On Jul 1, 9:57 pm, Engineer wrote:
I could use some help with a
BC-348-R "boatanchor".

Can anyone point me towards a schematic for the "R"?

I think I have the entire "R" manual in electronic format. Send me an
email and I will reply with the manual as an attachment if I have it.

Some comments on the BC-348R:

1) If yours is original, it probably still has Those Dern Black
Plastic Capacitors. These are small units that look like micas but are
actually paper, and they are notoriously unreliable. They fail open,
shorted, and every way you can imagine. They are 0.01 uF 400 volts or
so, and should all be replaced. Besides being annoying, some are in
parts of the circuit where a failure takes several other parts with
it. Replace them all and save yourself a lot of cussing. Yes there are
over two dozen of them - I found them all.

The 348 design is such that you can have several iffy caps and the set
will still work, sort of.

2) Note that the B- in the BC-348R is *NOT* grounded. It floats some
volts negative of ground. This is done because the 6K6GT output stage
also functions as a shunt regulator. Do NOT try to power the receiver
with B- grounded, because the last stage will draw excessive current
and possibly cook the audio output transformer.

The audio/regulator stage design is explained in the manual, and is
quite ingenious. You can mod the set to eliminate the feature, but I
always found it easier to use a power supply with ungrounded B-

3) The receiver isn't meant to drive a speaker. Not enough audio gain,
and pushing the set to do so makes it sound awful IMHO.

There are three options:
A) Use only headphones, as was intended. You have a choice of 300-600
ohms or 2000-4000 ohms by changing transformer taps
B) Modify the audio chain to a more-conventional design
C) Use an external audio amp, with an appropriate resistor load to
avoid frying the output transformer.

4) The dynamotor probably needs the bearings repacked. 60 year old
grease isn't much of a lubricant!

5) Probably your best option is to build an external AC supply. The
heaters will run on AC just fine - 24 volts at 0.7 amps (Note that the
6K6GT draws 0.4 amps for the heater, while all the rest of the tubes
draw 0.3 amps). The B+ should not exceed 220 volts, and the set will
work fine at 190-200 volts. With a little care, you may be able to
avoid wiring changes in the rx itself by making up a dummy chassis to
replace the dynamotor, and powering the rx through the rear plug, with
the AC mains switch on the power supply.

6) If the receiver design seems a bit odd, remember that it was meant
to be used in unpressurized planes like the B-24, under extremes of
heat, cold, shock and vibration.

If you need any tubes for it, let me know. I had three BC-348Rs at one
time, but sold them all more than a decade ago in a moment of extreme
weakness. (sniffs, looks away, rubs speck of nonexistent dust from
eye, sighs.....)

73 de Jim, N2EY