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.

 


RELATED COMMENTS TO MY ORIGINAL POST (RELEVANT)

Editor’s note: About 1/4 of the replies I got tell me I should GRAB the TEST SETS, or GRAB THEM and sell them to THEM! You guys must like these ARC-5s things, too.

And here’s what some said about the ARC-5s (relevant to the BC-348):

Q: The now SK had an OS-8B scope, URM-25D sig generator, a mil audio generator (I can’t remember make or model right now) and… the BC-348. He also had some ARC-5 stuff, including a BC Band one. But, I have some questions about the BC-348:

  • As to the ARC-5 receivers, they’re in demand today – they were the “command” radio set for shorter range HF communications on many aircraft in WW2 together with the ARC-5 series transmitters. The BC band ARC-5 receiver was generally used by hams as a second, tunable IF for the BC-348 (which has an IF frequency of 915 KHz) to provide additional selectivity to the BC-348. The ARC-5 antenna terminal can be coupled to one of the BC-348 IF stages with a small capacitor, and use the ARC-5 for the audio output. This gives you a nice selectivity and tunable second IF without hacking into the BC-348 which is getting scarcer and scarcer.

 


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 :-) :-) :-)

 

  59 Responses to “BC-348 FAQ”

  1. Avatar

    I am trying to find out the current selling price for a BC-375, modules & Dynamotor all brand new in original boxes. Matching serial numbers. No tubes as boxes were never opened until I took a peek.

  2. It’s now 2013 and I can’t see any recent comments…so, to come out of the closet in 2013 as a BC348 lover on a site written 14 years ago…

    The preferences range around easier access for service and the tubes for the three of the Q type. The argument also is that one less IF is less noise. All those are valid. On the other hand the Q series (it being the most well known letter) look more like an aircraft set than a communications receiver. I like the 4 IF’s, I can come out anywhere anyway to my BC 453. With the BFO working well ssB is a dream. I am not one of those lazy modern minded hams that wants everything the factory can produce…What’s the point…Ham Radio isn’t all about communicating and we used to have rules on subject matter, 3rd Party messages, hogging the air, fould language, politial debates, using the airwaves as a telephone. HF Radio started to feel the pressure around 1963 as VHF was so much simpler, Acorn tubes (954/955 and 717A’s) were abundant as was radar gear) but that started some arrogance in the radio fraternity, the VHF lot (who only had 2nd grade licences “Z” calls and scorned CW to justify it (heheheheh) . My only VHF interest was in 276 mHz (why I don’t know!! probably I liked a Radio and Hobbies receiver I built ..and soon TV took that band!..Grrrrr! Other things changed too, people began to see themselves as special and more sophisticated than the old HF “fuddy duddy’s”though the brilliand guys like Tony Mulcahy and Terry Thorpe were on HF though Terry was a brilliant UHF technician on Radar and all aircraft gear. Terry’s construction work was like Mil-Spec and he used an AR8 /At5 until his Home-brew Geloso front ended gear was up and running.

    Once the VHF lot took over the WIA it was all downhill. My fondest memory of there is Old Norm Bearde…a gentleman badly treated in that farago. Norm was disgusted at my being thrown out during a meeting out by two of them for attending meetings as a schoolboy on holidays…without paying fees (and with 16 kids there was no money but I loved to go and watch and listen) and told not to return until I paid fees .This was in the era when CQ and QST were bibles and the WIA had a real library to which I later contributed manuals I had had declassified. Dick Smith was still a radio mechanic underneath the “Big Bear” at Nth Sydney and I have some amusing sories about Dick.

    My meeting with the inquisators was actually a personal issue with one of the adult two bully boys who wanted a new order there, “VHF” “men” at Atchison Street. Norm heard of this outrage and held an AR7 with all coil boxes under his desk for me until “one day when you can come up with a couple of dollars”..far less than fees. My die was cast then. I realised what a real man was, and I met some great ones, Ted Barlow, Terry Thorpe, Sandy Bruce-Smith, , Tony Mulcahy and everyone of them gave me time…I believe in that kind of tradition for Ham radio that includes cw and tube sets somewhere in one’s kit…preferably pre 40’s…for this is where Hams got their delights post WW11 even if only the humble AR8/AT5. I can’t see the slightest matter of personal science in digital-binary nor in VHF repeaters…that’s all glorified CB. The old 348 is harder to find today…I suppose thousands have followed their SK’s to the ground and probably hundreds are in tractor sheds attics and garages. It’s sites like this one (and a couple of dozen others) which keep the heart pumping and the love flowing. That’s Ham Radio..it was the like no one under 40 or 50 will ever see again as the world is made to change into something it never should be.

