[Milsurplus] The BC-221N Freq Meter

Thu Feb 1 16:58:29 EST 2024

Hi

The gotcah is that doing this or that with the coil (like adding a brass slug) is going to impact it’s Q and it’s tempco along with any change in inductance. 

One thing that could have changed is the input C of the oscillator tube. It’s right across the tank. Any change there *will* shift things a bit.

Bob

> On Feb 1, 2024, at 4:22 PM, David Olean <k1whs at metrocast.net> wrote:
> 
> Hi Bob,
> 
> A bit more information.....I got the BC-221 in 2016. One of the radio club members was going to throw it out. I could not bear to see that so I took it home instead.  I first noted that it had some residual drift over time. I hoped that the drift was just getting the thing heated up and running, to drive out moisture etc. so I left it on. Then I noticed the oil.  One of the bathtub caps was leaking. They all were somewhat lossy when tested on a good capacitor bridge.  I ended up removing the old bath tub caps and re stuffing them with new polyester caps, so it still looks original. I took this Philco BC-221 to the Gilbert PA  Red Ball Express fest a few years ago after the repairs, and took home 1st place in measuring frequencies on 80 and 40 meters. I am not too sure how accurate everything was there, but the BC-221 worked very well. I had calibrated the 1000 Kc crystal and used the calibration points. I could adjust every one of the check points with the corrector. I think I was within about 35 to  50 Hz for each crystal that was measured there at Gilbert.  I reset the corrector to the nearest cal point and then interpolated using the book.
> 
> So, today, after noting how stable it is, I am trying to improve on the calibration chart with the simplest solution available. I figured that I would try out the calibration points and see how they fit the cal book. but that is when I noticed that the inductance had possibly increased and the tuning capacitor needs a bit more turning to cover the original range.  The proper thing to do, I guess, is to generate new crystal calibration points, but that is a lot of work. I could use the existing CAL points and adjust the corrector, but I worry that things could get mixed up doing that. Which checkpoint is the correct one? Is it the nearest above or below the unknown frequency?  Without re adjusting the capacitor travel, I see an awful lot of work ahead for me.  I took the BC-221 apart, and I will experiment with a small amount of brass slug in the coil and see what that does. It is very true, that I do not really know what is wrong. It could be the tuning cap that has changed value and has less C now only 81 years after it was built. The coil could have some deposit on it that is altering inductance slightly.  We are not talking about any huge drift, but I want the BC-221 to be as accurate as possible. I just do not want to re invent the wheel, if someone has addressed this already.    I have repaired a number of R-390 and R-392 PTOs but I am no expert. I have a feeling that I will make my own calibration chart from scratch! I will then calibrate everything based on the initial crystal cal point at 2.000 MHz and reference everything from that point.
> 
> The bottom line is that it is fun and much better than watching TV.
> 
> 73
> 
> Dave K1WHS
> 
> On 2/1/2024 11:26 AM, Bob Camp wrote:
>> 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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