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”

Paolo Gramigna IK4YNG says:

2017/11/11 at 10:40

Hi all,
I went further another step.
I decided to check the voltages in all the pins of all the tubes, comparing the values with the manual.
I found that the voltage at the G2 and Plate of the 1st RF tube was only 34 volts. Not good at all!
I found that the 15.000-ohm resistor, leading B+ to G2 and Plate, had increased to 150.000-ohm value.
Replacing it, the voltages are now more correct, and I have more RF gain, of course.
At that point, I decided to go for brain-surgery. I disconnected the Xtal filter, and after donning white cotton gloves I opened it and cleaned the surfaces with the best cleaner I had, then blowing away any residue.
Now the Xtal filter is working as expected; I had to retune the IF chain exactly on his frequency, and I still have a loss of about 6-7 db when inserting it, but the selectivity is now VERY good.
The only problem left is that I cannot obtain any result when I try to neutralize it; there is simply no reaction when I try to move those “capacitive lugs” as explained in the manual.
As a side effect, my “useless” S-meter is now more useless than before; it does not react anymore to the changes in the signal strength.
Not a big deal, but it appears that bringing the B+ to the proper value, on the first RF tube, cancelled the variation of voltage I was seeing before, on that point of the circuit.
I will leave it as it is; the only reason for having that S-meter was to avoid the hassle of replacing it with a black metal patch. My BC-348 it’s not “original” in many other ways, but at least I have a sensitivity of 1 to 3 microvolts on all bands, and that was my goal! About authenticity, “somebody” had even drilled some dozens of small, neatly aligned 3mm holes in order to increase the ventilation, on top and left sides, and I’m not even thinking to close them.
I was thinking to replace the power transformer with another one wired for 220 volts, but at the end, I’ll use a step-down transformer 220/115 that I have and I’ll avoid any further nightmare.
Quite soon I’ll close the BC-348… If I don’t come up with another idea…
- Jack says:
  
  2017/11/11 at 14:02
  
  Hey…don’t leave yet Paolo….don’t close up that set….have faith…believe in Murphy…
  
  Another case will turn up, in the meantime as the cabinet holes are small….pretend they are just illusions..or think of them as perforations from a direct hit nearby on the aircraft….
  
  You wrote…..”there is simply no reaction when I try to move those “capacitive lugs” as explained in the manual.”…do you mean can’t move them or moving them makes no difference?
  
  Paolo on the meter, I have a suggestion which may or not appeal…Replace with say a 300 or 500dc voltmeter. In a safe spot or render one so, thoughtfully put a 12 position rotary switch with a BC 348 pointer knob…nots some ring-in plastic chook-beak thingo….and switch the 8 x plate voltages and if careful perhaps 4 x (B+ minus) grid voltages to have an occasional check of what’s going on down there in the engine room…..just a thought.
  
  There are some sites here which might be interesting for a few, or some less exploratory folk but may have some avid interest for the atrophied brains of long term BC-348 anti-fungus or fried cockroach sniffers.,,especially the ‘am phone’ site.
  
  http://www.hpfriedrichs.com/radioroom/bc348/rr-bc-348.htm Friedrich including cleaning crystal, replacing crystal switch (a possible loss area) a cracked RF resistor and so on….I’ve found that every site has some little thing of interest. Friedrich cleaned with alcohol….commercial cleaners leave a residue, however imperceptible.
  
  I think it was Friedrich who answered the question posed as to why the B/C band was not included. Morale would be the first.. focussing on the mission, from the Echelon workshops…morale in realising this was a dead serious receiver..not for entertainment, to not have music blasting through any speakers during flight…and not further reducing the tuning-compressed band-space for the other frequencies, as happened when the 200 kHz band was ordered to be included.,,,
  but he also mentioned that 915 kHz is within the broadcast band.
  
  https://www.nonstopsystems.com/radio/pdf-radio/article-bc348-BoSam.pdf….there’s a blank page so we think it’s the end …but wait!! …there’s more
  
  http://www.fracassi.net/iw2ntf/manuali/RADIO%20RECEIVERS%20BC-224%20and%20BC-348.pdf This site, a handbook revision in 1957, has a massive amount of information In a form interesting to an owner including a logical approach to the crystal which we might not do with serious intent. I can’t immediately recall whether there is a padder in the crystal circuit but adding one could be useful..it just occurred to me…even though it might add to loss it might gain some from being periodically peaked to the all-corrected IF setting.
  
