[Milsurplus] The BC-221N Freq Meter

[Ken Gordon]

I am with you on the BC-221: I also have two here and used to use them regularly. I am 
continually amazed at their stability and accuracy.

On 1 Feb 2024 at 10:43, David Olean wrote:

> Hello surplus friends,
> 
> I have a pair of old BC-221s. One is in a wooden case, and my Philco
> BC-221N is in a metal case. I did make an AC supply for the units and I
> recall that I used a solid state regulator for the filament voltages
> for the metal cased BC-211N. I recall having serious drift issues on
> VFOs with varying filament voltages in the past, so figured 6 VDC
> regulated on the filaments was a good idea. 

Yes. IMHO, it is a good idea: however, you might think about reversing the polarity on a 
regular basis.

> Of course the bigger question is why even bother when there are freq
> counters today that can sense frequency shift when someone swats a
> mosquito in Sumatra!  

Yeah. Right. But what fun is that?

> I can't answer that question, but I am amazed at the level of accuracy
> available in the 1940's with these instruments and I want mine to be as
> stable and accurate as possible. 

Likewise. I am having trouble with one of mine in that after it has been on for awhile, 
something in it shorts out the HV and ruins the rectifier tube. I have not yet tracked that one 
down.

> So I decided to make my own abbreviated calibration sheet for my
> particular unit.  It seems that the BC-221 suffers from the same
> disease as old Collins PTOs. Over time, the inductance changes and it
> takes more than ten turns to negotiate the 1 MHz span. In the BC-221,
> it takes 12.5 more divisions on the scale to traverse between 2.000 and
> 4.000 MHz. The calibration drift is proportional across the range. Has
> there been any discussion about this? 

None that I have been aware of...yet.

> Is there a way to fix it? 

Hmmm...I don't know.....yet. I had not thought about that problem before you brought it up. 
But I can sure see that could be a problem now.

> I see the dial "expansion" on both the low and high settings.
> 
> I am thinking that I should make all new crystal checkpoints rather
> than rely on the old check points.

That sounds like a good idea. I HAVE noticed that the crystals in my units tend to drift a small 
amount, depending on how cold it is in my shack.

> I was hoping that maybe I could slightly adjust the inductance to bring  the original crystal 
> check points back to where they were in 1942.  
> Am I wrong in suspecting the inductance?    

No. At least, I don't think so. 

>  I hate to mess with the temp compensating capacitors. 

Don't bother with those. Usually, they are just fine.

> I was thinking That maybe a brass slug near the coil might suffice,

Yeah. I was thinking the same thing.

> but implementing that is a tall order for sure.  I am looking for
> ideas.   Please do not tell me that I am wasting my time. I already
> know that.

Ha ha! Yes, but as I said, what fun is NOT working with them? :-)

> As it sits now, my BC-221 drifts about 25 Hz over a 24 hour period at
> 2.5 MHz. I rate that as very very good for a VFO. These units need to
> be preserved and their stories told.

I most certainly agree. 

I think I'll dig mine out and, first, track down why the one is destroying rectifier tubes, then go 
from there.

Both of mine are -AK units, which are one of the few versions which have "modulation" 
included, making them more closely like a good signal-generator.

I paid $5.00 for one and the other was free. Both still have their original calibration books too. 
One has a homebrrew AC supply, the other has no supply, yet. Both are in the wooden 
cases.

It is really nice to hear from another person who respects these units.

BTW, I also have at least 2 (maybe even 3) of the LM units but without calibration books. I 
originally bought them (for very little) to use for parts, but after receiving them, they all were in 
such good condition that I decided to try to restore them and to build new cal books for them.

Later,

Ken W7EKB