[Milsurplus] Frequency meters, LM - and BC-

[Brenda Gentry]

There are probably no people still left who can answer a few questions. 
The BC-221 was a part of the radio installation on the B-17 and B-24 
bombers. With the vibration and other stresses, how accurate did they 
remain? How often were they checked or recalibrated? Just how close did 
they need to be?

    B.Gentry

On 9/3/25 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 
>>> <mailto: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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