    Like yours, my 348R has character and I am a traditonalist. Even the sound of the toggle switch for the CW has a nostalgic sound. These sets can be repaired and have a special nostalgia for us who’s uncles and fathers died in Yankee marauders or somewhere else. Like mmny I have some ‘mea culpa’s”…I turfed thousands of new mil/CV/VR tubes and boxes of components when it looked as though they were gone forever but that was a hard lesson learned. I threw away radiospeople claw-for today I still have a few hundred worthy tubes though…

    Sure the 348 Filter doesn’t seem to work (unless you align it and the IF’s correctly) sometimes and sure the performance is down on my mod’d Kenwood 820S and my 130S maybe even my Drakes but I get most fun from it……I’d rather tbe fighting to 337 on it than 599 on a Kenwood. For 40 years for outstanding reception I used my URR391 which I bought during Vietnam and sure my AR88D was magic but the old 348…I don’t know…it has something special. Sure too the BFO can be broken by people who think they should turn 360 degrees and sure we hear such pitiful (and often!) complaints that the dial lights don’t work so well especially when filaments changed from 28V to 6V but for me…it’s my Susan Hayward , My Vivien Leigh….been around the traps but always welcome back.

    My 348N (same as Q) , which I may sell could be repainted on the box top but looks ok. I just like the R!!! and I really want one in the series BC348 A,B,C orD and might swap. This N was used post WW11 in an Australian Catalina I believe and has a “refurbished by (????) factory sticker/decal on it. It is a yellow and blue label. Voila

  3. Hi interesting for me that I have just bought the only 348 I have seen (and there could be scores of them for all I know….someone might advise) with RAAF indent number and matching serial number on a plate. It was a 1953 restoration by the services…seems to have been a momentous year.Unfortunately it is a “Q” but then…is it likely to be the g.c. model…(?).

    My R is still my favourite and I’ve done little to my 342N as have been studying. I sold to a minister of some Christian Religion, the ex-TAA-Catalina “Q” with the interesting refurbishment transfer however I think he has done nothing to it after 2 years or so. I’d like a couple more jones-plugs if anyone has some.

  4. Avatar

    Hi, congrats for the fine infos on this page, I have a french BC-348 marked STTA ( services techniques des transmissions aériennes) end of waranty feb 1957 ! No version markings so I guess, looking the tubes installed that it’s a H model, to be confirmed. I’ll restore it for matching it with a “Air France” ART-13 and a US Navy tremandous TCZ power supply Col-211101 .

    best regards ,
    73 de F6GTC, Henri

  5. Hi Henri…trss bien…je vous comprends ok ..ou habitez-vous?.en France?

    J’ai une maison ‘ au compagne’ la bas.

    I made an error in my long dissertation ..348 has 3 IF’s! not 4 ….I also have an ART-13 ex Panam I think…has the Crystal ‘Mod’….and also a 500kHz (kC)crystal. I have also the original 500kHz crystal (3 prong) from the ART 13….good stuff. (in zee Freench ‘superbe!! or just ‘supa…)

    This ART 13, good condition (good ‘nick’ we say here colloquially) has a couple of mods I have to investigate when I get a break from University. It came from a Radio Inspector (RIP) in Tasmania. I’m no longer a purist but I do believe if you are going to mess around or sdo mods….do it neatly and record the changes in a way they will stay with the set.

