Hi, all.
I thought I would anchor the the known (by me) history of my BC348R
receiver... and share it with any interested parties in this group.
Any updates and/or corrections will be gratefully received. Warning:
long post...!
Cheers
Roger

__________________________________

A BC348R history

"We are only the temporary custodians of our collectibles..." Anon.

Early history
1943: RC348R S/N 16743 was built by Belmont Radio under a large
contract with the US Army Air-force. It was delivered to the San
Antonio air-force base and likely installed in a B-series bomber (B24,
B25, B26... no information on this.) Subsequently, S/N 16743 may have
flown thousands of hours in WW2 to who knows where... Britain,
Germany, China, Pacific islands campaign? We know it was not the
Doolittle raid or it would be scratched up a bit!

1945: Continued US service in peace and local wars... could have been
anywhere... Europe, Asia, Korea, US home front? Whatever happened to
that aircraft and her crews, S/N 16743 survived without a scratch.

1960's (speculation) Aircraft was decommissioned and S/N 16743 put in
storage or sold surplus to, I suspect, someone in aviation or
avionics. It was used in receive-only mode at some point since the
power socket jumpered where the "mute-receive/transmit" switch is
wired. Some time afterwards it was taken to the town of Shawinigan,
Quebec, Canada.

My history
Early 1970's: I got his unit in the early 1970's. I lived in
Montreal and had recently got my private pilots licence. My air
transport pilot friend, John Ainsworth, said there was a couple of
surplus "aircraft radios" in an FBO's office at the Shawinigan
airport, that we could buy "as is". He closed the deal at CDN$50 for
them both (almost the same as US$50 back then.) I flew John up in a
Cherokee PA-28-140 to pick them up (still have the logbook entry.)
They were both "R" models and were alleged to have come out of
decommissioned US Army Air-force B25's or B26's, but I have forgotten
the details. We put them in the back seats of the Cherokee (checked
the weight and balance!) and flew back to Montreal. John took one and
I took the other, SN16743. I have no idea what happened to John's
BC348 and since then we have lost touch.

Mid 1970's
For a while my unit was a static display, I was too busy to look into
getting it working. A year or so later I got around to restoration -
it did not seem to need much visually. Everything was there,
including the dynamotor. Someone had bridged pins 2 and 6 of the
power plug so it had been at least tried out on receiver-only mode,
i.e. no transmitter hooked up.

Of course, I needed a 28 VDC power supply so I built one... but not
very well, I'm afraid! I built a regulated P/S using a series
regulator transistor (a T-03 based OC28, IIRC) and a zener diode in
the classical circuit after a transformer and FW rectifier. I made
two mistakes, one easy to fix the other nearly fatal. The easy one:
the base current of the OC28 at full load was too high for the zener,
so I added an extra transistor as a current amplifier (IIRC, as an
emitter follower after the zener.) Problem solved. The second, and
more serious, mistake was using a power transfomer with too high a
secondary voltage. I had purchased a 250 VA Hammond transformer that
had a 40 VAC secondary with the notion of having enough volts to drop
across the series regulator transistor. Of course, there were too many
volts and it got too hot! (it had a decent heat sink, too.) I tried
various work arounds, e.g. using choke input (but I did not have the
right choke, it was too small), and series resistors ahead of the
regulator - but they got too hot. Finally, I put a separate wirewound
resistor in the 120 VAC lead to the MT and adjusted it so that only 3
volts or so was dropped across the OC28 at about 1.5 amps... about 4.5
watts in the OC28. It worked but was a nasty kluge to my mind. I
actually put the ww resistor in a separate porcelain jam jar away from
the chasis, always meaning to put it on posts top-side later.... never
got around to it. Anyway, we got power!

Back to the radio... I took it down to our then country place near
Magog, Quebec, planning to put up a decent SW antenna... in the
meantime I strung a longish wire out of the window to a nearby pine
tree. Radio worked on all bands but the dynamotor was quite noisy so
I did not listen much. Then I got distracted from the project and the
BC348 stayed unused for over 15 years...

