Other new toy

Discussion in '"Boat Anchor" & Classic Equipment' started by K9STH, Aug 7, 2018.

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  1. KC8VWM

    KC8VWM Moderator Volunteer Moderator QRZ Page

    I also have a BC 348 "R" in need of some work. (A lot of work?)

    I picked it up at a hamfest for $20. One of those deals where the radio was no longer original, had holes drilled in the front panel and was missing screws to hold it all together lol.

    I thought it would be good for parts, but I actually got the thing going again. So now there's no turning back right? lol.

    Someone installed additional controls to the front panel. One is an RF gain control and the back half of the original volume control ( double gang type) was separated resulting in another new adjustable control installed on the front panel. So now it has dual volume controls ! LOL. Actually the circuit itself has something to do with the AVC / MVC.

    The original MVC / AVC "handle switch" is missing. Just a hole there now but oddly a third hole was drilled to install an on/ off toggle switch for power? I think I would have used the existing hole where the MVC/AVC control switch used to exist myself, but I suppose there's no point in trying to make sense out of it all.

    It has some kind of modified BFO and an additional tube and circuit was added inside. It's kind of an interesting mod I never ran into before on BC 348's.

    I thought about removing the "mods" plugging the holes and restoring it to its former ugliness, but it's missing some parts like the spindle rods used to secure it to the cabinet. The MVC / AVC control is missing and some of the control panel knobs are not original.

    Power supply and audio transformer is a bit of a botched job, (like many) but at least it actually works now. The wire to power the LM27 lamps was cut off and disconnected so the lights are there but they don't work. Wonder why that was done? I'll have to figure out where it's supposed to be connected.

    Anyways, I am just playing with this $20 BC 348R just the way it is for now. I put it back on the bench and played with it last night after reading your replies, but i'm not really sure what I am going to do with it exactly.
    Last edited: Aug 10, 2018
    WN1MB likes this.
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    For the era in which they were designed (early 1940s), the BC-348 series are a pretty good receiver. The difference between the BC-348 and the BC-224 is that the BC-348 is designed for 24-volt to 28-volt aircraft and the BC-224 is designed for the 13.8-volt aircraft.

    Some people prefer the BC-312 / BC-342 receivers over the BC-348. The BC-312 and BC-342 receivers do not have the lowest frequency band (200 kHz to 500 kHz) but do have a vernier control in addition to the main tuning knob. I do have a BC-312M that I brought back from the dead


    as well as having a BC-348Q that I have owned for decades. Frankly, I do not see all that much difference in performance.

    Up to, and including, the BC-312J model the only difference between those models and the BC-342 models was the fact that the BC-312 models were designed for operation from battery sources and the BC-342 models were designed to operate from 117 VAC power. Then, starting with the BC-312K models, the crystal filter was eliminated and, as such, there was a difference in the receivers.

    For operation in the 1500 kHz to 18,000 kHz segment, the performance of the BC-224, BC-312, BC-342, and BC-348 are much better than any of the lower tier general coverage receivers and at least equal to, if not better than, a number of the middle tier general coverage receivers even into the 1950s. The surplus receivers are more stable (frequency drift) than a lot of other receivers that were of newer designs.

    Glen, K9STH
  3. KC8VWM

    KC8VWM Moderator Volunteer Moderator QRZ Page

    Worked on it again a little this evening.

    Here's the $20 BC-348R on the bench in operation with all its crazy extra knob mods.

    What kind of work / rework does yours need?
    WN1MB likes this.
  4. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    I am slowly making progress on the sensitivity. First of all, there was a wrong tube in the mixer stage. Someone had put a 6K7 in that position instead of the proper 6J7. Getting the correct tube into that stage improved the sensitivity by over 30 dB. However, the sensitivity is still quite a ways from where it is supposed to be.

    Also, the "ground" connector next to the antenna connector had been moved and there was no connection to the chassis. This was because of the paint, on the outside, plus the fact that several rusty washers were in place as well.

