[Milsurplus] Frequency meters, LM - and BC-

[Dave Olean]

I am pleasantly surprised at all the interest in the BC-221 and its' 
variants.  I wrote up an article for Electric Radio back in the spring 
and sent it off to Ray . I have not seen it in print yet, but I took a 
semi deep dive into the BC-221 and looked at the typical accuracies that 
were possible with such a meter today. One thing that I found was that 
the VFO coils had drifted somewhat, so that the calibration books did 
not align with the errors inherent in the tuning capacitor, so I found a 
way to bring the crystal checkpoints back in line with where they were 
in 1944. A few folks thought I was crazy to expend so much energy with 
obsolete tech. (I put up no defense) My interest was just what you guys 
are now discussing. I wanted to see how good it could be. Re calibrating 
the coil and capacitor improved the accuracy a bit. It was not earth 
shaking, but I found that the original books along with a BC-221 that 
had a regulated AC supply and was not bouncing around in a B-17 at 
18,000 ft could produce accuracy much better than advertised. The manual 
states 0.034% at 4.000 MHz.  I was seeing accuracies of well under 0.01 
% in a controlled environment. Many measurements were at 0.003%.  A few 
spots produced larger errors and I attribute that to errors in the 
calibration book itself. I found some mistakes there as I re checked my 
unit against the book. I found myself easily making mistakes re 
calibrating the thing as well. After awhile you can go cross-eyed 
looking at that dial. It is not an easy job.

I have two BC-221s, an "N" version and an "M" version. I re packed all 
the bath tub caps and looked for other bad components. Not much goes 
wrong with them. The two versions needed different techniques to re 
calibrate the VFO coils, but it was possible and simple to do. Basically 
I used metal near the coils to tweak the inductance. One unit was low, 
while the other was high!  BC-221s are a lot of fun.

Dave K1WHS

On 9/3/2025 3:45 PM, Bob Camp wrote:
> Hi
>
> You can (with some care) measure the frequency of an LM or BC-221 to 
> at least “as good as” the original calibration setup. I came across 
> the frequency standard rack from one many years ago. It was state of 
> the art back then. We’ve moved on.
>
> The other side of the process is very accurately “spinning the dial”. 
> You can build up a fancy drive setup. Most folks don’t go that far. 
> Without it, it’s *very* tedious to get enough points to do as good a 
> job as they did “back in the day”. The tuning capacitor isn’t quite as 
> linear as one might think ….
>
> Yes, this is really a “that depends” sort of thing.
>
> Is a cal book that’s “close” good enough? For a lot of things it most 
> certainly is.
>
> What on earth are we comparing to?
>
> How accurate is that 80 year old cal book today? Does the meter still 
> have the same tubes in it as when it went through cal? (some very much 
> look like they do). How much does the cal book change when you swap 
> tubes? How long do you keep the unit powered up before you use it? Do 
> you “trust” the crystal or do you zero it with every use? If so how do 
> you do it?
>
> Lots and lots of rabbit holes to wander down.
>
> Bob
>
>> On Sep 3, 2025, at 1:40 PM, Bill KA8VIT <ka8vit at ka8vit.com> wrote:
>>
>> These days you can easily recreate the CAL book using an NanoVNA.
>> There was an article posted about it somewhere, (I thought here).
>> 73 - BILL KA8VIT / W8COD
>> ====================================
>> Bill Chaikin, KA8VIT
>> Chief Radio Operator
>> WW2 Submarine USS COD SS-224 (NECO)
>> ka8vit at ka8vit.com
>> http://ka8vit.com
>> http://www.usscod.org
>> ====================================
>>> On 09/03/2025 1:44 AM EDT Bill Pileggi <wpileggi at juno.com> wrote:
>>> The LM and BC series frequency meters might be more popular than we 
>>> think... I had one stolen from my table at the Sellersville, PA 
>>> hamfest sometime between 2010 and 2020. Absolutely astonished me. 
>>> The meter did NOT have the CAL book on the front, I had the CAL book 
>>> with me in electronic format (it's a complicated story as to why I 
>>> didn't have the physical CAL book (for this serial number) ). The 
>>> CAL book remains in my electronic archives...
>>> Bill KA3AIS
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