BC-348 FAQ

Feb 161999

BC-348 FAQ

by Buzz Harrah, KE0MS
February 15, 1999
Originally posted to the MilSurplus Mailing List
Reprinted with permission

Calling all MIL-itants,

Recently you all came to my rescue when I needed “Fatherly” advice on a BC-348 I’d found. Thanks to your info, I’ve worked a deal with only pickup yet to take place. Your pricing information especially helped the deal “gel”.

I was asked by several thru direct mail if I could gather together my info and publish it for all the other BC-348 “wannabe owners” out there, kind of as an FAQ or something. I got almost 2-dozen responses over the weekend to send it, so, (not knowing how many are on this list) I decided it’s easier to let you all get it and judge for yourself if you need it. Delete it if you don’t.

All of this info was contributed by YOU who responded, the members of this list. You may recognize some of your comments. I moved them around to most logically answer the questions posed in the FAQ I came up with.

However, this info is presented to you AS IT WAS PRESENTED TO ME. I could not/did not attempt to verify every fact I received. (How could I?) And, you all know the “BA-Mantra”, imported from another source: “YOUR MILEAGE MAY VARY.”

As we all have a perspective, I included all the pertinent data I got and grouped in into the general categories you will see.

Enjoy. I did reading it as well as cutting and pasting it again. And save it; as I think it’s a good BC-348 primer.

Buzz
http://www.ia.net/~harrah/
harrah@ia.net

THE BC-348 RECEIVER- SO, WHAT IS THIS THING?

The BC-348 was the main HF receiver in US Airforce bomber aircraft in WW2, in conjunction with the BC-375 and ART-13 transmitters. The original design was the BC-224 (US Army Signal Corps) and the BC-348 was the adaptation for the requirements of the Army Airforce. The receiver as issued, runs on 24VDC (the standard DC voltage on aircraft) and generates the required plate voltage (around 250VDC) with a dynamotor which is inside the receiver.

The BC-348 was my first “real” HF receiver in 1964 when I first got my ham license and worked about 100 countries with it. Now I have 2 of them, with an ART-13 transmitter, to functionally duplicate the B-17/24/26 type bomber radio compartment and I use that station quite often, mainly on the East Coast military collectors’ net on 75 M.
These are pretty fair radios for AM and CW use. Not real selective, but they do have a xtal filter and with an added Q-Multiplier, they do nicely.
Be prepared to replace every coupling/bypass cap. Not a hard job as most are visible and usually strung between two posts on a phenolic board. The filter caps may need help also as will some of the resistors. I did a complete cap changeout and found only 3-4 resistors out of tolerance. The whole job took a few hours, but she played nicely when I finished.
There were many other modification articles and books out regarding the BC-348, so you can expect the receiver might have some or all. One of mine was completely rewired by a previous owner, redone top to bottom, and equipped with an AC power supply. Oh, yes it is also painted bright shiny lime-green with gold Dymo labels. It worked quite well, and after awhile the looks grew on you (like a fungus?) so I never changed it. The other one is more original.
The BC-348 is a great receiver, a little broad in the selectivity department, but a neat way to cruise the bands! My first exposure to shortwave came from a BC-348 so the radio has a little sentimental value to me.
My reason for knowing is that I had an unrestored one in High School (a loooong time ago). It still had the dynamotor.
Ah, the BC-348! One of my first military BA’s. Used as a liason receiver in B-17’s and some other aircraft. We gave our boys good stuff.
Single largest problem with all the radios of this vintage is paper capacitors. The ‘348 O uses about 18 or so of them and they’re all subject to failure. Normal procedure is to replace them all, check tubes, etc. and an alignment. If the radio has seen regular use, vs. sitting for years they seem to last much better.

THAT FUNNY 117VAC WIRING DYMO THING

Q: OK. This had one of those nasty DYMO label-maker-sticker-tapes saying “117V WIRING.” It plugged directly into the wall. Was this unit usuable for different voltages with a tapped xfmr arrangement, or had he modified his do you think?