    I prefer the ‘R’and the other grid-cap series as I said . Since we spoke I bought a Q…I’m not greatly attached to them irrespective of their easier servicing etc. To my interest and then rising alarm I see it has extensive mods…separating RI and Audio controls is ok….but there are wires going everywhere and a pretty ‘average’ (pauvre) mains p/s….I’ll have to rebuild it to give proper access. He’s also clipped the terminals off the ends of the original power supply wires….which is not only stupid but ‘vilain’. One component has been ‘snipped…a coil I have to identify…and a transistor hooked into it…whether as a transistor or a diode I don’t know as just on quick glance two wires are bridged….. The original Jones plug has been replaced by a modern one, that’s ok but a component attached to it is a complete mystery…maybe there’s a 348 group who might have seen these mods…rather than I put the set back to standard. Curiously the electrolytics whoever did it has installed, are British!….which might explain the confusion!…..I wish I could find him…would short cut the process…He installed an audio stage but curiously, instead of using a 12AU7 he used the most expensive and rarest version…a 13D5 from memory.. Contact me at goldmort@onthenet.com.au maybe we can compare notes and set photos….

  6. Oh and Henri…why model “H” rather than say, R or M or some other….By the way there are Utube videos on doing some 348 repairs…I’m not recommending…just saying . Do you have a lot of 348 information? I have the handbooks etc but BC-348 dynamotor is a great site….http://www.radioblvd.com/bc348_dynamotor.htm so is http://hpfriedrichs.com/radioroom/bc348/rr-bc-348.htm….of course there are many but these two have a lot of information. Je vous remercie, Monsieur mes salutations distinguises

  7. Here’s a novel power supply and maybe amplifier?…..on a pretty grubby g-c 348. I’d have polished it up first but…very different…and lethal!, if not careful

    http://www.philipstorr.id.au/radio/twenty/three/bc348_06.jpg

    Info available here…http://www.shortwaveradio.ch/radio-e/mil-usa-signal-corps-bc-348-test-e.htm

    ART 13….(with a few French words thrown in at the end) http://www.qsl.net/i0jx/ART-13_BC-348.pdf….Don’t do substantial mods on the ART 13 as it will ruin its value …some do I think 20M conversion…forget it!!….who wants 20M any way…that’s a bit like CB!!..80/40…donc…ces sont bien..tres bien!!…mielleur qu’eau de vie!!

  8. Hi again….this is a very clear manual…a lot are not, Henri….http://ekladata.com/PAr3yVT4nQiJS80hemrGhErCT64/bc348_manual.pdf a bientot…

  9. Henri if you know the tubes you probably know why your’s might be an ‘H’… neanmoins……this could be handy if you don’t have it….
    https://www.nonstopsystems.com/radio/pdf-radio/article-bc348-kf6nur.pdf

  10. Is this the place to find the answer?
    The ID-plate on my BC-348 is missing. I have identified the unit by checking circuit diagrams and tubes. I have found that my radio belong to the “H-family”. But how do I know if its H, K , L or R. What separate them?

    Goran

  11. Avatar

    Hi Goran. I’m not personally sure of every detail…differences can be ‘minor’ like rerouted cabling or a component or component supplier change which affects an indent number as opposed to just a ‘mod'(modification)change…Someone will know and the original manual covering all models will help…

    I had one in the 1960;s…orange cover… probably donated it to the WIA who’s have thrown it out by now….as it’s just a ‘modernist’ organisation now. Some sets changed say the output tube for a 6K6 which someone may have done since production on yours….blurring which model it is.

    If there is a separate ‘mod’ plate you might be able to track some history which enables you to make a fair decision on model….Anyway…You’ll find some ‘model’ information in the tables here https://www.nonstopsystems.com/radio/pdf-radio/article-bc348-kf6nur.pdf I’ve seen another table, less colourful but may have had more information… but have to recall where

    However….whatever plate you put on it will not be truthful…..it will almost certainly not be the correct serial-number, order, date, maybe even model and manufacturer…. and so on….Was pulled off in echelon workshops for some reason…or sold by that huge radio spares place in your Arctic area…Personally I’d leave it blank,,,,There are a few reasons people pull the plates…to put on another set,…intend to repaint and lose it….or sell it without plate. Some even think it looks better….very queer!! These plates do come up for sale from time to time at ham-fests and eBay, or buy a wreck and snatch the plate, pull for spares even check one against the other to legitimise your set some what against the one you stripped.. If I get any further information I’ll post it Regards Jack

  12. Thanks Jack,
    Learning curve gets longer by the age… But I can now to almost 99% claim it is a BC-348-R, thanks to the informative page you linked to.
    It’s a CCC tube version +ceramic HFO socket. Two ** also tell that a plastic label probably was fitted on my radio. There are threads for four screws in the front on my radio, so no rivets. So I guess screws was used for plastic labels. I assume the Order then was 11414-43. I have also the base and Order is 11415-WF-43.
    So I can stop digging further into this. It is now up and running after I changed most of the capacitors.

