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

[David Olean]

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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