[Milsurplus] The BC-221N Freq Meter

[David Olean]

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