Other new toy

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

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  1. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    I just started on the BC-348 that I obtained in the radio auction. It is a BC-348R version.

    I have seen bad workmanship before, but the wiring changes, modifications, etc., put all of the rest of the bad examples to shame and that is in a bad way! Wires have been disconnected and then connected back with horrible "globs" of solder, half of the heater ("filament") string has been rewired for 6.3-volt operation and the other half is still for the original 25.2-volt operation and then a 26-VAC wallwort was connected to provide the voltage for both! Fortunately, with half of the heaters in parallel, the voltage was dragged down to where those wired for 6.3-volts did not get enough voltage for the heaters to go open!

    The power transformer is pretty well mounted, but the diodes and filter capacitors are very poorly installed and the soldering is even worse!

    Now, using the 26 VAC walwort is a novel idea and it is not a bad approach. But, why half of the heater string was changed to 6.3-volts and then the original voltage applied to the entire heater string is beyond imagination!

    Some of the capacitors, including the electrolytics, and resistors have been replaced with modern versions. Of course, how they are connected is another matter!

    There are penciled notations on the r.f. and local oscillator modules with 1944 dates saying that they were checked and found to be OK! That is a bit unusual, at least I have not run into such before.

    At least the receiver is not smoking even though it is definitely old enough to smoke!

    There is definitely hope for the receiver although it is going to take some work to get it working.

    Glen, K9STH
    KC8YLT and K9ASE like this.
  2. KE0ZU

    KE0ZU Ham Member QRZ Page

    That kind of thing is amazing. Just when you thought you'd seen it all, you get another example of what is worse.

    I got a free TCS Tx/Rx from a friend, who had it given to him some years earlier. Someone had pulled the 1625 RF output and modulators, and replaced them with 807s. Now these are essentially the same tube, but one is a 7 pln, and the other is a 5 pin, so there was a lot of effort in that process. it looked as if he just quit after making the switch, and left the thing as a Rats nest of wires and unmounted parts. That also took a while but its been a fun rig.

    Good luck with your project, hope you'll enjoy the result.
  3. KC8VWM

    KC8VWM Moderator Volunteer Moderator QRZ Page

    Doesn't the BC-348 usually require 250v B+?

    This must be a 28v version.
    Last edited: Aug 7, 2018
  4. KE0ZU

    KE0ZU Ham Member QRZ Page

    From the 348 Doc.;
    I bought my 348 from Fair Radio in 1974 for $75, new in a box. Most likely one of the last ones, and still going strong. I learned CW on one in '53.
    Last edited: Aug 7, 2018
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  5. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    The B+ needs to be in the 225 to 250 volt range. The 26-volts is for the tube heaters ("filaments"). There are 8-tubes, each with a 6.3-volt heater which are wired in a series-parallel arrangement. That is, 4-tubes are wired in series for each group, thus requiring 25.2-volts and then each group is wired in parallel with each other.

    One of the common modifications is to remove all of the heaters from the series-parallel arrangement and then wiring all of them in parallel so that a 6.3-volt winding either on the same power transformer used for the B+ or by a separate filament transformer.

    Remember, the BC-348 series were for use in aircraft that had 24-volt to 28-volt systems.

    Here is a photograph of the dates penciled on the sub assemblies in the receiver:


    I was exactly 6-months old when the assemblies were checked!

    Glen, K9STH
  6. KC8VWM

    KC8VWM Moderator Volunteer Moderator QRZ Page

    I'm aware of changing the heater voltages from series to parallel for 6.2v, but i'm trying to figure out how a 26 volt wall wart is somehow powering up the rig ?

    Does this mean you have the Dynamotor?

    Amazing to see those pencil markings still intact.
    Last edited: Aug 7, 2018
    KC8YLT likes this.
  7. K6LPM

    K6LPM Ham Member QRZ Page

    I suppose thats Amateur radio.... well I like to imagine that this piece of work was an inquizzative and budding youngster. I can imagine my own craftiness might resemble such a mess when I was around nine years old and without proper guidance.
  8. KE0ZU

    KE0ZU Ham Member QRZ Page

    I wasn't very old either, just 13 days shy of my second birthday.

    Looks like your receiver also has the early double ended tubes and a separate glow tube regulator.
  9. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    The wall wort was there to provide the voltage for the heater series / parallel arrangement. However, 1 of the arrangements had been rewired for 6.3-volts and the other was still wired for the 25.2-volt arrangement. Then, the wall wort was connected to the entire heater load. With 4 of the heaters in parallel, the total load on the wall wort dropped the voltage to around 9-volts. Way too low for the half that was still in series and too high for those in parallel.

    The dynamotor has been replaced with a pretty shabbily wired AC power supply.


    The BC-348R was the last of the BC-348 series but has most of the circuitry of some of the earlier models. I don't know if this was done to use a quantity of the earlier components that were still in stock or what.

    The 2-r.f. stages are not working. I can inject a signal into the grid of the "detector" (mixer) and everything works fine. But, each r.f. stage, working towards the antenna, has less and less gain. I can force a signal through the antenna jack, but it is greatly attenuated.


    My Heath RX-1 Mohawk came from a college amateur radio station and there had been repairs made that my then eldest granddaughter, who was 2-years old at the time, could have done better workmanship! You would be surprised at how bad workmanship that adults can do.

    Glen, K9STH
  10. K6LPM

    K6LPM Ham Member QRZ Page


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