The BC-348 was primarely an aircraft receiver with the dynamotor as the only INTERNAL power supply. There were external rack mount 110 vac supplies available in the military to run the radio on the ground. This is probably a homemade conversion.
When these receivers got to the surplus market, the first things hams did, was to yank out the dynamotor and build a small power supply that mounted where the dynamotor used to sit, for 115VAC to supply the filament and plate voltages to the receiver.
BC348s were aircraft receivers and used a dynamotor for the high voltage generation. He modified it by removing the dynamotor and replacing it with a power supply.
Sounds like the BC348 has been modified enough to allow for a built-in power supply. This was common practice when these things came out on surplus market.
I do know there were some mods that allowed 110 volts, but most radios were just changed by removing the dynomotor and installing a small power supply. I have not put a 110 supply in mine because there is no way I can see to get the 110v into the case with out some type of hole or rewiring the power plug, so for now I run it off the dynomotor and it is a nice little radio with plenty of volume, but the dynomotor is a little loud. I guess that wasn’t a problem with 4 1200 HP engines running.
The “117 wiring” label you noticed probably indicates that someone modified the unit to have a 117-volt AC power supply inside. The original BC-348s used a dynamotor to convert low-voltage DC (28v, 3a for DM-28 dynamotor typically found) to the voltages needed by the receiver. The open space intended for the dynamotor is a good place to build an AC supply and many hams did so.
Lots of them around, and there was a 117 volt model, but most were modidifed to run on 110 by hams – they took out the dynamotor.
Second, all BC-348s were bought by the military as 28 volt aircraft radios, there were no military versions that operated on AC. There was a 12 variant called the BC-224(actually as its number implies, the 12 volt version came first, the 348 was the 28 volt version, but a heck of a lot more of the 348 were built). All the AC conversions you run into are most likely to be done after they were released from military service. Note that a lot of the 348 were bought by the airlines right after the second world war, so some of the conversions were probably very professional looking, most of the ham conversions that I have seen were very sloppy.
There is a rumor, but no hard evidence that I have seen, that Hallicrafters specifically made a drop in AC supply that replaced the dynamotor, this would be easy to do mechanically-it would just be a screwdriver operation, but if this is true there may be a concern that one side of the AC line would be tied to the chassis-a definite safety concern.
The AC supply was a common mod and there were many variations. They were normally built onto the same chassis that the dynomotor was mounted on. If you can get the original mount bracket with shock mounts, that is a real plus.
I have a BC-348Q that is in great shape. Still has the dynamoter in it, and the ac supply is very neatly tucked up underneath it.
Normally they were 28VDC with a dynamotor. You can bet it was modified.

THAT OUT-OF-PLACE-LOOKING 2 X 6 PANEL

Q: The rig had a approx. 2″ x 6″ panel screwed on the right side, centered, covering what may have been some sort of — what? Was there a knockout on these rigs that allowed adding something?

This pannel is standard equipment on the BC-348. It provides access to the underside of some of the tube sockets. They ALL have them.
The panel you describe allows one to reach the tube sockets and some capacitors. Designed to look “plain” with 6 screws holding it down.
I have a BC-348Q and it is complete from the box(no mods) the 2X4 panel is to get at the tube bases for alignment, checking etc.
The plate you mentioned is original, it is just an access plate to allow service on components in that area. Once you dig into the radio, you will see why they put it there. It was also a good place for hams to mount extra pots and switches.
This is quite a nice receiver when working properly. The plate you mentioned on the right side of front panel is used to cover the socket connections for several of the tubes. The chassis is an aluminum CASTING, neat stuff for 1943!
That plate on the front panel is supposed to be there and allows access to the bottom of the tube sockets mounted along the front panel.
It allowed access to the RF and Mixer stage components under the tube sockets.

VARIANTS OF THE BREED

Q: Were there several variants of this rig? This one went .1-.4 MCs, Then went from 2- about 20 MCs if I remember right. Skipped the BC band.

BC-348 Receiver, 200-500kHz and 1.5-18mHz, six bands, AM/CW, crystal filter, 915 kHz IF, BFO, MVC/AVC, 28v, 3a for DM-28 dynamotor, 10.5″x18″x9.5″, 44lbs, variants:

Model	Manufacturer	Tube line-up
E	RCA	41, 6B8, 6C5, 6F7, 6K7 (3)
H	Belmont Radio	6B8G, 6C5, 6F7, 6J7 6K6GT, 6K7 (3)
J	Wells-Gardner	6SR7, 6K6GT, 6SA7, 6SJ7, 6SK7WA (4)
K	Belmont Radio	same as H
L	Belmont Radio	same as H
M	Stromberg-Carlson	same as E
N	Wells-Gardner	same as J
O	RCA	same as E
P	RCA	same as E
Q	Wells-Gardner	same as J
R	Belmont Radio	same as H
S	RCA	same as E

[different mfrs got their own variant letters as they sometimes used different tubes and had different wiring schemes.]

Yes, there were some variants, frequency wise, but the one you describe is standard. (Seems that they ALL covered basically the same bands, but variants involved manufacturer differences, or layout differences BECAUSE of different manufacturers. The bands they covered (by my info) seems to tell me they ALL skipped the BC band.)
The letters (L, Q, S to name a few) in the variants, to keep it short, denote some tube differences (for example some models use 12K7 for the RF amp while others use the 12SK7, etc.) The performance is the same for all models. They all had the VLF band, for picking up Navy distress signals.
They came in several models, but are all basically the same circuit wise. Mine is a BC348R and is one of the older models. Uses double ended tubes. The newer models use single ended tubes. The 348Q is one of the newer ones and is seen often for sale.
They do skip the BC band. Guess the AAF didn’t want the pilots listening to the radio while attacking Jerry……..
To answer your questions though, the frequency range is .2 to .5 and 1.5 to 18 mc/s, although a very few of the early ones did not have the .2 to .5 band on them. There were a few ham conversions that rewound the low freq range for 10 meters or the broadcast band, but these are all ham conversions, not done by the military.
there were two major versions of the 348. Externally they were interchangeable, operated the same, covered the same frequencies, etc. The J, N, and Q series were made by Wells Gardner and were built differently internally than the others, a different tube lineup and some mechanical differences, plus a different arrangement for the CW OSC on off switch on the front panel.
Apparently the ID tag is still in place, and that will tell you what series the ‘348 is…..there were many variations with different suffix’s. Mine is an O, which was made by RCA. The series suffix is important because the circuits were different as was the tubes used. Some models were very similar and some were markedly different.
The Broadcast band is skipped in all of them I think.
Several models but they all covered the same freq range as you stated.
Did find a Schematic in the old CQ Surplus Manual – but there wasn’t anything else in the mag just a Schematic. I did get a TUBE line-up though.BC-348-R
.1-.4 MCs
.950 -> 18 MCs (915 KC IF)

