BC-348-0 Help Please

[wlindsay1984]

Hello Folks,

I have in my possession a US Signal Corps, BC-348, model O, built by RCA. serial No. 579. It has had the dynamotor removed and AC power supply installed. The valves seem to have the additional 6.3 volt rewiring done. A number of other components have been removed and substitutions installed. The 101ABC audio transformer has been removed, and it appears some sort of replacement has been put in its place which looks like a small choke. This choke has 5 pins. Does anyone know where these should be connected to? Two of the pins from the choke, pins 2 and 4, go to separate connections on VT-48, the audio output amplifier, the remaining wires, I pulled, when I had originally planned to replace everything and start from scratch, there also appears to be a small capacitor and resistor on this choke. The valve in VT48 has been removed and it appears a glass British manufactured one, a Mullard EL84, has been put in its place by the original modifier. On the resistor board, board no. 272, there is a missing 1,500 mmfd capacitor, are there any modern equivalents that could be purchased in Maplin etc?, I am new to all of this, and slowly learning, it is quite difficult. Also one or two of the 10,000 mmfd Micamold caps don't look in great condition either, is there a modern equivalent? I was hoping to replace the power supply with something a little more modern, I think it dates from the 1950's, one of the metal capacitors has a date of 1955. Any advice on building a modern custom power supply would be great. The set is also missing the So-143 plug.

Thank you all very much for your time and patience, I am looking to get this set in some sort of working order for a small museum I have been working on, dedicated to the U.S. Army Air Forces that were on a local station here during the war years, it would be great to be able to demonstrate the radio powered up.

Best Wishes

Will

[ms660]

That "choke" could well be the audio output transformer, if so then two of its connections (the secondary winding) should go to the 'phone jacks and then to the loudspeaker via the connector socket.

There should also be two connections for the primary, one should go to the anode of the audio output valve and the other to the main HT rail.

The primary could measure anything from 200 to 500 ohms (hard to be specific) the secondary would normally measure less than 1 ohm.

I think an equivalent for the original audio output valve might have been a 6K6gt, might be worth putting that back to original.

Can you give a circuit reference number or a location in the schematic for the missing capacitor? From your description it's a 0.0015uF (1.5nF) If that value is not available then two capacitors connected in parallel to make up that value, alternatively a 1nF or 2.2nF might do, it all depends on it's circuit function, hence the request for it's circuit reference number/location in the schematic

Suspect all micamould types.

EDIT: The resistor and capacitor on the output transformer will be there for tone correction, they are normally connected in series across the transformers primary winding.

Lawrence.

[wlindsay1984]

Hi Lawrence,

Many thanks for the speedy reply. Would I be correct in thinking, that the first two pins are the primary, and the secondary the 4th and 5th pin, there is no connection to the third on the choke?.

There are two 1,500 mmfd caps on resistor board no. 272, the first is connected to pins 2 and 18, the second one, which is missing, goes to pins 4 and 20. Its function is listed as the Audio Coupling Condenser.

There are 18 10,000 mmfd, Micamold capacitors within this model. A number seem to have seen better days. I seen someone use Sprague orange drops as a replacement, I think the originals were rated up to 500v, is it common to be able to use a rated value higher than 500v?, for example, if a 600v rated cap was available?.

Whoever originally had the radio has really stripped out a lot of the original components. I will get some pics of the original power supply also, maybe it can be salvaged for use?.


many thanks

Will

[ms660]

The way to identify the primary and secondary windings is by doing resistance checks. Most probably two connections for the primary winding and two connections for the secondary winding, eg: no tapped windings

The original transformer assembly also had a filter choke incorporated, the voltage dropped across the choke was used to supply the grid bias in the original set up, it would be best to see what (if anything) that choke was replaced with, if it wasn't replaced with anything then I'm guessing the modified audio output stage uses an automatic bias arrangement (cathode resistor) to obtain the grid bias.

Replacing a capacitor with one with a high rated voltage is no problem.

Lawrence.

