Hi, Military Vacuumlanders,
I'm more a denizen of RAR+P and RAT but I could use some help with a
BC-348-R "boatanchor". I acquired this eons ago but am only now
getting around to refurbushing it. It's totally original with the
dynamotor and it actually works but I'm not sure how well (I can get
some stations on a short wire antenna but there's lots of harsh hum on
the "long wave" band). BTW, I brewed up a 28 VDC bench supply from
which to run it. A long time ago I downloaded a few manuals in a
hurry but did not do a thorough check on the version - so I've got
documentation for versions J, N and Q, but not R. It seems that there
are several different schematics and tube line-ups for BC-348's. My
schematic is for a BC-348Q and it's not at all the same as for the "R"
- different tubes for a start !

By my forensics, the BC-348R has the following tube line-up:
Military tube Civilian equiv.
Function
VT-86 6K7 1st RF
VT 86 6K7 2nd RF
VT-91 6J7 Mixer
VT-65 6C5 Local Osc
VT-86 6K7 1st IF
VT-70 6F7 2nd IF & BFO osc
VT-93 6B8 3rd IF & Detector
VT-152 6K6 Audio O/P

(I hope this "table" holds together!)

Can anyone point me towards a schematic for the "R"?
All replies gratefully received
Thanks & cheers,
Roger

"Reclaim, repair, refurbish, rebuild, reuse, recycle"
Above address is a spam trap...
Any replies to: analogdino "at" rogers "dot" com

Try: http://bama.edebris.com/manuals/military-kg7bz/bc348/

The BAMA alternate site (above) indicates a manual including the "R"
version.

de K3HVG

Engineer wrote:

Can anyone point me towards a schematic for the "R"?
All replies gratefully received
Thanks & cheers,
Roger

On Jul 2, 5:53 am, K3HVG wrote:
Try:http://bama.edebris.com/manuals/military-kg7bz/bc348/

The BAMA alternate site (above) indicates a manual including the "R"
version.

de K3HVG



Engineer wrote:
Can anyone point me towards a schematic for the "R"?
All replies gratefully received
Thanks & cheers,
Roger- Hide quoted text -

- Show quoted text -

Many thank, K3HVG, but I have this one, viz:
"Maintenance manual for BC-348-J, N, Q. rev. 21 June 1948."
....but it is for models J, N and Q only.
The "R" has a different tube line-up (see my OP).
Cheers,
Roger

Engineer wrote:


Many thank, K3HVG, but I have this one, viz:
"Maintenance manual for BC-348-J, N, Q. rev. 21 June 1948."
...but it is for models J, N and Q only.
The "R" has a different tube line-up (see my OP).
Cheers,
Roger

As K3HVG *correctly* noted - the BAMA site has the schematic for the R
(as well as the others). For most purposes - any schematic that
includes the 991 tube (which isn't in your list) will be usable for the R.

--
Sherry

On 7/2/07 9:21 AM, in article ,
"Randy or Sherry Guttery" wrote:

Engineer wrote:


Many thank, K3HVG, but I have this one, viz:
"Maintenance manual for BC-348-J, N, Q. rev. 21 June 1948."
...but it is for models J, N and Q only.
The "R" has a different tube line-up (see my OP).
Cheers,
Roger

As K3HVG *correctly* noted - the BAMA site has the schematic for the R
(as well as the others). For most purposes - any schematic that
includes the 991 tube (which isn't in your list) will be usable for the R.

The AN which is said to include the R model, does not show a 991 tube.

Don

OK, Roger... I checked with the site again and your receiver, the "R"
model is listed as one of two manuals included. Please have a closer
look, under TO 12R2-3BC-112.pdf
The manual cover sheet and contents clearly state the "R" model.

Jeep/K3HVG

Just to make doubly certain, I re-loaded the above ref manual on BAMA
and compared it with my original AN-16-40BC224-2 dated 20 JULY 1945.
The ref des for the regulator is 125. In the BAMA manual, its located
on the last schematic sheet part and in the parts list as 2V991.

Jeep/K3HVG

K3HVG wrote:
OK, Roger... I checked with the site again and your receiver, the "R"
model is listed as one of two manuals included. Please have a closer
look, under TO 12R2-3BC-112.pdf
The manual cover sheet and contents clearly state the "R" model.

Jeep/K3HVG

On Jul 2, 3:52 pm, K3HVG wrote:
Just to make doubly certain, I re-loaded the above ref manual on BAMA
and compared it with my original AN-16-40BC224-2 dated 20 JULY 1945.
The ref des for the regulator is 125. In the BAMA manual, its located
on the last schematic sheet part and in the parts list as 2V991.

Jeep/K3HVG



K3HVG wrote:
OK, Roger... I checked with the site again and your receiver, the "R"
model is listed as one of two manuals included. Please have a closer
look, under TO 12R2-3BC-112.pdf
The manual cover sheet and contents clearly state the "R" model.

Jeep/K3HVG- Hide quoted text -

- Show quoted text -

Many thanks... and to other posters and emailers, too.
I have retrieved the "R" schematic! Now the harder work starts...
My plan is to keep it as original as possible but find a way to take
the dynamotor "off line" (to preserve it in the radio for "all
time"... and to cut the mechanical noise), then run the B+ from an
external regulated tube supply and the heaters from an external 4 x
6.3 = 25.2 VDC supply - I can adapt of my 28 VDC dynamotor supply,
but now only 0.7 A total. I'll go the other side of resistor "76-A"
that appears to drop 28 - 25.2 = 2.8 volts for the series/parallel
heater group (resistor "76-B" accommodating the higher (0.4 A) 6K6GT
heater current.) On second thoughts, the 25 volts could now be AC,
even easier (perhaps using a recycled s/s receiver power transformer)
- the 28 VDC supply staying untouched for "dynamotor days"!
Very best,
Cheers,
Roger

"Reclaim, repair, refurbish, rebuild, reuse, recycle"
Above address is a spam trap...
Any replies to: analogdino "at" rogers "dot" com

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

wrote:

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.

I'll second the motion from N2EY, except that I have good success using
LS-3 (Hi-Z) speakers and setting the receiver output Z to the correct
tap. The LS-3 is not intended for the BC-348 but, as with the '312 and
'342, they work just fine.

de K3HVG