[ARC5] Info sought about output transformer in BC-221-AK

[J Mcvey]

Wow this stuff is all over the map!One says 10K pri, 256 sec., but the turns ratio entry implies a primary impedance of about 15K for a secondary of 256 ohms!Are the output tubes in these models all the same?I did notice that they used a 15K load resistor in the early models that didn't have a transformer. Perhaps they were figuring that the configuration was a balance between required volume and battery drain rather than optimal matching?Then again, the external matching transformer adaptor to use HS-30 on the earlier models, had a 8000 ohm secondary....

OK, here's what we can do:With the transformer disconnected,Feed a 1khz sine wave into the secondary , measure the amplitude with a scope.Then note the amplitude of the secondary. The voltage ratio should be the turns ratio.Since the secondary is pretty well established to be 256 ohms, the primary impedance for your specific transformer can be calculated.
                                      *****or****

If you are fortunate enough to have an impedance meter , measure the primary with the secondary open, and the secondary with the primary open.The square root of the pri/sec inductance ratio is the turns ratio.


 

    On Saturday, January 6, 2018 1:19 AM, WA5CAB--- via ARC5 <arc5 at mailman.qth.net> wrote:
 

 Belay my last.  It has been maybe as much as 40 years since I last read the 1944 edition of TM 11-300 through.  Since then, I've only gone to the appropriate chapter, usually to the schematic.  What I wrote originally agrees with the statements in Paragraph 10 - Headsets, which says that early models used Headsets P-18 or P-20 and later model use Headset HS-30 with Cord CD-605.  Which is the long Hi-Z cord.  What it fails to say is that that only applies up through BC-221-AE.  BC-221-AF through BC-221-AN use HS-30 and Cord CD-874, which is the long Lo-Z cord.  The reason for this is that up through BC-221-AE, the audio output is RC coupled.  From BC-221-F the audio output is transformer coupled.  Further confusion is caused by the list of components that make up each SCR-211- model.  All of the lists save one include both the CD-605 and the CD-874, with no note indicating when you should use which.

The 1944 edition has no proper parts lists, those having by that time mostly been moved to SIG 7, SIG 8, or SIG 7&8.  So you can't look up the specs on any of the parts, including the audio output transformer.  However, when all of the BC-221's except the -AN were built, each came with a digest size manual on only that one model.  I don't happen to have the small manual on the -AK, but I do have ones on about a third of the 27 models, including four of the six that came with the output transformer.  They all have proper parts lists.    Two say primary 5645 turns, secondary 747 turns, and the other two say primary 10,000 ohms, secondary 250 ohms.  So that's the full answer.

Robert Downs - Houston
wa5cab dot com (Web Store)
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