[ARC5] BC-221 and LM on eBay

[Kenneth G. Gordon]

On 28 Mar 2022 at 12:23, Bob kb8tq wrote:


> The answer is very much a "that depends" sort of thing.

Oh yes!! There many things which should be taken into account if one wants the maximum 
accuracy from these instruments. Still, for something designed and built so many years ago, 
they ARE, really, amazing instruments.
 
> The internal crystal isn´t temperature compensated. If you zero it today indoors
> and use it outdoors a month from now, there will be an error. If you let it warm
> up / stabilize for a couple days, zero it, and then use it ... maybe not much error 
> from the crystal.

Again, yes. You really should zero the crystal before every use if one really wants high 
accuracy. For general use it is quite adequate without doing that, though..
 
> A lot of stuff with one of these depends on zero beats. Just how close you can 
> get on a zero beat depends on multiple things.

And yet again, yes. Especially with us old folks failing-hearing.

Now, in that 73 Magazine article on "Hand Calibrating the BC-221 and LM Frequency Meters" 
January 1961, there is a "A simple zero-beat indicator...." using a 1629 "magic eye" tube 
shown, which would easily replace our old ears. I intend to build one asap.

That device will give us a visual indication of zero-beat right down to sub-cycle levels.

One should probably take into account the inaccuracy of the HF signals from WWV if you 
want the best accuracy, although I don't how I could do that. 

I understand that not too long ago someone used either a BC-221 or an LM in the yearly 
ARRL Frequency Measureing Test and placed quite high in the rankings. I know it was within 
a few Hz.

It also seems to me that it should be fairly easy to add some temperature compensation to 
the crystal oscillator circuit.

Lastly, SOME versions of the BC-221 have the crystal adjustment capacitor under a small 
plate on the front-panel in the center near the bottom. My -AK models have that. Others 
require removing the instrument from its case, which introduces other problems. 

IMHO, the -AJ, -AK, and -AL models are the most usable ones. Those are the only models 
which include modutation. ALL LMs have that feature.

Ken W7EKB