    Thanks agn and best regards
    Goran

  13. Hi ok thanks to you and to Mark. I have an ‘R’…very nice….with plastic label and 4 tiny screws. I don’t like the ‘Q’ as much but it has obvious service advantages. The ‘R’ is more like a ‘ham’ set which keeps me mindful of not being one of the ‘buy and talk CB-Hams’….who think AR is a substitue for a telephone conversation (taboo in real ham days) and comparing their Jap sets, antennae and armchairs…(chuckle) then finishing with “73’s” OM which was a cw sign-off “73”…. never plural…..ah, nostalgia!!….how green was my valley……

    The Q I have I bought using eBay as it was an RAAF rebuilt set in 1953, complete with tags. The plate on them is riveted. When it arrived it had so many mods (not RAAF) I didn’t know where to start. I wish the guy had left a circuit in there as some wires go I think to test points or some plug in control….Thank god he didn’t go all the way to despoilation and install an s meter….it will turn out quite nice..one day….when I finish this Masters….

    Others would just rip it all out and go back to the set as it was. I’ve decided to rebuild the work sensibly and neatly and use the advantages. At some stage the RF and Vol controls were separated…but the original idea connected to frequency via the tuning cap was quite a clever idea…The power supply and extra audio transformer work is just appalling…but that can be fixed. What beats me is people cutting the spade terminals off the dynamotor supply cables and soldering them to sit in mid air….”Rough as guts” we call it in Australia…..

    I like the gc tubes…nostalgic and I have no obsessions about having the greatest radio on the planet…just to have it working really well and run through BC 453 for cw. I have had already owned the best and almost every disposals Rx ever made (chuckle) but the fun for me is bringing the signals up by dextrousness. By the way when peaking IF’s in the 348 determine the actual crystal frequency first then centre IF’s around that frequency. Doing it the other way around can find levels drop noticeably with crystal switched-in. Regards.

  14. Goran, Fair Radio sales (Idaho??) might even have a plate but were I doing it I’d get the original screws too…they just don’t look ‘right’ at all with anything else. I have seen two on eBay over the years but maybe Hamfests for one or a wrecked set..or contacting one of the serious 348 lovers and putting out the word. ‘Everything comes to him who waits’ ..sounds ominous actually,…. but ‘you know what I mean’.

    There’s a really nice g.c. set(‘P’) for sale right now on eBay…with frayed mains wiring..and with shock mount…it has riveted the nomenclamature plate. I wonder whether different manufacturers took different approaches or maybe your set and mine are earlier…or maybe the difference between a P and an R is the plate…There are so many things I’d like to know. One of them is why people put an s-meter into 348’s maybe because CQ put it up as a mod….Whilst I can think of a couple of technical aids (e.g. in transformer ‘peaking’) I don’t think I’ve needed an S meter for RST in 50 years, and Tone and Readability don’t have meters….still…it’s a good value ‘P’ at the moment if anyone is looking for a set with nice paint and with the shock mount an so on….Not being in USA and having 2 already the postage would be a deterrence for me but if I had none….well…this one has most of the wok done and perhaps a round meter could be fitted…look better.