1st RF VT-117

2nd RF VT-117

1st Det/Osc VT-150

2nd Det/avc/CW osc VT-233

1st IF VT-117

2nd IF VT-117

3rd IF VT-116

Audio VT-152

PRICING THUMBNAILS

Editor’s note: My unit described as “kinda dirty”, but should clean up ok; but no obvious mods other than 117VAC mod. The 117VAC modification was the “ONLY” MOD that seemed to not significantly detract from the desirability of buying one of these units. While the original DYNAMOTOR was preferred by the masses, this mod is so common that it was felt (or at least I detected) that getting one that HADN’T been modded in this way was highly unlikely and should NOT be cause to pass a unit by.

Also, any comments relating to EBAY are indications of PRICING PHENOMENON in today’s market ONLY, and do not constitute an endorsement of EBAY selling or indicate that EBAY sold-equipment is better. It simply indicates that more is paid for these rigs on EBAY than at the normal hamfest-type outlets available.

Q: What is one of these things worth? (ballpark estimates are fine.)

I got one of these beauties (BC-348-R) and it was converted to AC and works very well/ Going prices range from $80 to $200 depending on condition, etc.
Typically bring $70 to $150 at swap meets, depending on condition, but sometimes up to $200+ on the Ebay internet auction site.
I paid $45 for mine. Seen them on e-bay for up to $150. A modified one in fair condition should be about $35-50. A pure, unmodified, original condition one with dynamotor might go for $175 or so and in my estimation would be worth it.
The case should have no other holes, just the cut out for the 8 pin power-audio plug. There are several sites that have the manual on-line. As for price I would say $150-200. E-bay has changed the price of things lately. I paid $100 for mine about a year ago.
Value – anywhere up to $140.00 depending on condition. Most go for $50 – $75.00 at flea markets, $75.00 up on the internet.
As far as price, the typical price I have seen for a ham modified unit with no added holes on the panel, an AC supply and missing the shock mount and connector that mounts on the shock mount, is about $75. Less than that for added S meters, front panel switches and other mods, more if it has the shock mount, connector and original dynamotor. I would say that a military original complete unit would be about 150, but I haven’t actually seen one of these offered for sale so that’s just a guess.
The BC 348’s go – depending on condition and the owner’s willingness to part with them – for anywhere between $50 for a heavily modified or good parts unit to $150 for a very good, unmodified radio. It may be even a tad more if the receiver is mint, as issued and has the original dynamotor in it. The dynamotors are scarce and sometimes cost almost as much as the radio itself. I personally paid $100 for each of my BC-348’s, with homebrew AC supplies in them (good workmanship, however) and good cosmetic shape, in working condition.
$50.00 to $100.00. Sounds tike your find would be on the low end.
I have seen them (348s) occasionaly for sale, and I think that Fair radio had some a few years ago. I seem to remember they were around $100. Could be wrong, but I think thats right.

TO PERSUE OR EVADE?

Q: Can anybody help me with specific info, advice to persue or evade, etc?

GET IT………. you’ll love it and you’ll have a piece of history.
They are OK radios, but modern radios are MUCH better. On the other hand if you are a tube nut, then they are a must have along with an R390A.
Just remember that SSB was not in use when they were used, that came later. Even the R390A receiver did not support SSB.
If you like old tube collectables, then it might be right for you.
I have a BC-348Q that is in great shape.
All in all a neat radio.
I’ve had several over the years and always enjoyed modifying, using, and abusing them.
I got mine for repairing a BC radio for a friend, cost me a resistor, two 40 UF 450 volt caps, and two diodes. Had to solid state an old Circa 1930s radio. The rectifier tube had a burnt out filiment. No possible spares. I was given the BC-348 even thought I didn’t really want it at the time. Now I wouldn’t mind having it here [while away from home].
It is a lot lighter than my R-390A!
Looks like I will be able to buy 2 BC-348s this weekend. Matching transmitter too. There are Antenna tuning units too.

Editor’s note: I got almost nothing on Antenna tuning units, other than it is believed there are several, as part of the “Liason Sets”, using the ART-13 transmitter, of which this unit is part.

