Wanted: BC-348R info
 

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.

Wanted: BC-348R info
 

Note to all:

The complete manual for the BC-348R can be downloaded from the BAMA
mirror site:

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

Scroll down and click on:

TO 12R2-3BC-112.pdf

(Maintenance manual for BC-224-F, K, BC-348-H, K, L, R. rev. 11
February 1955)

This is a 10 MB PDF, so it will take a little time. Lots of other
manuals there, too.

http://bama.edebris.com

On Jul 3, 10:16?pm, Engineer wrote:
On Jul 3, 11:29 am, wrote:
On Jul 1, 9:57 pm, Engineer wrote:
Those Dern Black
Plastic Capacitors. These are small units that look like micas but are
actually paper, and they are notoriously unreliable.

************** I did not know that, I thought they were micas!

Me too. They are reliably unreliable. That doesn't mean the
ones in yours will fail, it just means they have a bad reputation.

*************** 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.

You figured it out a lot quicker than I did!

? 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.

The trick is that the 6K6GT isn't biased the way it would be to
deliver a couple of watts as a speaker driver. Sure it will work, but
it will be running flat out.

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!

Put them in the PS unit and put the whole thing in a speaker cabinet.

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.

The problem with that is the dynamotor, if not running, will present a
near-short circuit to the power supply. And the 24 volt heater line is
connected to the dynamotor input, so it will be shorted too.

By simply removing the dynamotor assembly and putting in a dummy
chassis so the loose wires don't short, you can retain the rx wiring
and run from an external supply. Then if you ever want to drop in the
dynamotor and run from 24 volts, you can do that too.

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

Not amazing, really, If the set wasn't used much, the tubes could be
almost new.

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

I can send as an email attachment but it's 10 MB. You can also
download it from the above site.

73 de Jim, N2EY

Wanted: BC-348R info
 

On Jul 4, 11:20 am, wrote:
Note to all:

(snip)


************** 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.

The trick is that the 6K6GT isn't biased the way it would be to
deliver a couple of watts as a speaker driver. Sure it will work, but
it will be running flat out.

**************** I'll check that and, if needed, rewire it. I
noticed the screen voltage exceeded the plate voltage by quite a bit.
A 6K6 o/p stage is dead easy to brew up.

(snip)]


The problem with that is the dynamotor, if not running, will present a
near-short circuit to the power supply. And the 24 volt heater line is
connected to the dynamotor input, so it will be shorted too.

************** No, it is now totally disconnected

By simply removing the dynamotor assembly and putting in a dummy
chassis so the loose wires don't short, you can retain the rx wiring
and run from an external supply. Then if you ever want to drop in the
dynamotor and run from 24 volts, you can do that too.

************* B+ P/S is a separate Heathkit reg. unit and I don't want
to lose the dynamotor over the next 50 years! So. I'll just leave it
in the chassis for the next collector...

(snip)

Cheers,
Roger

Wanted: BC-348R info
 

On Jul 4, 1:10 pm, Engineer wrote:
On Jul 4, 11:20 am, wrote:

Note to all:

(snip)



************** 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.

The trick is that the 6K6GT isn't biased the way it would be to
deliver a couple of watts as a speaker driver. Sure it will work, but
it will be running flat out.

**************** I'll check that and, if needed, rewire it. I
noticed the screen voltage exceeded the plate voltage by quite a bit.
A 6K6 o/p stage is dead easy to brew up.

(snip)]



The problem with that is the dynamotor, if not running, will present a
near-short circuit to the power supply. And the 24 volt heater line is
connected to the dynamotor input, so it will be shorted too.

************** No, it is now totally disconnected

By simply removing the dynamotor assembly and putting in a dummy
chassis so the loose wires don't short, you can retain the rx wiring
and run from an external supply. Then if you ever want to drop in the
dynamotor and run from 24 volts, you can do that too.

************* B+ P/S is a separate Heathkit reg. unit and I don't want
to lose the dynamotor over the next 50 years! So. I'll just leave it
in the chassis for the next collector...

(snip)

Cheers,
Roger

PS
Heaters now powered by a 24 volt HVAC control transformer - connected
between chassis and pin 2 of the 1st IF tube, VT-86/ 6K7 (bypasses
resistor "76-A".) BTW, these 24 volt HVAC control transformers are
easy to find as scrap units (look in old house demolition bins.) I had
two in my junk box - one 12 VA (not enough) and one 40 VA (too big but
OK, so I used it.) You need 17 VA minimum.
B+ remains from a separate vintage Heathkit regulated tube supply -
one of 3 such items here, so I can dedicate one to the BC348. My big
28 VDC supply is now surplus!
Next things to do:
1. Install a terminal block and a 4 conductor power cord to the
external P/S's (DC and AC)
2. Check all those old paper caps I was warned about...
3. Check the alignment, tweak as needed (the manuals have full
instructions for all those 1940's radio techs - RIP, most of them!)
Support the military! (Who is watching "Dogfight" on Discovery
channel?)
Cheers,
Roger

Wanted: BC-348R info
 

Like your idea of leaving it 'as original as possible'. While having
it work for today using mains electric power. Good for you.
Just a thought the original system worked off 28 volt aircraft
batteries. And of course the aircraft 28 v engine generators.
Batteries have a low impedance and might have done much to 'shunt'
brush noise of the various dynamotors? Could you try adding more shunt
filter to your 28 volt supply just to see if it could reduce the hash;
presumably coming back from the input brushes of the dynamotor? Or if
from brushes on the output side check hash filter caps there?
Finally could you unplug the dynamotor and have a cable from external
PS plug into that socket? No other changes whatsoever! Retain
dynamotor (perhaps attached to PS?) for future re-installation.
Good luck with your project.

Wanted: BC-348R info
 

Terry, see below in your text after ************

On Jul 7, 2:53 pm, terryS wrote:
Like your idea of leaving it 'as original as possible'. While having
it work for today using mains electric power. Good for you.

************** Thanks. We are only temporary custodians of "our
stuff".

Just a thought the original system worked off 28 volt aircraft
batteries. And of course the aircraft 28 v engine generators.
Batteries have a low impedance and might have done much to 'shunt'
brush noise of the various dynamotors?

************ True, but my 28 VDC supply is also very low impedance.

Could you try adding more shunt
filter to your 28 volt supply just to see if it could reduce the hash;
presumably coming back from the input brushes of the dynamotor?

************ I don't think that is happening
Or if
from brushes on the output side check hash filter caps there?

*********** Ditto.

Finally could you unplug the dynamotor and have a cable from external
PS plug into that socket? No other changes whatsoever! Retain
dynamotor (perhaps attached to PS?) for future re-installation.

*********** No, I can't risk the physical separation - the executors
of my estate won't go looking for the dynamotor! Neither do I want to
rewire the rear P/S socket - keep it original. My solution was to
installed a new 4 screw terminal plate (2 new small chassis holes...
sorry, can't help that!) with a captive power cord to the external
supplies. The dynamotor wires were just lifted carefully from the
original 5 terminal block to the new 4 terminal one (two are shorted
together in the original - see schematic. As we all now know for the
BC348, the external B- is NOT to chassis due to the odd biasing of the
6K6 o/p stage (it's about 14 VDC negative to chassis), so keep
external B- isolated!

Good luck with your project.

************* Thanks, keep 'em playing, cheers, Roger.

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