[Milsurplus] BC-474 matters

[Hubert Miller]

Re the BC-474-A :   IIRC, Bro. Stinson reported previously on sparking at the send-receive switch, is this correct?
I was looking at TM11-4006 BC-474-A Repair Instructions May 1945. This is not that great a manual, and has me
wondering if i own the TM11-250 or have just misplaced it. TM11-4006 is pretty dry. I suppose the transmitter
modulation checks and receiver alignment info may be useful.

I note that p. 36 says
" (4) Additional checks. Operation of the REC-TRANS switch causes high peak voltage across capacitor C13 which
sometimes causes break-down of this capacitor. This can be prevented by connecting the capacitor permanently
across the B+ voltage as shown in figure 21. This change is accomplished by unsoldering the lead from C13 at the
terminal strip, and soldering it to the B+ lug on the EMISSION SELECTOR switch. "
I noted the page number for this paragraph, as the paragraph is well camouflaged and going without the page
number, it took me about an hour to find it again.

Does this address the issue Dave raised? The current rush when C13 charges, when switched to TRANS, i think
might cause sparking at section 4 of this switch S8; which would be another way of looking at the bad effect of
C13 location.

BTW, if you're looking at the "transmitter practical wiring diagram", p. 42 of TM11-4006 and p. 44-45 of TM11-250
at the Radionerds PDF  http://www.radionerds.com/images/a/a3/US-TM-11-250.pdf
and you're not seeing C13, it's not your eyes that need checking. C13 is located on this drawing just to the right of
V1 in the upper left. In the PDF its C# designation is kind of blurry and in TM11-4006 it's printed small to provide an
entertaining puzzle for you. The sketch of the wiring change is TM11-4006 p. 39.  If any interest i maybe can scan
it and post. I note that the schematics and practical wiring diagram in TM11-4006 however still show C13 in its
original unmoved problematic location.

This may be of interest also, p. 26 :
" (h) If oscillations or fringe howl are heard when the receiver is operated, the fault may be remedied in the
following manner: shield Tube JAN-1D8GT (V-4) with tube shield and ground the No.1 pin of tube V-4 to the
chassis, using the shortest possible connection. This will result in the tube shield being grounded through its
clip to pin NO.1, and through the corresponding connection to the chassis. "

That paragraph was of interest to me, as the last time i operated a BC-474, around 1980, i encountered what
sounded to me just like regenerative receiver "fringe howl". I cured it by adding a "de-Qer" resistor across the
primary of the output transformer, something like 10 or 20k.  I believe i read that others inserted here a
capacitor instead; but i think that is a misapplication and a de-Q resistor is the smarter, theory-wise way. Of
course, the Army way of the paragraph preceding is maybe the best way, but the resistor works too, with no
noticeable degradation of performance. The capacitor solution i would avoid as that 'may' add an undesirable
audio frequency peaking to the transformer.

The particular set i examined last night had a homebrew power supply with it. The builder had provided a voltage
regulator tube for 90 volts, but he had tied the receiver B- to chassis ground, which made the output transformer
go west. This is the set i used back then; i can't remember exactly what i done to the PS; i think it involved adding
a 7 or 8 volt zener to the PS and maybe another lead coming out for the receiver B-.
-H








-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/milsurplus/attachments/20170309/b8e2ba46/attachment-0001.html>