BC-224 Receiver

Discussion in 'Technical' started by W5HRO, May 3, 2017.

  1. W5HRO

    W5HRO Administrator

    Anyone got's a BC-224 receiver? Just had to start a topic for one :biggrin:

    Below is the original BC-244-A until they went to the B model. The B model looks like the BC-348-B

    BC-224-A-1.jpg
    BC-224-A-3.jpg

    Here is a later D model which looks like the B model

    BC-224-D.jpg
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  2. Dnminbax

    Dnminbax Member

    I happened to purchase a BC-224B earlier this summer. I've had communications receivers for years. Always wanted a tube shortwave radio so I bought this BC-224B. Do not know anything about it other than it seems to have the standard mods. What is unique is that a faceplate has been fabricated to allow it to be rack mounted. The plaque on the front indicates an H Model but the cab shows B and it does not have the lower band. It worked the first time I tried (GFCI adapter just in case) seems to pick up stations fairly well. However the VT-48 and 5Y3GT tubes is sizzling hot, I replaced the 20 mfd power caps and filter caps with new ones. I attached the power supply pic and I get 273v on the B+ and -28v on the B- (terminals 5&4). I am self taught and early in my electronics education, do not understand how B- "floats" and was wondering if anyone could look at the pics and help figure out the correct way to get the voltage where it belongs. It looks like whoever had it refrained from hacking it up too bad, it has the original 101 output xfrmr and the power source is exactly as I got it, save the new caps. It uses the dynamotor base and wiring harness. AC is at terminals 1 & 2. I have pics of the wiring but didn't want to overdue the pics.
    Thanks in advance

    bc-224_front_panel.jpg bc-224b_dial.jpg bc-224 power2.jpg bc-224 xfrmr.jpg bc-224b cabinet.jpg
     
  3. W5HRO

    W5HRO Administrator

    The BC-224 was the original +14V model. The BC-348 came out after, but was +28V instead. That was the main difference. The military went to 28V aircraft systems instead of a 14V system so that's why RCA had to change it.

    I'm pretty sure the BC-224 tube filaments run off of +14V (+12V) and not +28V (+24V). Have you checked that? If you run them at +28V the tubes will overheat and the filaments will quickly burn out if I am right. Power the tube filaments up with a +12V DC regulated supply with enough current first and see if the tubes light up and run correctly.
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  4. W6MQI

    W6MQI Member

    Correct
    The original design of the BC-348 audio output stage was to ground the cathode and to negative bias the grid (fixed bias.) To achieve a negative bias the negative from the dynamotor was connected through the choke inside the audio output transformer unit to chassis. The resistance of the choke was about 250 ohms which elevated the negative above chassis and provided about minus 16vdc bias voltage. When designing your AC power supply, you will have to connect the CT of the power transformer to the B- line which then connects to the choke inside the audio output transformer unit. Then, in order to achieve good filtering, the negatives of the filter capacitors should connect to B-, not to chassis. This will provide good filtering for the -16vdc bias voltage and also will result in a power supply that has no perceptible hum. The exception is the last filter capacitor in the dual section filter which should be connected to chassis. I'm assuming the BC-224 is wired the same way I didn't check to verify.
     
  5. Dnminbax

    Dnminbax Member

    I guess I'm gonna need to trace all the wires to understand what has been done. Terminal 3 (brown tracer) from the dynamotor is now wired directly to the heater/filament on VT-93 and I do not get any voltage on terminal 3 when I ground to the chassis, same with the filament voltage. The only voltage I get terminal 3 is 27v when I check across terminal 4. I still get -28v on terminal 2 and 273v on terminal 1 (I am using the B-224-B instruction book as reference).
    Not sure I understand enough to know why, but the all tubes seem very hot, the plate voltages are 250v and more. Its crazy though, I pick up signals in all bands and the CW/BFO and crystal filter work. Maybe an entire new power system is the way to go.
    I apologize for my rudimentary knowledge but I figure the best tact is to understand what wiring changes have been made. I appreciate the help and hope ya'll can help me solve the issue when I get you better info. I'll holler back soon.
     
  6. W5HRO

    W5HRO Administrator

    I think what it is, is someone made that grey front panel and put the other old original black wrinkle finish painted hardware parts on it from a junked unit. That’s what it looks like so I doubt it is original. The grey front panel looks modern. Chances are they used the original front with the name plate on another unit or they sold it and just installed the front parts from a junked unit they had laying around along with it's name plate.
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