[Milsurplus] The BC-221N Freq Meter

[Bob Camp]

Hi

There are a *lot* of different versions of the BC-221. Many folks made them. Even with the same suffix letter, you can see differences between them. Some outfits made more stable units than others. I also suspect that some batches of components resulted in less drift. 

If your crystal is still on the correct frequency, then the calibration points in the book still “work” ( = they do what they are supposed to do). Yes that *assumes* you can get to them. 

If you can’t hit the points is the inductor to blame? These gizmos are tuned very differently than a PTO. The same “gap change” issue does not apply here. It could be the coil, the fixed caps, or even the tuning cap. 

Drift is going to be dependent on a lot of things. One often overlooked one is humidity. Leave the unit out in that damp garage for a year. It will soak up a lot of moisture. Normally the “soak up” process is faster than the “dry out” process. Temperature is the one we all point to. It has a nice easy to observe first order effect that moves pretty fast. Does all that dust inside the unit matter? … hmmm ….  How about the oil slowly drying out on the tuning cap plates? … hmmm ….  Lots of weird little things going on.

25 Hz at 2.5 MHz is 10 ppm. TC caps are doing well at 30 ppm / C. A typical room struggles to hold 1 to 2C over an hour or two. 10 ppm per day is indeed good for a VFO. I wound not mess with any of the parts inside the box ….

Bob

> On Feb 1, 2024, at 10:43 AM, David Olean <k1whs at metrocast.net> 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. 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!  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.
> 
> 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? Is there a way to fix it?  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.  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?  I hate to mess with the temp compensating capacitors.  I was thinking That maybe a brass slug near the coil might suffice, 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.
> 
> 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.
> 
> 
> 73
> 
> Dave K1WHS
> 
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