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BC-348Q Issues

Discussion in '"Boat Anchor" & Classic Equipment' started by WB0LDJ, Nov 21, 2021.

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  1. WB0LDJ

    WB0LDJ Ham Member QRZ Page

    I have a BC-348Q that's been setting in the back room for eons, and I decided to get it out and see if I can get it on the air. Initially, it was dead, then I found the relay line that someone had removed during the a previous "conversion". I connected it to B+ and actually heard some VERY weak signals. The receiver has a couple of strange problems, though.

    First, it's very unstable. The tuning is as broad as a barn and the only way I can get it zeroed on my sig gen is to use the fine adjust on the generator! To say there's backlash is a gross understatement!

    Second, when I turn up the Audio Gain pot, I get a terrible oscillation that goes from a rough hash to a motorboating sound depending upon how far I turn up the gain control. In AVC mode, the oscillation is always present, regardless of the gain setting. In MVC mode, I don't get the oscillation until the gain pot is almost all the way up.

    I realize there's some strange oscillations going on inside the receiver, and I'm planning to swap out some caps in hopes that it will reduce, if not eliminate the problem. I do have some questions about the caps, however.

    The "Q" model doesn't have the dreaded "paper" micamolds, but it does have a bunch of the oil-filled tubular caps. Should I swap those out? Someone once told me that they last forever, but I'm dubious about that. I have plenty of the yellow poly 630V caps - it's just the hassle of getting in there and changing them out.

    I've checked the tubes on my TV-7D, and they all seem to be OK.

    I'm thinking of separating the AF and RF gain controls, if I can figure out where to put the extra shaft without drilling any new holes in the panel. Concentric pots would be nice, but finding a 350K audio taper pot ganged to a 20K linear pot is like searching for hen's teeth.

    Does anyone have any experience with this problem? Any suggestions as to where to look would be much appreciated.

    Mike Harmon, WB0LDJ
    mharmon at att dot net
  2. WA1GXC

    WA1GXC Ham Member QRZ Page

    Yes! I have experience! With everything you've said. I own 3, by 3 different mfrs.

    I gotta run and have a busy day but will re-visit this perhaps tomorrow.

    The "motorboating" is exactly that. You have one or more bad bypass caps.

    The paper-oil caps are presumed all good. Presume they're all bad. All but about 3 out of the entire complement will be.

    Mechanical bacllash--should be smooth tuning. Clean and lube.

    GL See you later

    Change every cap you can find
  3. W9GB

    W9GB Ham Member QRZ Page

    You did not mention whether this BC-348Q is powered by the original Dynamotor, OR
    has been converted to 120 VAC.

    This is your WINTER Project …
    It will require Patience, Attention to Details, and Acquisition of Missing Parts
    You May Find Earlier Repairs (or BAD Modification work) you will have to Correct.

    Radio Boulevard
    Western Historic Radio Museum
    Rebuilding the BC-348 Family of Receivers

    Beginning Restoration of BC-348Q
    Cliff, KD7NKN
    Antique Radio Forum (2016)

    Robert Sumption, W9RAS … a former Heathkit engineer
    YouTube video (December 2013)
    WW2 BC-348Q Receiver Restoration (acquired in 1972)

    Robert Sumption, W9RAS (Part 2)
    WW2 BC-348Q (Rattle, loose screw - shorted B+)
    December 2020

    Ray Fantini, KA3EKH
    YouTube video (June 2009)
    BC-348Q Receiver Restoration (4 modifications shown)
    Last edited: Nov 21, 2021
  4. W9GB

    W9GB Ham Member QRZ Page

    It is NOT off-the-shelf , can be DIY built (accomplished).
    Already done by countless restorers, especially music industry restorers (guitar amps).
    START by measuring the Size of the existing Potentiometer and Space behind panel.
    Most likely this is a 1 inch (24mm) potentiometer.
    You already know the Resistance and Taper.

    CTS (old Chicago Telephone Supply) : Potentiometers, 24 mm (~1 inch) common
    as well as 16 mm sizes (model 270, 280, 282, 284)
    Musicians and Guitar Amplifier restorers (major buyers) use the 24mm size (450G).
    Other 24 mm models: 450, 295 and 026 (5 watt)

    You can disassemble and build what you need (CTS offered a Build-a-Pot kit from 1950-70s)
    Not a DIY type Restorer (no time or skill set)
    STATE Electronics in New Jersey (yes, a USA company) does custom work.
    Their web page is: !!
    They are an Alps and Bourns distributor.

