[Milsurplus] The BC-221N Freq Meter calibration.

[David Olean]

Hello Hue,

I am keeping my BC-221 exactly as it was made in 1942. I have been 
leaving it turned on and set at 4.000 MHz for about a week. I check it a 
few times every day and am amazed at how it is rock stable now. It 
varies +/- about 25 or 45 Hz over many days.  When I first got the 
BC-221 in 2016, I noted that it had some significant long term drift and 
did not behave as it does now.  I did pretty much nothing to the 
circuitry except replacing the bathtub caps. There is a bypass function 
with Capacitors 10-1 and 10-2 in the BC-221N VFO that could cause some 
long term drift. They bypass the screen and plate of the VFO tube. 
Leakage in those caps would vary the voltages to the VFO and cause 
significant drift. (Other BC-221s have similar caps with different  IDs) 
One of the failure modes of these leaky caps is that the leakage 
increases with increasing internal temperatures within the cap. Then 
there is the fact that an unused BC-221 can absorb moisture when stored 
in damp and cold areas. My BC-221 had an unknown background, but the 
constant frequency drift was telling me that there was something 
happening within the unit as it was drying out.

I have a 2nd BC-221 made by Bendix in WW2,  that I will try out with 
some accuracy checks. It has been in a warm environment for a few years, 
but has not had any bypass caps changed. It might be a good test to 
evaluate it for some before and after measurements. I really believe 
that  long term excessive drift of the VFO components (mostly the 
inductor) causes the crystal markers to go to a different portion of the 
tuning curve, and that might reduce the accuracy of the calibration 
book.   I am still looking into that.

We take frequency accuracy for granted these days.  Today, there are 
even frequency police who alert you to when you are 20 or 30 Hz off of a 
net frequency.  I ran a signal shop in the US Army back in 1969 and 
1970, and we did not have a frequency counter anywhere in our TOE gear. 
We relied on signal generators and crystal calibrators entirely. Now we 
only did 3rd echelon maintenance, but fast forward to today and even 
Johnnie Novice gets upset when you are a few Hz removed from where you 
should be. heh heh. It is expected that my 10 GHz weak signal gear 
should be within about 100 Hz for best results now.   Just 20 years ago, 
we were tuning +/- 25  kHz and then +/- 50 or 100 kHz if we could not 
find the other guy!  My hat is off to those people in WW2 who relied on 
the LM or BC-221 to keep those aircraft and tank radios on frequency. It 
was not easy.

My BC-221 is going back on the shelf, but I am putting a note inside 
outlining what has been done to it. Maybe I could drive my Central 
Electronics 20A as a VFO?


Dave K1WHS


On 2/9/2024 3:22 AM, Hubert Miller wrote:
> It is a pretty simple matter to change the valves to FETs. I think 73 Magazine did the writeup, which i have somewhere.
> Change vacuum tube test equipment to FETs, yes. For any communications device, no, or just with plug-in replacements,
> is what i say.
> -Hue Miller
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