ADDITIONAL REFERENCES AVAILABLE

There is a ton of information at http://netnow.micron.net/~kj7f/boatanch/bc348.htm check it out.
Contact W7FG at www.w7fg.com for a manual. Only $18 and a great investment.
I can find the exact model /manufacturer/tube lineup list for you if you need it, I just have to look into some of my Internet links. mbendror@villagenet.com
Look for a schematic in the gentlemans files. Also, the Surplus Conversion Books from the late 40’s and up thru the early 60’s have tons of info on this radio.
I have some of the CQ Magazine books on surplus equipment mods for hams, so if you see something that doesn’t match the original schematic we might be able to figure out what it was intended to do. (SBJohnston@aol.com) More on these in a bit…
There is some good info on BC-348s at the Military Commo List site(s):
http://www.telalink.net/~badger/millist/mi.html
(specfically) http://www.telalink.net/~badger/millist/m7.html#a1253
There is a lot of information out there in old QST’s, 73’s, and the Surplus Conversion Manuals I, II, and III……..
Fair radio does have [copies] of manuals for all of the models for $12. (http://www2.wcoil.com/~fairadio/)
The CQ Conversion Manual listed sources for ‘Conversion Data’: CQ MAGAZINE:
September 1956
February 1959
March 1959
http://netnow.micron.net/~kj7f/boatanch/bc348.htm
http://www.aade.com/hampedia/military/military.htm
http://www.qsl.net/wf2u/
http://www.qsl.net/wd8das/

FINAL CLOSING THOUGHTS:

This kind of sums all this old MIL stuff collecting up pretty well. I’d made an off-hand comment on how excited I was finding this little receiver, and here is the comment that I got back. I couldn’t have made a better “straight” guy on Vaudeville!

Q: Thank you for all your answers. Very interesting reading. And, this “mil” stuff is starting to grow on me.

A: That’s what that ‘MFP’ coating was for inside the old gear :-) :-) :-)

73 Responses to “BC-348 FAQ”

Jack says:

2017/11/05 at 01:33

HI Goran…I sent Paolo some schematic extracts. He didn’t actually say the set worked or didn’t work as I recall. If the surplus manual said ‘across those contacts’ it was asking for B+ and the other lead to the 2nd RF anode….I told him better to get rid of the meter….when there were real hams doing real experiments yes they couldbe ‘handy’and nomorethan just ‘handy’……but today with glorified CB operators who don’t like this and don’t like that and have no tradiiton or skills other than buybf stuff off the shelf or elsewhere and no technical brains no one needs an s-meter…actually I have only used them for direct comparisons but even then there are second to second atmospherics and noise issues. I suggested he find another cover plate and very finely cut it to cover the hole, use tiny black screws and then fit an RG gain pot there. I have a terribly modified set which I have to repair a piece at a time but as I am flat out with my Master’s degree I have little time and much fatigue. Your advice is good and he can always try a different circuit…I’d just snip it out….also they reduce the value of a set to maybe 10-20% of one without it. He can also if he wants by a replacement plug and refit it….there’s a couple of people selling them on eBay..more modern but can be fitted is the original bracket is still there.

Paolo…does your set work?…forget the meter…..Is it working as a radio?
Paolo Gramigna IK4YNG says:

2017/11/05 at 04:42

Hi Jack & Goran (and all the others…)
First of all, thank you for the kind advice.
I think I shall present myself: I’m IK4YNG, I’m 71 years old and now retired.
I have a fair laboratory, most of the instruments are old but working well (as myself, I presume) including HP RF generator, HP Sweep generator, oscilloscope and so on.
I’ve been involved, many years ago and while in the Army, in the activity of servicing all kinds of military radios, all vacuum tube of course.
So now I’m spending a lot of time in servicing and repairing what I find interesting.
I stumbled on a BC-348-J on eBay, for a good price; I knew it was not in mint conditions, but I cannot afford to buy perfect items on my retirement plan (my wife is holding the family purse with an iron grip…) so I started to recover it.
I’m not planning to perform a “philological” restoration, my goal is now to have it working properly; I want to place it in my studio, to listen to the sparse remaining SW stations.
To begin with, it was a mess; Dynamotor replaced by an AC power supply, handmade with electrician’ wires connected and insulated with sticky insulating tape; one wire missing in the CW switch cabling; bad electrolytic capacitors… But now the BC-348-J is on and working; at least, I can hear several stations on the 12 Mhz band, and that was a relief!
I still have to perform an alignment; I downloaded the manuals, and I have the instructions; so probably I’ll be able to follow them.
One of the previous owners removed the external connection plug completely and rewired that part of the circuit.
A big hole was drilled in the upper right corner, and a quite nice instrument was placed there, but the relevant wiring was not installed at all.
So, before trying to close that hole, I decided to try (at least for a while) to make it work.
I have the schematic that Jack sent to me. The problem is that I don’t have access to pins 2 & 6, because of the above-said rewiring.
So I painstakingly traced the circuit, and probably I have found where the modification was done.
The connection to B+ is simple, but I still have doubts about the Second Grid connection, because I don’t know if I have to connect directly to the pin 6 of 2nd IF, or after the (97-2) 35000 Ohm resistor, originally going to the Pin2 of the power plug. Instead of that resistor, all I have found is a 50000 resistor connecting Pin 6 of 2nd IF (and pin 6 of first IF, of course) directly to the B+bus.
Now I’ll spend a little time trying to install that S-meter circuit, and in any case, I’ll proceed with the alignment.
I’m planning to find the exact frequency of the Xtal filter, and align the IF on that frequency. I suppose I’ll have to inject the RF generator (trough a capacitor) to the plate of VT-150 and trace it to the diode of VT-233. Am I correct?
For the moment, I thank you for the encouragement and advice.I’ll keep you posted!
Cheers,
Paolo
Jack says:

2017/11/05 at 12:36

Hi Paolo, was that the one for sale on ebay about 6 weeks ago or thereabouts…I have some photos of that one.
I recall it showed some alloy showing on the LHS of the meter. The incoming power lead was an abomination but…
everything can be fixed one way or another (wry smile!!) . Does it work at all?…

I like the sets but only have two.Each is in nice shape physically but one looks as though modified by ‘metaphorically’
a blind man with one arm.