  Ah yes there is one (p10)
  h. CHECKING CRYSTAL OPERATION. (1) Leave the “CW. OSC.” control in the “ON” position. (2) Place the “CRYSTAL” switch in the “IN” position. The noise should be greatly reduced. (3) Rotate the “TUNING” control on the receiver slightly to each side of the signal. The signal should be tuned in and out with a smaller movement of the “TUNING” control knob with the “CRYSTAL” switch in the “IN” position than with the switch in the “OUT” position. The volume of the signal should be slightly less.
  
  There’s some nourishment in here for dissipated owners
  http://amfone.net/Amforum/index.php?topic=28945.0
- Jack says:
  
  2017/11/11 at 14:08
  
  I forgot, Paolo,…what was the crystal frequency after the cleaning? Ciao
Paolo Gramigna IK4YNG says:

2017/11/11 at 14:55

Hi Jack,
The frequency remained more or less the same. Only the efficiency of the crystal was much better. It appears that moisture or contaminants were degrading the performance.
I used a strong degreaser for cleaning, then rubbed the crystal with tissue paper, blew away all moisture and lint using bottled gas normally used for cleaning optical lenses, and reassembled it in the same way. I got an increase of several decibels, and as far as I can see a slight increase in the Xtal frequency, less than one kilocycle.
I have to say that the crystal, when dismounted, was still shiny without traces of oxidation. I did not seal it around after reassembling, I’ll wait for a little to see if there will be any change.
The most surprising fact was the reduced voltage I saw on the G2 and Plate of 1st RF; the tube was still somewhat working with only 34 volts!
Now I have several other voltages not respecting the indications in the manual; but the previous owner had modified the bias circuitry, adding an RF gain control (he used the dial lamp potentiometer for that) so I’ll have to study a little more tomorrow.
It’s a pity that there are so few stations transmitting in the SW nowadays! Apart from Radio China International, Radio Rumania and several religious stations from the Middle East, the bands are nearly a desert!
- Jack says:
  
  2017/11/11 at 15:15
  
  Hi Paolo…I noticed Friedrich only used alcohol…On voltages I have a BC1206 (I think it is) here….1930’s 6 tube used for WW11 airplane direction finders (excellent Q5 owing to small-size)..they run off 28 volts…including plate voltage.Women delivered ‘planes from factory to recipients…we forget their contribution to easily in the loss of 100m people to the types who benefit from creating and financing war. These little sets are an absolute wonder…like the command sets….Perhaps the women kept their ‘own’ but had them serviced…I don’t know but there’s a woman pilot story somewhere on internet about the 1206.
  
  Just an idea of them is here…and they’d fit in a lady’s handbag…. http://vk2bv.org/archive/museum/bc1206.htm http://radionerds.com/index.php/BC-1206.
  
  There’s also some information on many sets including 348 and 1206 here. My 1206 is ‘mint’.http://www.radioblvd.com/WWII_Communications_%20Equipment.htm
Jack says:

2017/11/11 at 15:16

sorry…5 tube…
Paolo Gramigna IK4YNG says:

2017/11/11 at 15:29

I did an exact reading of the Xtal frequency, as it is now: 915,24 KHz.
The IF is tuned exactly there now, and if I insert the Xtal I get a reduction in signal strength of a few db (difficult to be more precise, with my equipment) and the selectivity is indeed better; there is no audio distortion, apart from the clipping of the higher tones.
At this point, I can say that I’m happy with those results.
Tomorrow morning (it’s half past midnight here) I’ll check if the resistance values on the tubes are what expected in the manual.
I would like to see if there is any capacitor leaking; I think not, but it’s better to check.
Jack says:

2017/11/11 at 15:33

HI again Paolo….I also forgot to suggest you get your supply voltage down. You probably know all the function voltages are in the manual…I’d stick to them….but then that’s because I know the amazing designers knew their onions and increasing voltage may give only the imagination of better performance…whilst reducing MTBF.

A comment from ‘am phone’ might interest you and others as it interested me, though I already realised it…the levels he mentions…” Also a lower voltage secondary transformer is better than a 250 volt or above being it makes little difference in the radio s performance between running 190, 200 or 250 volts on the plates but the radio runs cooler with lower plate voltage.”

I’ll stick to 220-230V.

