BC-610 - The Great Transformer Mystery

[ashart]

Help!  I'm in need of transformer advice!

My pal just acquired a BC-610 and I'm trying to help him get it on the air.  It makes carrier but has no modulation to speak of.

(1)   The modulation transformer primary DC resistance is spec'd as 670 ohms.
(2)   My reading on it, with 3 different instruments, is about 220 ohms.

Obviously, lots of shorted turns, right?  Read on.

(3)   The impedances as in the books and marked on the transformer itself are:
Z(pri) = 21,000  and Z(sec) = 8000 ohms.  Thus, the spec'd turns ratio calculates as N(p) / N(s) = 1.62.  It's in step-down mode.

(4)   When applying about 5 V(rms) @ 1 Kc. to the primary, the secondary voltage is such that the voltage ratio calculates to 1.64:1. 

(5)   Surprised, I re-did the voltage test at about 8 V(rms) to the primary and this time the secondary voltage again produced a step-down ratio of 1.64:1.

QUESTION FOR TRANSFORMER EXPERT OR BC-610 MAVEN:  How can the primary DC resistance imply so many shorted turns, and yet the tested voltage ratios show a turns ratio close enough to be accounted for by measurement error???

I's sure appreciate some technical insight to the question above.

Thanks for any answer!

al hart
al@w8vr.org
www.w8vr.org

[KA2DZT]

What does the sec DC resistance measure?  You right, doesn't seem to make sense about the pri DC resistance being low and yet the turns ratio seem to be correct.

Measure the DC resistance on the sec,  could be the specs for the DC resistance is wrong.  See if the sec DC resistance also reads low.

Over 600 ohms for the primary seems to be way too high.  A few hundred ohms for the primary DC resistance is more in the ball park for a mod xfmr of that size.

Also, measure the DC resistance from the pri CT to each end.  The resistance should be close to being equal, but may not be exactly equal, this is normal for a xfmr with windings over each other.  Outer windings take more copper to complete the same number of turns.

Fred

[ashart]

Hi Fred - Thanks very much for your reply.

The Plate-to-centertap readings are abt 116 ohms and 103 ohms, reasonably adding to the 220 ohms plate-to-plate.

You're right about the secondary resistance being off too.  The specs say 160 ohms, but we measure only 107 ohms.

I'm not convinced the specs are wrong, as they're posted in several parts of the Army tech manual for the 610.  I would also think that if that large an error were the case, many 610 owners would have written about it here or elsewhere on the web, and I've seen nothing.

So, the mystery remains. 

73.

-al

[W7TFO]

Al, the 610 had several different builds in the iron department during production.  Your reference lit may not jive with the iron present.

73DG

[ashart]

Hi Dennis - Good to hear from you.  The last time we "talked" you weren't feeling so well.  Let me know how you're doing now.

Yes, I know the 610 in its different versions, had different transformers.  Yet, the version my pal has is an "F" and the transformer is marked Zp = 21,000  and Zs = 8000, which is totally consistent with the manual.  The manual, but not the transformer, is marked Rp = 670 ohms DC and Rs = 160 ohms DC.

The problem transformer measures Rp = 220 ohms DC and Rs = 107 ohms DC.

To make such major reductions in R, and maintain the same Z, I would think the transformer would be wound with much heavier wire than the spec'd transformer.  Would there be room in the case?  Also, as I've mentioned, that would be the subject of much internet chatter, and I've seen none!

I'm moving further toward recommending that the transformer be removed from the transmitter, put on a bench, and somewhere finding or building the gear to give it a ringing test.

Thanks for the input, Dennis, es 73.

-al

[w4bfs]

here is a simple test you can run and will build confidence in the servicability of your xfmr

you have already verified the turns ratio ....good

I would excite the secondary of the xfmr with 120Vac with a series incadescent 60W light bulb ... a good xfmr will allow the lamp to remain dark and drop only 20Vac or so .... this is how the bc610 xfmrs I have have tested and were consistent .... the transformer size and style varies widely by manufacturer