1999: Fast forward forward. We wanted to divest ourselves of
property in a potentially "separatist Quebec" and get out (should have
done this in 1976, or at least in the 1980's... never mind!) We sold
the country place and stored the radio in our Montreal townhouse. In
1999 my wife and I moved to Thornhill, north of Toronto (what a
relief!), since I had been offered a great job as VP Engineering with
a company in North York (Toronto) just south of the 401. Radio was
moved, of course, along with tons of other stuff, but that's another
story. Put the BC348 in storage again.

2007: Eight years went by and I spent time on other things, including
my engineering work, restoring commercial tube radios, building tube
amplfiers, skeet shooting and relearning the piano. Circa 2007 the
BC348 at last got onto my "do list" as "test tubes and refurbish". In
the Spring of 2007 I actually did something about it! I had already
searched out manuals and schematics on the 'net, so the first task was
to test all the tubes. All tested "good" to "very good". Then, fix
that bloody P/S! The regulator was not needed. The BC348 takes a
nearly fixed current of about 1.25 amps at 28 VDC so a choke input P/S
properly designed would do the job. Fortunately, I now had the right
choke to hand so I simply gutted the old P/S and brewed up a simple
unregulated unit. It worked fine with very minor fixed resistor
voltage correction. Now to the radio...

It was, of course still working. I used a matching transformer from
the 300 ohm "Low" phones output to an 8 ohm loudspeaker but the sound
quality was very bad and power very low (designed for phones.) To keep
the radio original, I fitted a small commercial radio OPT (6550 ohms
to 4 ohms) into the chassis (not much room, you have to be very
creative with space.) Using a spare 6 ohm LS this matched the 6K6GT o/
p tube quite well. The old OPT was left in, of course, for future
change back to original. I wired the 4 ohm secondary to the jack
sockets to one of the original phone jacks so that I could plug in a 4
to 6 ohm speaker. It all worked perfectly, lots of audio power now,
around 2 watts.

Now for the important part... I wanted to bypass the dynamotor so that
it would not wear out - it was too noisy anyway. But I did not want
to remove it from the chassis as it could get lost over the next 200
plus years... (This note will be kept with the radio, inside the
chassis.)

I disconnected the 5 dynamotor leads from the dynamotor terminal strip
and connected them to a new 4 terminal strip (one point is common),
carefully mounted under the chassis near the dynamotor - there's not
much room, I used standoffs and two new holes (allowed!) I connected
this terminal strip to a 4 wire cable to the external P/S's. This
cable is actually a 3 wire plus insulated shield cable (shield, red,
black and white.) Red goes to what was dynamotor B+; black goes to
what was dynamotor B minus (NOT chasssis, it's NOT the same - check
the schematic, you "smell a rat" when you see the 6K6 cathode is
connected to ground); shield goes to chassis and white goes to pin 2
of the 6K6 output tube (NOT to the old 28 VDC + as there is a resistor
("76-A") that drops the 28 volts to 25.2 VDC for the series parallel
tube heaters.

For the B+ I used a Heathkit regulated power supply set to 200 VDC (up
to 220 is OK.) The heaters are in series-parallel across the 28 VCD
supply with resistor "76-A" to drop 2.8 volts and another one to even
out the 6K6 heater's 0.4 amps and the other tubes' 0.3 amps. PIn 2 of
the 6K6 is at 25.2 volts above chassis so I used a 24 VAC HVAC control
transformer to run the heaters - AC is fine for this, no hum at all.

That's about it to date., No alignment done - it seems good enough
for now. No paper caps replaced yet as all voltages across them are
normal. It receives on all bands but I don't have an outside antenna
(no space.) I plan to take it over to a ham friend's house shortly
and try it on his 160 foot "long" wire to see what it will do.

So, some more history has been preserved for posterity... It's an
honour to own... correction: to have temporary custody, of an original
piece of equipment used by those brave men of the US Army Airforce in
WW2... somewhere under the atmospherics of today's SW stations your
can just hear the faint echo of those big Wright-Cyclone radial
engines, the bursting of flak and the roar of twin 1/2 inch
Brownings...

Roger Jones August 2007