    The one modification that was actually done right was the splitting of the audio gain ("volume") control away from the r.f. gain control. This is the first modification that is often done when the B-348 is demilitarized.

    A large percentage of the resistors have changed value and some of those have gone as much as 80% higher in value and there are still more of them to be checked. Getting those resistors replaced has also contributed to getting the sensitivity where it should be!

    Glen, K9STH
  5. KC8VWM

    KC8VWM Moderator Volunteer Moderator QRZ Page

    I often find changing out of spec. resistors seems to "clean up" the quality of the signals.

    What are your thoughts about the micomold caps?
  6. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Micamold capacitors are paper capacitors, not mica types. Those are an automatic replacement!

    Glen, K9STH
    KC8VWM likes this.
  7. K5DH

    K5DH Ham Member QRZ Page

    BC-348s were made by four contractors: Stromberg-Carlson, Wells Gardner, RCA, and Belmont. The E, M, O, P, and S were similar and are covered in the same T.O. The J, N, and Q were similar. The H, K, L, and R are similar. I have scanned copies of all three T.O.'s on my hard drive. The three major variant groups I just mentioned are surprisingly different internally, and not a whole lot interchanges between them. All BC-348 models ran on 28 VDC as they were intended for use aboard aircraft. The BC-224 series was the same receiver but set up to run on 14 VDC for use in ground vehicles. Despite what the "experts" on eBay might claim, no BC-348 or BC-224 ever came with a mains-operated power supply. (The reason I mention this is because I've seen them listed on eBay as having the "original AC power supply" installed). The DM-28 dynamotor unit is very scarce, especially in restorable condition. Despite the different letter suffixes, all DM-28 models will interchange. The FT-154 shockmount tray is pretty hard to come by, and the power connector (which is part of the FT-154) is really scarce. I've seen the dynamotors and shockmounts sell for more than a working receiver. The last couple of power connectors that I bought (several years ago) were $25 each -- used -- and that was a bargain.

    It's rare to find a '348 or '224 that hasn't been "ham-ified". As Glen mentioned, some of that Hammy Hambone workmanship is truly atrocious and often downright dangerous. Before you power up any vintage equipment, scan it carefully for "death waiting to happen", and make the necessary corrections before you flip that switch.

    I recently picked up an unmolested BC-348-R with a DM-28-Q dynamotor for $50 at a surplus sale. It's pretty clean inside because it had been stored for several decades in a dry warehouse, which means no significant rust or corrosion. I haven't powered it up yet. Lots to do before that! After I sold the last of the seven BC-348s that I had, I told myself that I was done fooling with them. Then this one came along. . . :)
    N2EY, KC8VWM and WN1MB like this.
  8. KC8VWM

    KC8VWM Moderator Volunteer Moderator QRZ Page

    Same problem here. :D:p

    This is the last one I restored from a hack job. It was "Q" version.

    I even put a brand new wrinkle finish paint job on it and here's how it turned out. This was just before the lines were repainted on the knobs.



    There's a secret for preserving the existing lettering i'll share... If you look really, really closely you can tell the lettering was masked before painting.




    This masking of the lettering was tediously accomplished using small strips of silly putty and a toothpick. :)
    Last edited: Aug 11, 2018
    K6LPM likes this.
  9. KC8VWM

    KC8VWM Moderator Volunteer Moderator QRZ Page

    I agree but there appears to be mixed feelings about replacing them for the sake of preserving originality for some apparent reason.
  10. KE0ZU

    KE0ZU Ham Member QRZ Page

    There may never have been any come from the factories with an AC supply, however, there was a post war field mod to add one. I have two versions of instructions, on "generic" and the other specifically for the "Q" version of the receiver. The supply schematics for both are the same.

    Click the pic to see all the sheets.
    Last edited: Aug 11, 2018

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