Note also that if the wiring to the 8 pin plug is disconnected, making sure the relay wires are properly dealt-with for one’s circumstances (eg running aTx might require the 348 rendered inoperative on transmit)…and I’d be more inclined to neatly terminate them…eg using insulated ‘twist-on’ electrical connectors.. rather then remove them….”who knows what tomorrow may bring luconoe”… (“Carpe Diem”…Horace)
Paolo Gramigna IK4YNG says:

2017/11/11 at 15:50

Hi Jack,
My B+ is still a little too high, being 245 volts. I’ll try to reduce it by adding some resistance in series with the rectifying diodes (I already have 1000 Ohms in series between the capacitors).
The problem is, adding resistors will increase heating, but I’ll try to go down to 220-230 Volts.

SURPRISE! I found just now that my first RF tube is a 6AC7 and not a 6SK7. I did not notice until now! It appears that those tubes are at least similar, looking at the Radiomuseum data sheet; Do you know if there are more subtle differences to take into account? Who knows why it has been changed…
Paolo Gramigna IK4YNG says:

2017/11/11 at 15:53

The 8-pin connector is completely missing, and the wires have been re-routed by the previous owner. I had to spend a whole day tracing those modifications, and insulating a few wires left dangling!
Jack says:

2017/11/11 at 16:44

Hi Yes tracing wires without undoing forms can be time-taking on modified 348’s!…especially the g/c versions but they are mor like a ‘real’ radio than the SET’s (heheheheh)Tthe twist-on or “GARD’ in US connectors I mentioned I would never use for electrical consumer wiring, but they are fine for terminating single wires as in the 348, if well fitted. Easy to find on eBay. If previous owner left the 8 pin frame there you can fit a european (ie like computer) 3 pin socket to it readily….great mod and very convenient but mark its voltage. Still no photos arrived…was hoping for some..
Paolo Gramigna IK4YNG says:

2017/11/11 at 16:57

Hi Jack,
I’ll find a way tomorrow to post some photos and I’ll send you the link.
Jack says:

2017/11/11 at 17:04

Paolo…I mentioned the 6SG7 as a tube said to bring higher gain…. but some of those replacements require a pin mod….or more. It does require a change….I bought a dozen for my 348’s and command sets, but now think I blundered. I’d use a 6SK7 where the set had one. Granted sometimes tubes were later improved-upon but the designers used the tubes which gave the performance they wanted. You might be able to improve gain on the 6SK7 with a minor fiddle but to be honest, I’d improve antenna hook-up, ensure all components are on value and use orange drops or the mustard Phillips as caps. Remembering these were made for a trailing wire of a particular Z..good soldering and clean sockets and pins might see better s/noise.

This is from a 1970 discussion note well the comments. Also if filaments are still 24v watch circuit voltage with increased 6aC7 filament current.(http://www.antiqueradios.com/forums/viewtopic.php?t=37411). To optimise a 6AC7 as replacement might require some work and note comment below on cut-off and oscillation…see whether your set has its original RF components.

“Those tubes are interchangeable but here are some differences.6SK7 & 6SG7 are remote cutoff, good for RF & IF stages. They have different basing but haven’t seen this to be a problem.6SJ7 & 6SH7 are sharp cutoff, good for 1st audio but can distort as IF or RF amp in a radio with AVC.6AB7 (remote cutoff) & 6AC7 (sharp cutoff) were originally made for TV. These have higher gain and draw .45 amps filament current. The others draw .3 amps. (Use for parallel wired filaments only.) These are great tubes, will give more gain but some radios go into oscillation”
Jack says:

2017/11/11 at 17:10

Paolo (et al) look on the Friedrich site for the photo above this and see just how neatly the power socket sits in the ol 8 pin frame….and using one with mains filter on board…very neat

“Power was originally delivered to the BC-348 through a special plug at the rear of the set, comprised of 8 rectangular blades. The connector on my radio had itself been modified, and some of the wiring that left the plug went no place. In the end, I decided to remove it entirely. The upshot of this decision was that the plug’s support bracket provided the ideal place to mount a standard IEC power plug (the kind found at the rear of your computer.) I selected a plug that features an integral noise filter to shield the radio from any hash (EMI) that might be present on the power mains.”
Jack says:

2017/11/11 at 17:46

Forty years ago one evening I watched a boss/technician at department civil aviation Wentworth Falls NSW rebuilding a 348….completely using information from his brain. He substituted nuvistors for the RF stages…terribly neat worker too…
He was no bragger, but when I asked him he said sensitivity in one he’d done previously was <1uV…as good as 0.5uV. I have nuvistors and have been tempted………but like hotting up cars, just bolting-on this and that isn't optimising and could be going backwards…which is one reason for giving serious research and thought to replacing tubes in a set still using the original types 30 years after design.

