On Jul 3, 11:29 am, wrote:
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"?

Thanks for detailed reply. See below in your text after
*******************

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.

****************** Thanks, I now have one but I'd still like a look at
yours.

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.

************** I did not know that, I thought they were micas! Will
take a look and decide what to do.

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

************* Ah, ha. Well, mine is working but I don't yet know how
well.

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.

*************** Yes, I found this out early on when I saw that the
6K6 cathode went to ground. Smelled a rat and checked it out! The
6K6 stays properly biased with my ext. P/S.

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-

************** Me too, P/S is isolated. I have it running with
external P/S B- going to the wire that came off the dynamotor negative
and the B+ going to the wire that came off the dynamotor B+, and DC to
the heater with "76-A"still in. 6K6 bias about 14 VDC.

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.

************** I'm using a matching transformer from 300 ohms (LO) to
8 ohm speaker. I plan to install a proper OPT to match the 6K6 to 8
ohms but leaving the original there for posterity.

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

*************** See above.

C) Use an external audio amp, with an appropriate resistor load to
avoid frying the output transformer.

************** Not a bad idea... but needs etra bits!

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

************* Not used now but I may repack bearings to be sure

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.

************** That's the next step. My B+ is 220 VDC.

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.

************** I want to leave the dynamotor in the chassis "as is"
and have an external AC heater 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.....)

************** Thanks but, amazingly, all tubes test "good" to "good
plus"

73 de Jim, N2EY

*************** Cheers, Roger.

************ PS: I'll email you for the manual.