    They have a MOD-POT series,
    so that you can pick the specific parts/functions you need (Build a Pot).
    Selection Guide

    36 STATE ROUTE 10 STE 6
    PO BOX 436
    EAST HANOVER NJ 07936-0436
    USA Toll-Free Telephone: (800) 631-8083
    Local: (973) 887-2550
    Fax: (973) 887-1940

    CEO/Senior Vice-President
    Thomas Sutcliffe
    ext. 107
    Last edited: Nov 21, 2021
  5. W9GB

    W9GB Ham Member QRZ Page

    CTS Concentric Rotary Potentiometer, series 450

    CTS 450 series - 24mm Rotary Potentiometers

    NOTE: IF You use a Concentric Potentiometer, THEN you need to make Decision on KNOBS.
    1/4” and 1/8” shafts are common today. 3/16” shaft was an option in 1970s (and before).

    Disassembling a Bourns Potentiometer (Repair or modification)
    CTS Customization can occur by Swapping desired board from another 450 series.
    CTS are different from Bourne in assembly (as shown in video).
    Jeweler-like Hand-Eye Coordination a desired skill

    Last edited: Nov 21, 2021
    KA9P likes this.
  6. WA1GXC

    WA1GXC Ham Member QRZ Page


    Several random but notable pieces of info:

    I agree with the statement above, be extraordinarily observant as to picking out "improvements" and adaptations in wiring executed by previous ham
    owners. If you see something that appears crazy and makes no sense, it's probably because it's crazy and makes no sense. Do not rely upon prior re-wiring
    to power connectors/terminal boards to safely operate the gear.

    --As a corollary, one of the most common failings of attempted adaption for ham-use was to bring in the plate supply voltage incorrectly.
    Note in the schematic : the original B+ power line from the dynamotor is referenced to a "floating" B- line------The plate supply should not have a negative return to common
    ground/chassis potential. This is true of all BC-348 sub-types--both the original 'double-ended' tube line-up with grid-cap tubes and the BC-348 [J, N,Q] series
    with 'single-ended' tubes. Many conversion articles inexplicably say this pertains only to [ J, N, Q] variants. Not true--it applies to every BC-348 as manufactured.
    This isolated B- line is required for proper cathode-biasing, especially of the audio output stage. Note the choke between B- line and chassis ground.

    The capacitors as installed have great variability in their viability. Paper-oil are often trusted and that trust is often misplaced. The thing's 75 years old, for Pete's sake.
    It was the War and quality of components were a consequence of the suppliers contracted.

    Which brings us to trimmers--The air-variable capacitors in the shielded boxes are either good quality or will result in heartbreak. Some were made by ASP and
    they have press-fit (swaged) mechanical assembly to the rotor. If you're attempting an alignment, spray every one of those variable trimmers with solvent
    (CRC QD works swell as a safe mechanical solvent as well as a lower-cost electronic cleaner) before attempting to rotate. If there's any resistance to movement whatsoever--
    S T O P-- and wash it down again. Work the rotation back and forth gently. If your receiver has one of these press-fit components, the head will pop right off and the
    rotor and stator will be spring-loaded to a mechanical and electrical short-circuit with potentially disastrous consequences. If you've got one bad one, it's almost certain
    your builder used that type for every one of the trimmers. Beware and be careful.

    Don't be afraid to take off the front-panel "tube-shelf" access plate. Do ohmmeter checks on the screen, plate, and cathode resistors on all RF-circuit tubes and
    audio tubes. High probability they've drifted very high from age and abuse by---hams running your receiver with the B- to ground.

    Get yourself a complete reprint of the Signal Corps Technical Manual. For early receivers, there are editions that are quite complete listing all serial-letters
    [A, M, N, O, P etc.] and the -J,-N, -Q entirely different tube line-up. Many surplus outlets provide good-quality reprints. You need the book--schematic reference
    numbers vary with production run, manufacturer, letter-variant type.

    Breaking out the RF and AF gain is fine, but I don't think you'll find it will do that much for you. Not worth the effort in my judgement.

    Good luck.

    It's a very nice and fun receiver. And a piece of history. It won't compete with communications receivers even 10 years more modern, but you can use it
    on 80, 40, 30 meters and have a good time.

    Last edited: Nov 22, 2021
    KA9P likes this.

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