I think discovering what these sets actually had to do is very important in seeing what we have and how best to use it.Your set has 3 IF stages as I recall and should perform well with antenna matching…all else being ok and remembering it only takes one crook cap (or resistor) to drag-down performance….and some of them are hidden away. There is a U-tube series on replacing them.

I might have fitted a 2 to 1 geared band spreading capacitor in lieu of the dial light rheostat….were the sets as plentiful as 'the old days'.

I forgot actually that yours is a 'j' (aka 'Q') so single ended tubes….I think I'd put a good antenna tuner in front of it and make it as good as the g/c varieties (fiendish chuckles).

This comment came from 'jamminpower'

" Also, the gain in the RF stages seems to be deliberately set to a relatively low level. This keeps the signal small up to the conversion stage, while getting the band-limiting effect of the RF stage bandpass filters. I believe the point of this is that keeping the signal small reduces the intermodulation distortion and thus reduces the interference from strong stations caused by nonlinearities in the early stages."

Another from amfone http://amfone.net/Amforum/index.php?topic=8930.5;wap2

k4kyv:
"If the receiver doesn't have a separate agc amplifier, but feeds the agc rectifier directly off the final i.f. transformer secondary, parallel to the diode detector, the agc must be disabled whenever the BFO is turned on. If not done automatically, it must be turned off manually. Otherwise, the agc rectifier makes no distinction between the BFO output and a strong signal, and the rectified BFO output generates enough agc voltage to nearly cut off the agc-controlled stages. Turning off the BFO for CW and SSB is standard procedure with the classic single-conversion superhet where the i.f. feeds a diode detector and the BFO is coupled to the diode detector through a small capacitor. In some receivers, the same diode serves as detector and agc rectifier.

If the receiver has a separate agc amplifier stage, I have found that slow agc is best for cw and ssb, but for AM, fast agc is preferable, since it allows the agc to better track the carrier under QSB conditions.

Another problem with using agc with cw or ssb is that under key-up condx or between ssb syllables, the receiver gain recovers and the background noise rises between cw characters or voice syllables. Hence slow agc for cw or ssb. If I use agc at all for those modes, I use just enough to prevent the receiver from overloading on strong signal peaks, by turning down the rf gain control. Some ssb receivers attempt to get around this problem by using audio-derived agc.

Maybe you could find two pots mounted with concentric shafts to separate the af and rf gain controls. That used to be common with older tube type TV sets which often had concentric knobs to accomadate dual functions. Just make sure you use pots with the proper taper. The rf gain should be linear taper, while the af gain should be audio, or logarithmic taper. If a linear taper pot is used for the af gain control, all the audible signal variation will be squeezed into about the first 25 degrees of rotation of the pot."

From Radio Boulevarde site:
The BC-348-Q is an amazing receiver and the more you use it, the more you'll appreciate the fact that this great performing receiver "does so much, so well, with so little." Being an aircraft receiver, weight had to be kept to a minimum but this little receiver has two RF stages, three IF stages, a Crystal Filter, a BFO and a "1940s accurate" dial readout.

It's surprising that even today the old myths of "not sensitive" or "too broad" still turn up in contemporary reviews of the BC-348. The receiver's performance is directly related to how thoroughly it was rebuilt and how accurately it was aligned along with operating the receiver with the correct impedance speaker. I've used my two BC-348-Q receivers (and a BC-348-R) many times as the station receiver and have found them to be fine receivers. Can adjacent frequency activity be heard? Sure. But does it prevent solid copy? Of course not. Remember, you have a Crystal Filter. With the Crystal Filter "ON" the IF passband will be around 1KC wide. The Crystal Filter will reduce adjacent frequency interference on AM – just be sure to tune the incoming signal "on the nose." Obviously, the critics who keep promoting the "Broad as a Barn" reputation of the BC-348 have never used the Crystal Filter or they have never aligned the IF section of their receiver correctly.