I know there’s some logic in it but it’s not easy to see…He also (aaaagh!!) snipped the original power supply terminals
and soldered the stumps or left them in mid air with a bit of electrical tape as insulation. No other terminals look
‘right’ on the power terminations. His wiring is a dog’s breakfast’…but that’s part of the eventual joy of
rebuilding/restoration although I doubt many of us can replicate factory quality.

I’m same age as you but at Uni to compete in ‘the market’ (and for which I feel a bit guilty) for another 10-12
years work.Good on you with your workshop….it’s useful to have tools which will work to the specs of the
BC348 era.

Whist I believe the set is short on enough stability to install 80kHz IF’s, a BC 453 is a good hook-up for the 348
Cheers

My Regards
Jack says:

2017/11/05 at 15:07

Oh..ok so it is +/- working…I’d actually check the values of non electrolytics before replacement before alignment, so
as to find stability and best signal and the mica’s are the more likely to be crook… I suppose a start with all those
outside the coil boxes and enjoy the receiver for a while before a refurbishment….(you may have to get your wife in a
step-over toe hold!! “sign the cheque…sign the cheque..”…or take her out for a nice meal, plenty vino and then go lifting jar
lids etc. to find the hidden stash!!…as she slumbers, remembering how you used to court her…..”you spend too much Paulo..
tonight I will cook for us, save your money… ..no no no amore….tonight we go to Sibilla, next time I have leave, to Reale…
and look I have some pretty earings for you…look, incantatore mio….put them on …yes spin around under the lamp….
ahhhh…bella..bella ..mi hai stregato…. ).

Back to our reality!!…I believe the resistors need checking in 348’s…I read sometimes or wide discrepancies and
occasional o/c’s and command sets. I’d serious leave alignment until all else is done…same as when you were in
the army….and like any set sometimes an IF slug can be broken… or the threads ‘bound’ just through age…at
least that is repairable and it’s a problem sometimes found in the 348 BFO. Some heart-warming info here
“http://www.eham.net/reviews/detail/6014″…Some GDO’s might have provision for measuring crystal frequency
and be close enough…however I have also read of the crystal being dirty and needing careful cleaning. ARRL
or RSGB handbooks may have crystal oscillator circuits for reliable measurement…I do recall one using a 6C4
but to be honest about it I have never yet checked a 348 crystal but may yet have to….what a pity the BC 221
didn’t have such facility …..but smarter blokes than I am may have devised a way to use that marvellous sig
gen to do. Here…in part for irony is a very sophisticated one. I doubt I’d build to check a BC348.I’ll find something….

http://www.arrl.org/files/file/QEX_Next_Issue/Nov-Dec_2013/Harris_QEX_11_13.pdf
Cheers
Jack says:

2017/11/05 at 15:17

On GDO’s and crystals…this might be interesting, stating measuring to say 0.2% in circuit ….and by the way the sophisticate version at the end of my last response also had a basic circuit…Ciao
Jack says:

2017/11/05 at 15:18

and I forgot the URL….I’m “losing it”..https://g4rvh.files.wordpress.com/2008/08/waht-you-can-do-with-a-dip-meter.pdf
Paolo Gramigna IK4YNG says:

