[ARC5] BC-221 and LM on eBay

[Bob kb8tq]

Hi

> On Mar 28, 2022, at 12:52 PM, Kenneth G. Gordon <kgordon2006 at frontier.com> wrote:
> 
> 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.

Many of us seem to “fall off” at the high end and still do pretty well at the low 
end. That works out pretty well in this case. Since the “accuracy” is really splitting
between two beat notes, you don’t have to be able to hear 20 Hz. If you can hear
50 Hz and spilt the 100 Hz delta to within 20%, that gets you the 20 Hz you are after.

> 
> 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.

As transmitted the WWV signals are “good” to parts in 10^-13 or so. As received it depends
a lot on the propagation conditions. With reasonable luck, you should be able to do < 0.1 ppm
as received. That’s way better than what you “need” to get one of these gizmos running. 

> 
> 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.

I suspect it would be a lot of work. You would need to know a lot more about the crystal 
than is commonly known.  

Some of the VFO circuits are temperature compensated. You can see an interesting coil 
spring and “shorting bar” arrangement on them. Just how well this worked / now works 
…. not clear. 

Bob

> 
> 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