[Colinaps]

http://hpfriedrichs.com/radioroom/bc.../pwrscheme.jpg

This bloke shows a reasonably competent PSU. Not sure about the exposed fuses though!

Cheers,

Colin.

[wlindsay1984]

Hi Guys

Many thanks for your help. I had a good look at the audio output transformer. The two outer tabs, tabs 1 and 5, appear to have a thinner wire going to the core, which appears to be insulated, the two inner tabs, 2 and 4, appear to have a thicker wire with no insulation, the central tab is not connected. So is this a step down transformer?. Would the primary appear to be tabs 2 and 4, and the secondary, tabs 1 and 5. If I recall correctly, I think tab 4, was connected to the Mullard EL84 on VT48.


Many thanks

(pic attached of transformer wiring)

Will

[Colinaps]

Hi Will,
The thinner wires will be the primary: high voltage, low current = high impedance to match the output valve anode. Thicker wires carry higher current and therefore low voltage to the speaker.
Cheers,

Colin.

Cheers,

Colin.

[ms660]

Not sure how familiar the OP is with valve stuff but here's the pinout connections for the EL84 to help figure out what's what around there:

http://www.r-type.org/exhib/aaa0028.htm

Lawrence.

[M0SOE_Bruce]

I have an unmolested BC348-O & manual. The output valve is a type 41. The majority of the caps listed in the manual are 500V mica, including the 0.01. Anything to do with RF circuitry was listed as mica, with paper type being mostly bypass & coupling.

The audio transformer secondary had 3 taps, common, 300R & 4000R. The 4000R tap went to the headphone jacks. As to the primary it would be the output impedance of a 41 valve (or close to it).

If you would like I can scan and send you a copy the the manual.

Bruce M0SOE

[wlindsay1984]

Hello Folks,

Thank you very much for helping me, I feel I have learnt far more in a few messages, than what I have done in weeks by myself. The fitting and wiring of the Mullard EL84, is rather different than what the schematic specifies for the type 41 wiring, this was done by the original modifier, its going to create a bit of a headache I think eep. Many thanks Bruce, good to hear of another O model out there, I have rarely come across them when doing research, I located a manual online, dated around 1948, which has some good information and schematics of the O model which I have been using to get by. Does anyone know where the black/white wire, from the 106 on/off switch, tab no.1, that originally went to tab no. 6 on the 101ABC Audio transformer goto?, since I do not have this 101abc component, is this connection just made redundant?, or is it connected to the choke type audio transformer in some manner?.

Many Thanks

Will

[M0FYA Andy]

There are some good scans of various BC-348 manuals available here http://www.jamminpower.com/main/bc348.html so maybe no need to scan another one.

Andy

[ms660]

Originally Posted by wlindsay1984

Does anyone know where the black/white wire, from the 106 on/off switch, tab no.1, that originally went to tab no. 6 on the 101ABC Audio transformer goto?, since I do not have this 101abc component, is this connection just made redundant?, or is it connected to the choke type audio transformer in some manner?

So far as I can make out, switch ref 106 is the BFO on/off switch, the wire from tab 6 of that switch was connected originally to the C section of 101ABC (the output transformer assembly) the C section was comprised of one capacitor, it would have been switched into circuit when the BFO switch was switched to on, it alters the time constant of the AVC line that controls the IF stage to suit CW operation, without it fitted AM reception will be ok but for CW reception it should really be fitted, there is no capacitance value given for that capacitor in the O version manual I'm looking at.

EDIT: But wait...hang on...I've got a hand annotated (not by me) schematic somewhere with the values shown for the Q version which I used to own...It gives the value as 0.05uF (0.47uF to you and me) so there it is...

Also regarding what I originally wrote in this post...I'm not absolutely sure if that capacitor is switched in the on or off position, perhaps someone else can check the schematic etc to clarify either way whether it's for CW or AM benefit, anyways for the cost of one I'd fit one unless it was already fitted during the modification.

Lawrence.