2017/11/05 at 15:30

Hi Jack,
Probably it is the same, I bought it indeed on eBay about 6 weeks ago!
You know, I was looking for a BC-348 from a long time ago, but here in Italy, you can get either collector’s item at a horrendous price (more than 600$) or junk item, missing even the knobs.
I noticed the S-meter, the power cable and other blemishes, but the price was interesting even after adding the shipping cost.
The package was very good, and when I opened the BC-348 I found that at least all the major parts were there, tubes included.
Now, after a few days, I’m at least quite happy.
As I told before, the AC power supply was installed with electrician’ tape, all wires flying around. First of all, I decided to repair the BC-348, not to restore it. That means using modern spares, not the original ones, beginning with the electrolytic capacitors. I want to keep it in my lab for listening SW broadcast, not as a display unit.
Quite soon the radio started to receive stations, but the BFO was always on, and inspecting the double switch I found only three wires on it; quite strange for a double switch. No trace at all of the missing wire, that was replaced according to the schematic, and finally, the BFO was off.
Then I decided to try to have the S-meter working. It had two wires connected, flying down and interfering with the gears, and ending in nothing. I know that the S-meter is useless, but a black patch on the front panel is worse (in my opinion) so after a good day of attempts now it’s indicating something related to the signal strength. I followed the schematic I have got here, but I had to adapt several values.
At that point, I attempted a calibration; I did not use the sweep generator on the IF, for the moment, but found the Xtal frequency (910,8) and peaked the IF to that frequency.
I found that the IF was all awry, two to three turns out in most IF.
Then I went to the trimmers, and also they were way out.
Now I have a sensitivity in the order of some microvolts on all bands.
What is remaining?
Well, It should be a good idea to replace the power transformer, since we have 220 volts here. I have one that will fit with three millimetres to spare…
Then I shall protect all those B+ parts (the diode rectifiers, for example) that are sticking out, with 300 volts on them. I have been careful, and I have an isolation transformer in my lab, but I got zapped once while trying to read voltages on the tube sockets…
Next step, I’ll map all the voltages and resistance, comparing the values with the manual. This way, I’ll probably identify other parts that need to be replaced; and I’ll check the tubes too, even if the reading you get in a tube tester is not always an indication you can blindly trust.
It appears that a noise suppressor was installed (i see a couple of orange drop capacitors and a couple of germanium diodes in the audio section) and I would like to see if they are responsible for the not very loud audio that I have.
Finally, the Xtal filter is attenuating the audio a LOT, even now that the IF is exactly at 910,8 kHz (the Xtal frequency). Probably I’ll not use it, anyway.
I was unable to calibrate the IF with the sweep generator until now, I have to find where to pick up the signal for the oscilloscope without disturbing the calibration.
So, there is still a lot to do… but this is the real purpose of a restoration… when travel is more important that destination!

Cheers!
Paolo

p.s. Is it possible to post photos here? Maybe I can show you something…
- Mark J. Blair, NF6X says:
  
  2017/11/05 at 15:36
  
  Is it possible to post photos here? Maybe I can show you something…
  
  My WordPress site doesn’t support hosting photos attached to post comments. If you have photos hosted somewhere else on the Internet, then you can include links to them in your comments, though.
Jack says:

2017/11/05 at 15:48

Thanks Mark…he can email them directly to me…I’m sure I have photos of it before he worked on it that doesn’t help others of course….cheers.
Jack says:

2017/11/05 at 15:50

HI Paolo…I may have kept some of the photos of it …I do that !!….Your mentioning the replaced caps rings a bell….I wrote to the vendor and said it was overpriced owing to the meter and work done….I don’t think he replied….The postage costs these days are stupendous. By the way I paid about $400 plus postage for a complete BC 1000 in Italy some 10 years ago…it never was sent….and have lost nearly $8000 to eBay crooks, so good yours arrived.

You could email me photos to goldmort@onthenet.com.au Your IF cans might hve some leaky caps of course…but great that it is working. One other option might be to have another crystal cut or to advertise for a couple to play with…..Ok on the suggestion I made….maybe you could crackle-black that area…that would be better and not draw the eye….
Paolo Gramigna IK4YNG says:

2017/11/07 at 10:02

Hi all,
I went a little forward on my BC-348.
I did a complete re-alignment of the IF using my HP8601A sweep generator, and my oscilloscope on XY setting.
The signal was injected by simply putting the RF output coax, with a short lead protruding, near the oscillator unit, where a red striped wire goes to the first IF transformer; I assume that I created a very loose, capacitive connection.
The oscilloscope probe, set to 10x, was connected to the pin 5 od the VT-223.
The BC-348 was switched on on MVC.
First of all, by switching on and off the Xtal filter I found where the filter was (approx. 910,8 KHz) then I tuned, several times in succession, the IF nuclei bringing them exactly on the Xtal frequency.
The selectivity and sensitivity are now well improved. I can hear my RF generator around 3 microvolts on all bands. The Xtal insertion loss is VERY high, I read somewhere that it can be opened and cleaned but since I’m not planning to use it (I’ll keep the BC-348 in my studio, as a SW receiver) I’ll leave it alone, for the moment.
Next step: replacing the transformer with one that I have for the 220 Volts.
What’s the optimal B+ voltage for the BC-348? Noe it is 300 Volts DC after the filter capacitors.
Jack says:

2017/11/07 at 12:51

That sounds all good to me Paolo, a job done well by a man with a purpose… but for one thing after all that work…Tuning to the crystal frequency was to reduce its ‘off frequency insertion’ loss’.

If not planning to use the crystal filter why not re-set the IF’s to the 915 kCs intended by manufacturer, who’d not have had such crystal deviation. You might then at some stage find a crystal or have one cut to 915kCs and solder or socket it in then do the possibly minor peaking.

Voltages
On voltages the dynamotor was 220V, generally the idea was 220-230v B+. I work on the absolute certainty that the people who designed these sets actually knew what they were doing unlike most of the Hams and magazines that went berserk on fantasy-mods. Reliability and respect for designer expertise.. if not genius… which is seen throughout the set, would suggest that to me that the optimal voltage is 230v which is perhaps what eventuated at the filter output. Thousands of hours would have been put into design optimisation, longevity and reliability not ‘finger in the air’ introduction of ‘a circuit I saw somewhere’. Mine will be both at 230V…There are better ways than cranking up voltages.