  15. Avatar

    Hi All,
    I’m sorry to jump in with an argument that is not so fashionable, but I have a problem…
    I’ve got a BC-348-J that I’m trying to fix; I’m planning to use it as a general coverage receiver in my study.
    The previous owner did a LOT of modifications<<, some of them in an ugly manner, like a power supply connected here and there with electrician's wires, with joints covered with sticky tape…
    That is fixed now, and a misconnection on the CW switch (one wire was totally missing) is fixed as well.
    Now to my problem… The previous owner installed a meter in the upper right corner, to be used as sn s-meter (I know, it's nearly useless, but now it's there already…)
    I found a schematic on the Surplus Manual Conversion; but due to the modifications that I found, I'm at loss on how to connect it. It says to connect it to posts 2 and 6 on the power plug, but that plug is not there any more…

    Another question: the volume control has been rewired, now only one potentiometer is connected (as a volume control) and the Bias control is missing. Should i try to restore it?
    Thanks for your patience and your help. My email is paolo.gramigna@controllo.it, if you want to answer directly.
    Paolo from Italy (IK4YNG)

  16. Paolo,

    Its impossible to give any advice and tell what is wrong when the receiver is modified. Post 2 and 6 on the power plug is for mute from an external TX-relay. Just keep the wires shorted. If power plug is missing you have to find the wires in the circuit diagram. Instead of using the diagram in the Surplus Manual Conversion I should stick to this manual instead http://ekladata.com/PAr3yVT4nQiJS80hemrGhErCT64/bc348_manual.pdf

    Goran

  17. 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?

  18. Avatar

    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

  19. 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

  20. 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

  21. 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

  22. Avatar

    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…

    • 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.

  23. 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.

  24. 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….

  25. Avatar

    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.

  26. 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’.

  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

  28. Gee, what happened to the graded URL’s I just spent an hour researching and ‘submitted’?…?

    • 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.

  29. 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

  30. 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…..

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

  32. Avatar

    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…

    • 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

    • I forgot, Paolo,…what was the crystal frequency after the cleaning? Ciao

  33. Avatar

    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!

    • 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

  34. sorry…5 tube…

  35. Avatar

    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.

  36. 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)

  37. Avatar

    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…

  38. Avatar

    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!

  39. 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..

  40. Avatar

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

  41. 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”

  42. 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.”

  43. 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.

  44. Avatar

    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.

  45. 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…..)

  46. Avatar

    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!

  47. Avatar

    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.

  48. Avatar

    Update: looking with my oscilloscope, on a certain position of the variable I can see a self-oscillation starting on grid and plate of VT-150, the oscillator tube.
    How can I neutralize it? it’s the first time for me, I have no experience…

  49. Hi Paolo, is it affecting performance?

    Presuming your scope lead capacitance is not implicated and a scope fault scope not initiating oscillation,

    I guess number one, try another tube and see if it remains. Check all component values close to the tube and replace
    the capacitors one at a time to see whether it is one and if so ‘which’. The 348 manual gives a great deal of resistance and
    voltage information on each tube. Bear in mind that at this age the 348 can suffer high resistance joints, BFO issues, fungus
    in IF’s, resistor values all over the shop including infinite (cracked), leaky caps, soft tubes, sagging elements in tubes,
    insulation break down in shielded cables and I suppose ‘issues’ from components acting as antenna when specs are out…

    I’ve never fully read the service manual however the American manuals often had quite comprehensive fault finding
    information. I am ashamed to say that I am one of those ‘when all else fails…read the instructions’ people and quite
    fatigued from study at the moment so I hope my general answers may give some direction…Oscillation generally
    begins from the grid input and cathode grid plate interaction, which is why I first suggested try another (known good)
    tube….then the components close to the oscillator. Cheers

  50. Avatar

    Hi Jack,
    Thanks for your help. I’m waiting for a new tube to arrive, and I’ll immediately try it.
    For the moment, I have replaced all capacitors in the oscillator box, and a couple of resistors that had a raised value (more than 20%) and now the voltages are quite as expected to be in the manual.
    Before dismounting the 1st conversion box – detector box, I’ll wait for the new tube to arrive.
    The receiver is now very sensitive, maybe too much. I mean, with strong signals it shows some audio distortion, if I don’t reduce the volume/sensitivity control.
    I did rebuild the receiver as per in the manual, so now I have coaxial controls for volume and bias.
    I’ll post any further information. Maybe it will be helpful for other restorers.

  51. Hi Mark, did my December 11 response to Paolo arrive mark?…I think I forgot to fill in email etc?……Hmmm if I didn’t, it wouldn’t ‘send’ would it!….I’d hate to rewrite it! so might not..Please advise.

    regards Tony

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