I've used the BC-348-Q on CW also. It's a very good CW receiver with very little drift. Of course, the lack of CW activity these days doesn't offer up much of a challenge to the BC-348-Q's selectivity. SSB is also demodulated quite well since in MVC the VOLUME is actually an RF Gain control.
I always find the performance of the BC-348-Q to be a pleasant surprise. Probably because, over the years, I have always heard so many negative comments about the receiver's lack of selectivity and limited features – but this just isn't the case. The BC-348-Q is a great performer that always delivers solid copy even in tough conditions.
Paolo Gramigna IK4YNG says:

2017/11/12 at 08:13

As promised, here are the links to some images and a short video:
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There are a few images “before” and a few “after.
The video is on the radio while receiving Radio China International, the only shortwave station that I receive 24 hours a day.
As you can see, the S-meter is inoperative. It has been disconnected, it was interfering with the AVC too much.
Jack says:

2017/11/12 at 13:47

Hi Paolo…I didn’t get any results from your offerings above other than ‘file dropper’ from any of the links and I won’t sign up to internet scenarios toget ‘on board’ unless e.g. like this forum. Thus I refuse to be involved with spy-sites like facebook, twitter, disqus and as little as possible with Google and Yahoo. That limits me, I can’t even make comment on internet articles sent by my engineering society. I can’t access what is saved into spy-cloud without my permission unless I make some microsoft account….inslaw promis…’echelon’…It has been a fairly seamless transition since operation paperclip got going..

I can only get pictures sent directly to me….you could URL me the video of the set perhaps…..

Interesting how closely to 915 the crystal became after cleaning….maybe there’s a message there for us all….don’t forget to reseal…not with silicon!!..I’ve never opened a 348 crystal can you make a paper gasket which will not disintegrate inside the case…?….probably not? I wonder what would be the ideal sealant.. remembering it might have to be opened again if something ‘stuffs-up’…Maybe it could be resealed just outside the case…perhaps a slice of the appropriate sized ‘spaghetti’ tubing carefully ‘forced’ on…so nothing goes on any faces.

Remembering that dB is the engineering status of reduction in hearing, 3 dB is a significant amount….You said “there is no audio distortion, apart from the clipping of the higher tones”….I think there is some ‘natural’ clipping …Chinese pitch is high, the set was made not for hi-fi but for typical western voice range to be clear and cw…which was a common wartime language. I’m not saying they limited the audible range but in design and testing would have been fundamentally interested in how good a communications Rx it was. Like that old Sophia Loren song (Agapo?) really wanted to say… Boom biddy Boom biddy Boom biddy Boom Boom Boom … ‘where does the clipping start’..? The audio transformer and its components? or before that. Is the trannie breaking down? cap leaky, Is a tube being biased just that little too much?, again a cap leaking…a grid resistor high……is a tube down on emission…..is a pin or a switch contact dirty….all that stuff…. Tube substitution might be interesting and temporarily using a dry modern audio trannie to see if that makes a difference……or is it the listening device range itself introducing the clipping ? Perfection is a frustration for the repair fiend….”there must be SOMETHING to fix”….that’s what sent CQ and the others on a 348 destruction expedition (chuckles…..)
Paolo Gramigna IK4YNG says:

2017/12/05 at 10:26

Hi Jack and all,
I’ve been a little busy, but finally, I was able to get back to work on my BC-348-J.
First of all, I’m sending you a link to my google drive. Hopefully, you will be able to see my photos there.
Here it is:
https://drive.google.com/drive/folders/1LXF8TGwH0k8xCIjByxngkVxLB0CBrr-t
I went ahead with my restoration, and I’m quite happy; there is only one problem left, that I’ll explain later in the message.
I replaced the power supply transformer. Now my BC-348-J works on 220 volts; the B+ voltage is about 230 volts.
I found a LOT of modifications, made by the previous owner. Following carefully the schematic, I tried to rebuild all the circuits as they were in origin.
I found several capacitors added, several resistors changed, and some of the originals had a higher value, more than 20%+ that I replaced as well.
I still have a separate potentiometer for the Bias voltage, the previous owner used the Dial Lamps potentiometer for that; i placed a 25Kohm there, wired as per schematic. Maybe later I’ll go back to the double potentiometer configuration.
I replaced all the 0.1 and 0.5 capacitors; I found that two were defective (DC loss) so I replaced all of them.
The sensitivity is now very good (about 1 to 3 microvolts) and the IF is aligned exactly with the Xtal frequency.
There is only one problem left:
On every band of frequency, near the top of the band, there is a region where the BC-348 starts motorboating.
It happens on all the three top bands, and in the same dial region.
So I suspect that something must be wrong in the oscillator section…
But opening the Oscillator sub-chassis is a major work! I must dismount half of the radio!
Does anybody have a suggestion of what to try?
Thanks to all!
Paolo Gramigna IK4YNG says:

2017/12/05 at 10:28

I forgot: if the photo link works, I’ll post a set of updated photos. The one you will see is still the previous ones.

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