I’d be looking to matching antenna to the set….and I recall my second ever-‘met’ Ham ex-WW11 Airforce Ted Barlow, VK2GQ at Spit Road Mosman in 1960…used a dipole with his but 348’s were designed to a trailing wire antenna impedance…He also used a 1930’s tx built on a breadboard with a huge tube that glowed like a lighthouse…standing vertically on it all fixed into a rack standing on the floor….which would be the cause of the need for the dipole…My first ever chat with a distant ham was through that 1930’s equipment. He had a 348 “Q’ which had no antenna trimmer but in those days one could actually find Hams on HF. The sun-god was merciful! Ted also had a lovely and very patient and supportive wife.

I plucked these comments below from I think one of the sites I sent to you (http://www.radioblvd.com/bc348_dynamotor.htm) …which does have a power supply schematic and voltages down past its filament wiring/rewiring sketches. It mentions the regulation…If the 991 has been removed I’d regulate the power supply (interesting is http://www.audioxpress.com/assets/upload/files/bicknell2890.pdf. but there will be simpler circuits…maybe today even a zener…) The 991 was near the 3 tubes on the sub-chassis where your meter now ‘is’….So many were pulled out you might find one (and a spare) at one of the big US surplus stores …or even through this site…

{and by the way, Paolo, whilst I remember..Last night .I went through my stored BC348 photos; is yours the set with the ‘upside down’ meter…or the quite large one with 4 recessed mounting screws?…could you email me photos of the set to goldmort@onthenet.com.au?}

“The BC-348 operates on 24-28vdc with the high voltage (~+220vdc) provided by an internal dynamotor. Over 100,000 BC-348 receivers were built during WWII by many different contractors building many different versions within that time period. The “Early” circuit used eight tubes with the heaters originally wired in series-parallel for 24vdc operation (two parallel strings of four 6 volt tube heaters in series would operate on 24vdc.) The “early” circuit provided two RF amplifiers, a Mixer, a Local Oscillator, an IF amplifier stage, a combination 2nd IF amp and BFO, a combination 3rd IF amp and Detector/AVC followed by a type 41 audio output stage (this was changed to a 6K6 in some later versions.) Early versions also will have a 991 neon lamp acting as a regulator on the local oscillator and will provide an antenna trim control. A selectable crystal filter was also included in the circuit. The dual dial lamps were adjustable for brightness and were wired in series through a potentiometer and fixed resistor. Frequency coverage was from 200-500kc (not on the B or C version) and 1.5-18mc. The audio output impedance was internally selectable at “low Z” which was around 300 Z ohms or “high Z” which was around 4000 Z ohms. Some BC-348s will have a decal on the front panel indicating if the “low Z” was optioned.”

The writer also noted this…wisely…in his power supply section:

“When mounting the “iron”(of the ac supply) be sure to avoid any physical orientation that might enhance magnetic coupling between the chokes and the power transformer”.

Performance
I’d also research the rf stages without going ‘ga-ga’ over some ‘top performance’ concept some re-designer might have had (notice my circumspection throughout). The dual RF stages had two purposes with which one would have done ok for RF purposes . The additional RF was to block set generated signals which would enable enemy location of the source. There’s a tube change which might help too…I think its a 6SG7…which commonly increases sensitivity.

It’s likely then that some improvement can be made to the RF stages, without going berserk….

Audio…or ‘faux gain’
Matching audio output to audio device would be another, personally I can’t see the need for an extra audio stage unless one is as deaf as Mr Magoo.

Antenna
I think the secret is in the antenna….which means ‘space’ Can you recall those early radio books with pictures of 100 ft or more antenna from house to tree or post…Unfortunately today the ‘zombie-makers’ have convinced gullible people that owning a house and yard ranges from ‘un-necessary labour through to selfishness when a hundred people could live there.. or be mentally entombed there. That Council manager salaries increase with population increases has also driven them to approve the loss, even guilt, for having the anti-social pleasure of having space, trees and greenery of your own.

FM has been a curse on the HF antenna in my view…no one except enthusiastic HF hams need antenna space anymore and the new order seeking controlled worker ‘citadels’ are even ‘digitising’ B/B band to no good purpose. I imagine arguing along the same disingenuous ‘better quality/no signal loss’ nonsense they used to have our entire nation have to ‘go digital’ TV costing vast sums. The latest digital con is the Australian NBN fibre-rollout which is an almost total waste of money and which it is now admitted ‘can never make a profit’…but we all had to pay increased fees to actually keep our telephone/internet going. “SocialMedia’ is a sort of ‘primal scream’ interfering with commerce’s time to think, make decisions, everything has to be ‘now if not earlier…Read Banjo Pattersons “Clancy of the Overflow” Paolo) Like everything government ‘sells-off’ to ‘more efficient’ private enterprise the consumer-costs just multiply. As well they gave the control-equipment contract to a nation known for its extreme espionage….a place not far from Gaza..

Perhaps its time for an ‘antenna led revolution’…’what do I want?’…antenna space…’when do I want it’…’now’ …all that stuff (I need some ‘smilies’ for this dissertation…. to indicate my chuckling away as I write!)