[ms660]

Ref my last post#12, looked at the schematic again, that capacitor is connected in circuit when the BFO switch is switched to on (as I originally said) Also the value in the annotated schematic was from the P version not the Q version, it just happens that the P and Q versions are shown in the same manual, apologies for any confusion.

Lawrence.

[Radio Wrangler]

Aircraft sets like the BC348 and R1155 were designed to feed a few headphones only and needed roughly a hundredth of the power a speaker takes, so a powerful valve really wasn't needed in the output stage. Headphones varied in impedance from 600 Ohms to several thousand. Speakers were usually 3 Ohms. This requires quite a different ratio of output transformer to a speaker stage.

Radio amateurs really wanted the ground-based receivers like the CR100, HRO and AR88, but they were expensive. The simpler, lighter airborne sets were available a lot cheaper, so there were hundreds of people beavering away, fitting mains power supplies where dynamotors once roamed, and adding audio power stages with output transformers to drive 3 Ohm loudspeakers.

The EL84 can handle a generous amount of power, and it has more gain than the WWII era audio output valves. Most modified radios will have been the recipients of types cheaply available on the surplus market rather than an expensive EL84.

If this mod was completed, then there must be an output transformer. It could never have driven a speaker without one.

It is possible that the transformer was mounted in the speaker cabinet. A number of domestic radios had their output transformer screwed to the back of the speaker's frame. If the modifier had got one of these out of a scrapped set, he might have run wires to the transformer on a speaker. In those days people were a lot less fussed about dangling wires with 250v on them.

I'm afraid you'll have to trace out the circuit of your set to find out what's been done. The manual only tells you what it once was, before it was changed, not what it currently is.

Post some photographs and people will be able to see what's there and help.

David

[G3VKM_Roger]

Hi All,

Just remembered that the BC-348 and other receivers were covered in a set of three "Surplus Conversion Manuals" back in the 50s, and these books are free to download, volume 1 covers the 348.

See:- https://www.google.co.uk/#q=surplus+conversion+manual

Part of the 348 modification article covers the rewiring of the valve filament chain, so that should be useful to help "back-engineer" your receiver.

Hope this helps,

Roger

[wlindsay1984]

Hello folks, here is a pic of the deck.

[ms660]

Looks like the original audio output valve 6 pin valve socket has been replaced with an octal valve socket, not that it matters.

Lawrence.

[M0SOE_Bruce]

The choke (101-B)that was mounted with the audio xmfr was a filter in the B- line. Note that the original B- was floating above chassis gnd. It provided bias for the audio output. With the changing of the output valve I suspect this area has also been modified. With so many mods though I can see it being difficult to follow the prints.

Here's a site that talks about restoring a couple back to original config.
http://www.radioblvd.com/bc348_dynamotor.htm

If you need photos of my "O" version let me know.

The manual I have is an early 1942 version.

Bruce

[ms660]

Here's an annotated schematic showing component values for the P version (first schematic in the PDF) Very similar to the O version, it might help.

Lawrence.

[wlindsay1984]

Hi everyone

Many thanks for all of your input and help, apologies for the late reply to you all, I removed the valve deck on Sunday, and only had a chance to quickly upload a pic as I was working on the radio all day. As much as I would like to return the radio to original spec with a dynamotor, I'm hoping to work either with what I have or what is currently available. I attempted to rewire the valves to 6.3v operation, using this schematic below, although I am a bit confused.

Below is the schematic for the original 28v dc wiring.




and below is a schematic for the ac 6.3v ac wiring.



On the 6.3v wiring sketch, the tabs on the valves have not been numbered, but I presume they are the same as on the first sketch, on VT86, there is an additional connection to make which is not numbered, I am guessing that tabs 2 and 7 are bus and go to ground. but what number is the connection above that?, which then goes to tab 9 on resistor board 274. The copy of the early wartime manual and photographs of the unmodified BC-348O would be great to see, many thanks for all of your help, I feel like I am getting somewhere at last.

Will