the
> 1000KHz crystal reference Oscillator with what would I replace it dear
> Lizor, with what?  What I think  need is a CAP that looses capacitance
> as the unit warms up to temperature.  If there is such a beast:
>   How to correctly "document" the change in frequency over time and
> temperature.
>   How to specify and select a capacitor that will stabilize the crystal
> Osc.
>
> Any help and or direction greatly appreciated.
>
>
> Thanks
> Hutch