[Milsurplus] BC-348 Questions

[WA5CAB at cs.com]

Mack, Hutch's figures are of course correct.  The B+ voltage and current 
drain are both too high.  For cross-check, the DM-28's were mostly rated 220 
VDC @ 70 MA output, and the receivers were sometimes used to power other 
equipment in the aircraft.  Common problems with ham hacked BC-348's were B+ too 
high and removal of the Audio Pack, replaced by a garden variety plate to 
VC transformer, with the B- tied directly to ground, removing the bias 
voltage from the audio amp.  Which causes the audio amp tube to draw more current. 
 So if the audio pack was removed, that could be the problem rather than 
more bad capacitors.

Hutch, minor correction.  101-A is the audio transformer and 101-B the 
coil.  There is also a 101-C capacitor.  Mack, if your set has had the audio 
pack removed, I have several of them.

General comment.  The audio packs for the BC-348-EMOPS  group and 
BC-348-HKLR group (and the associated BC-224's and early models) are all 
significantly different from those in the BC-348-JNQ.  The audio pack from the JNQ can 
be used in the others by adding the capacitor externally and grounding one 
end of the choke, but not vice-versa.  In all but the JNQ, the B- choke goes 
to ground inside the can.  In the JNQ the B- choke goes to the audio amp grid 
and the ground connection is through a pair of 50 ohm resistors in 
parallel.  The only differences between the majority of the audio packs are circuit 
symbol number (101- or 123-), terminal numbering, and the early ones do not 
have a Lo-Z tap on the audio transformer secondary.

In a message dated 4/18/2009 9:21:30 AM Central Daylight Time, 
olegerityincj at austin.rr.com writes: 
> Mack -
> Short answers 1.  To much voltage.  2. The draw should be about 50-55ma 
> @200V.
> The voltage should be about 220V - 17V = 203V to ground from the B+ 
> rail.  The negative lead off the power supply goes, went, to a coil then 
> to ground developing a ~ -17v bias for the audio tube VT-48, type 41 
> tube.   There should be part, 101 audio pack, consisting of 101-A is the 
> coil and 101-B is the audio transformer. 
> Hutch
> 
> 
> Mack wrote:
> >I'm restoring a BC-348-O. Can anyone tell me how much current the B+ 
> circuit should draw? I'm drawing 70 ma at 250V. Is that a bit high? Might there 
> be another leaky capacitor that I haven't found? It's also  pretty deaf 
> but I'll work on that after I'm sure it's not drawing too much current. 
> >
> >So far I've thoroughly cleaned the rig and replaced most of the non-mica 
> caps. I've also completely rebuilt the AC power supply. After I'm sure it's 
> working electrically then I'll tackle all the cosmetics with completely 
> stripping the paint and refinishing. 
> >
> >The BC-348 was the first receiver I owned as a Novice 41 years ago and 
> I'm trying to rebuilt a bit of that nostalgia. Thank you in advance for any 
> advice. 
> 

Robert & Susan Downs - Houston
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