I’m happily living without excitement over ipads, hero worship of ‘rock’ idols’ and all that..I think sometimes we are going too far forward and my adherence to my 348/342/453/ART13/B-40 etc. etc. gives me time to rest and stay in love with equipment which ‘age does not weary them, nor the years condemn’…they have a beauty which does not fade…and tradition and history all of which has even been largely abandoned in Amateur radio. I love that ‘clunk’ in switching frequencies on the 348 and the other sets…it has a sort of ‘reality reassurance’.
Jack says:

2017/11/07 at 13:27

This site I realise has my memory of the meter with some alloy showing and also has a comment on the B+…http://staff.salisbury.edu/~rafantini/bc348modifications.htm

Some common sense here…http://www.jamminpower.com/main/bc348.html

Handy….
http://www.morningstarobs.com/bc-348n-receiver.html
http://www.angelfire.com/electronic2/radiosean/tv/BC-348H/BC-348H.html
http://boatanchorpix.x10host.com/BC348Q_3.htm

includes a B+ story Paolo including this one I think confirms B+ range …Hallicrafters purpose- built powered 348’s

Solid state rectifier – Since the original power supply was a dynamotor, B+ would have been available almost immediately. I decide to try replacing the 5Y3GT in the Hallicrafters power supply in an easily reversible fashion. I soldered two 1N4007 diodes into a tube base adding a 500 ohm 5 watt resistor to make up for the increased efficiency of the diodes. The combination worked very well. I measured 240 volts B+ as the result, about 5 volts less than shown in the Hallicrafters power supply schematic but closer to the original design specification for the BC-348 at 228 volts.

Smile producers
https://swling.com/blog/2014/05/swling-with-heavy-metal-my-signal-corps-bc-348-q/
http://www.g1jbg.co.uk/mil3rep.htm

Blarney but interesting
http://amfone.net/Amforum/index.php?topic=28945.0
Jack says:

2017/11/07 at 13:31

Gee, what happened to the graded URL’s I just spent an hour researching and ‘submitted’?…?
- Mark J. Blair, NF6X says:
  
  2017/11/07 at 21:52
  
  It got held in my comment moderation queue, even though your other posts have been getting approved automatically recently. I don’t know why. Then on top of that, I was busy at work and didn’t have a chance to log in to my blog all day to approve posts waiting for approval. Sorry about that! I can often do that stuff while I’m at work, but sometimes I’m tied up enough that I never dig out my personal laptop.
Jack says:

2017/11/07 at 13:34

Hi well I back tracked and found them…amazing now it says I am duplicating but is so where’s the original? gone………

This site I realise has my memory of the meter with some alloy showing and also has a comment on the B+…http://staff.salisbury.edu/~rafantini/bc348modifications.htm

Some common sense here…http://www.jamminpower.com/main/bc348.html

Handy….
http://www.morningstarobs.com/bc-348n-receiver.html
http://www.angelfire.com/electronic2/radiosean/tv/BC-348H/BC-348H.html
http://boatanchorpix.x10host.com/BC348Q_3.htm

includes a B+ story Paolo including this one I think confirms B+ range …Hallicrafters purpose- built powered 348’s

Solid state rectifier – Since the original power supply was a dynamotor, B+ would have been available almost immediately. I decide to try replacing the 5Y3GT in the Hallicrafters power supply in an easily reversible fashion. I soldered two 1N4007 diodes into a tube base adding a 500 ohm 5 watt resistor to make up for the increased efficiency of the diodes. The combination worked very well. I measured 240 volts B+ as the result, about 5 volts less than shown in the Hallicrafters power supply schematic but closer to the original design specification for the BC-348 at 228 volts.

Smile producers
https://swling.com/blog/2014/05/swling-with-heavy-metal-my-signal-corps-bc-348-q/
http://www.g1jbg.co.uk/mil3rep.htm

Blarney but interesting
http://amfone.net/Amforum/index.php?topic=28945.0
Jack says:

2017/11/07 at 22:54

Thanks Mark…I also sent some information to one of your replies over which I accidentally stumbled….it was about measurements of an antenna a chap was wanting…if that rings a bell…..
- Mark J. Blair, NF6X says:
  
  2017/11/07 at 23:09
  
  Looks like a couple of your comments ended up in the spam bucket, probably because they included lots of links. I kicked them free.
Jack says:

2017/11/08 at 00:53

Thanks Mark, I kiss your cheeks Mwa….Mwa…Mwaa!….

Mark's Tech Journal

BC-348 FAQ

BC-348 FAQ

THE BC-348 RECEIVER- SO, WHAT IS THIS THING?

THAT FUNNY 117VAC WIRING DYMO THING

THAT OUT-OF-PLACE-LOOKING 2 X 6 PANEL

VARIANTS OF THE BREED

PRICING THUMBNAILS

TO PERSUE OR EVADE?

RELATED COMMENTS TO MY ORIGINAL POST (RELEVANT)

ADDITIONAL REFERENCES AVAILABLE

FINAL CLOSING THOUGHTS:

73 Responses to “BC-348 FAQ”

1st RF	VT-117
2nd RF	VT-117
1st Det/Osc	VT-150
2nd Det/avc/CW osc	VT-233
1st IF	VT-117
2nd IF	VT-117
3rd IF	VT-116
Audio	VT-152