[Milsurplus] LM/BC-221 stability

[C.Whitaker]

de WB2CPN
The BC-221 VFO is not expected to be all that stable, even over a few
minutes.  Stability is not in the design.
The crystal oscillator which is used to calibrate the VFO is expected to
stay on frequency, but needs to be checked with WWV regularly.
The accuracy of the VFO depends on the accuracy of the crystal
oscillator, and that crystal is adjustable.
So, when you use the VFO, (with the crystal on frequency), do the CAL
every few minutes or so, or every time you use it.  And always approach
the specific dial reading from the same direction, (i.e. clockwise or
counter-clockwise), as you approached the CAL point.
Temperature is the main culprit, but the battery voltage comes in as
a close second.
Bottom Line:  The crystal is supposed to be kept on frequency.
I've spent a lot of time with the family of BC-221's over the years.
73  Clete
++++++++++++++++++++++++++++++++++++++++++++++++

On 1/6/2014 7:49 PM, Larry wrote:
> I now have my LM-14 freq meter wired up and heated up as well.  One of 
> the things i've been watching the past few days is the stability of it 
> when calibrated or in the Calibrate mode.
>
> I notice that even with a good regulated power supply, i can set the 
> voltage to anything in the 200V range but presently its setting at 
> about 225V for plate and 24V for the filament.  It still has some 
> drift.  My question is, "how long after the unit was calibrated to a 
> particular check point did the techs wait before going back and 
> rechecking it"?  Was it allowed to drift 1 or 5 or however many KC 
> before they were concerned or did the do it regularly, say every 5, 10 
> or 15 minutes.  What would the requirement have been?  Saying that 
> they only used to to check one frequency on a bunch of receiver as an 
> instance?  Course if they were moving it from one freq to another i'm 
> sure they rechecked the Cal every time the moved it to a new freq.
>
> I'd think that with the wide bandwidth of the receivers that even if 
> it was off several KC it wouldn't have been a major item. Also, how 
> far off frequency do you think a receiver would have had to be before 
> it was reported to the main crew? I think it was said on here before 
> that sometimes crew members "diddled" with the tuning if they weren't 
> hearing other transmissions like they thought they should. Another 
> factor to consider.
>
> I've not yet put a freq counter on it to watch and see how far it 
> moves but that is coming up.
> Otherwise i think it's pretty reliable, if you leave the B+ on.
>
> Larry
> W0OGH
>
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