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

[David Olean]

Hello MIles

I have been making my own spreadsheet to use with my BC-221.  I seem to 
have trouble with the Royal Signals page in the past. They will not send 
me the password as there is some issue with my ISP not passing their 
"test". I could never resolve the problem, so I gave up the last time I 
tried.   I'll try again and see what happens.

Making my own Excel spreadsheet is tedious, but I can see the value in 
it. In re calibrating my own meter, I have found numerous errors in my 
book where someone did not get the cal points exactly correct back in 
1942. These may not have been errors as much as the limits of frequency 
measuring gear in 1942.  I am using a GPS locked frequency counter that 
is coupled to the BC-221. I zeroed the 1000kc crystal and it is within 1 
or 2 cycles.   Anyway, I checked the frequency every ten kHz on the 2-4 
MHz range, and entered it into my spreadsheet. The sheet then generates 
all of the 1 kHz checkpoints.  To make things easier, I adjusted the 
amount of dial travel to match the end points of the original 
calibration curve. (It has drifted over the 80 years ) In my case 0158.8 
was 2.000 MHz and 4699.5 was the end point at 4.000 MHz. This was 
accomplished by adjusting the position of a coil shield on the bottom of 
the instrument. I was able to remove most of a 13 dial division error 
down to about 1.4 divisions.  I am not finished with the spreadsheet, 
but it is going along OK. I have tried a few measurements on what I have 
done so far and results are fantastic. The VFO in the BC-221 is quite 
good for 1942. The manual states 0.034% accuracy under most conditions 
in the field. That amounts to a 1.36 kHz error at 4 MHz.  In a heated 
room, I am seeing 0.01% errors typically or about 40 Hz now with my 
newly calibrated BC-221 (regulated 6.0 VDC filaments)

Why I am doing this, I have no idea!!  I think it has something to do 
with the sacrifice and dedication that so many people had in the war 
years. It was an all hands on deck attitude that seems to be missing 
today.  A lot of effort went into the freq meters and they were a large 
part of what happened when things went smoothly!

Dave K1WHS


On 2/4/2024 2:53 PM, k2cby at optonline.net wrote:
>
> Guys,
>
> If you’re bound determined to bring a 1942 or so BC-221 frequency 
> meter into the 21^st Century by generating a calibration book, at 
> least make your job a little easier.
>
> Use either Microsoft Excel or one of the many clones available  on the 
> internet to store the results. Although I haven’t downloaded it, there 
> is a ready-made BC221 spreadsheet available on the internet at 
> https://www.royalsignals.org.uk/files/221/bc221cal.htm.
>
> Unless you already have a frequency counter, invest in one of the many 
> “cheap and dirty” counters available on eBay.
>
> Couple the output of the BC-221 to the counter with a 100  pF  capacitor.
>
> Fire up the BC-221 in the environment where it will be used, turn it 
> on,__and _let it sit there for 3 or 4 hours._
>
> By the way, the calibration crystal can be “tweaked.” On my 221 you 
> access the crystal trimmer by removing the nameplate on the front panel.
>
> 73,
>
> /Miles, K2CBY
>
>
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