[Milsurplus] The BC-221N Freq Meter calibration.

[B. Smith]

Most of us have several BC-221 type sets usually acquired at Hamfests 
for a  pittance  or we pick them up as a giveaway. And if you ever get 
tired of just letting yours set on the shelf the set it makes a 
interesting receiver/transmitter/VFO.I prefer to leave the basic set 
unmolested and to utilize and enhance the stable design with external 
circuits.
K4CHE
http://k4che.com/BC221/BC221pg2.htm
http://k4che.com/BC221/BC221pg1.htm

On 2/6/2024 5:10 PM, David Olean wrote:
>
> I did a few more experiments with my BC-221 and think that I may have 
> possibly made the BC-221 better than it was at least before I started 
> messing with it. That is, of course after it had sat around for about 
> 80 years. 80 years is a bit hard to comprehend, but they are that old! 
> By making a few mechanical adjustments I have brought this particular 
> BC-221 back into top notch calibration.
>
> I found that very slight movements of the VFO shield plate on the 
> bottom of the chassis can be used to adjust the inherent inductance of 
> the coil and bring it back close to the original settings in your CAL 
> book. There are four screws that hold the aluminum plate to the unit. 
> I put a pair of 0.030" spacers on the front two screws located under 
> the chassis. It moved the crystal checkpoints closer to the original 
> points on the scale. I then played with the spacing on the back side 
> where the screws go thru the top of the chassis and secure the shield 
> plate with two standoffs on the opposite side underneath. This is a 
> BC-221N made by Philco.  Some other units may not have the same 
> shield. All of these added spacers help to move the aluminum shield a 
> bit farther away from the ceramic coil. This affects the residual 
> inductance of the oscillator coil. I tried 0.020 spacers on the rear 
> and saw the 4.000 MHz checkpoint go above the original value by about 
> 4 or 5 divisions. The original error was  +12.4 divisions.  So I 
> increased the spacers to 0.040 and the crystal checkpoint at 4 MHz was 
> within about -1.6 divisions of where it was back in 1942. That is 
> awfully close.
>
> My method of testing involved calibrating the BC221 at 2.000 MHz on 
> the high range and then noting each frequency marker without touching 
> the corrector knob. My thinking was that the crystal markers would be 
> close to the "book" value if the coil was exactly the same as it was 
> in 1942. What I saw was a gradually increasing error as I moved up 
> from 2.000 MHz. At 4.000 MHz I was 12.4 divisions off what was listed 
> in the book. I then looked at fixing that error.
>
> Here is a listing of a few iterations across the 2 to 4 MHz range  I 
> compared crystal cal points from 1942, and then 2024 without 
> modifications. Then I show two examples of varying the coil shield 
> plate by .020" and .040". I also show the cumulative error from the 
> original 1942 book settings. The dial markings ate peculiar to only my 
> BC-221, but all of them will show the same trend I am sure.
>
> FREQ.           DIAL 1942 DIAL 2024   DIAL 020" SHIM   DIAL 0.040" SHIM
> 2.000
> 	0158.8
> 	0158.8 +0.0
> 	0158.8+0.0
> 	0158 +0.0
> 2.1666
> 	0566.9
> 	568.0 +1.1
> 	0567.9 +1.0
> 	0567.7+1.0
> 2.250
> 	0769.0
> 	0770.3+1.3
> 	0769.9+0.9
> 	0769.8+0.8
> 2.3333
> 	0970.1
> 	0971.5+1.4
> 	0970.6+0.5
> 	0970.6+0.5
> 2.500
> 	1369.2
> 	1371.6+2.4
> 	1370.1+0.9
> 	1370.1+0.9
> 2.6666
> 	1761.8
> 	1765.2+3.4
> 	1763.8+1.3
> 	1762.9+1.1
> 2.750
> 	1956.1
> 	1959.8+3.7
> 	1957.1+1.0
> 	1956.9+0.8
> 3.000
> 	2525.3
> 	2530.2+4.9
> 	2526.1+0.8
> 	2525.9+0.6
> 3.250
> 	3082.9
> 	3088.2+5.3
> 	3082.8+0.1
> 	3082.6-0.3
> 3.3333
> 	3266.1
> 	3272.8+6.7
> 	3266.1+0.0
> 	3266.0-0.1
> 3.500
> 	3630.0
> 	3638.2+8.2
> 	3630.1+0.1
> 	3629.9-0.1
> 3.6666
> 	3988.2
> 	3998.1+9.9
> 	3989.0+0.8
> 	3988.9+0.7
> 3.750
> 	4167.3
> 	4178.0+10.7
> 	4167.8+0.5
> 	4167.6+0.3
> 4.000
> 	4699.6
> 	4712.0+12.4
> 	4697.0-2.6
> 	4698.0-1.6
>
>
>
> Note the errors in that last column with all four shims under the 
> shield. Rather than a gradual increase when I started, you see errors 
> go from slightly positive to negative then positive and then negative 
> again as you get up to 4.000 MHz.  The coil is now very close to what 
> it was in 1942 and the accuracy is way up as a result.
>
> I looked at the low frequency coil as well and it moves in the correct 
> direction and is much better as well. original spreading on the low 
> band amounted to about 13 divisions.  After moving the shield, that 
> error dropped to about 5 divisions on the highest cal point. All the 
> others were under 2.5 divisions or so.  With the shield only very 
> slightly moved, the corrector still works on the low band.
>
> So how does it work?  It works great. The calibration book is now 
> quite accurate. I tried a few measurements across the range and saw 
> much better than 100 Hz accuracy. For grins I tried to measure my 
> signal generator on 14.211.700 MHz and the BC-221 got me within 300 Hz 
> at 14 MHz, so I am happy.  I checked accuracy on the low band and it 
> was very good as well.  As an indication of my progress, I have 
> scrapped my idea of a new calibration book entirely. The original book 
> now seems quite good and If I am off by over 100 Hz over the 2-4 MHz 
> range I probably screwed up so I measure again.
> 73
> Dave K1WHS
>
>
>
>
>
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