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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  7. Dnminbax

    Dnminbax Member

    I kinda figured it was a fabricated front with a different plate. Thanks for the corresponding assessment.
    I also did some more checking and I'm getting 6.3 vac on the heater circuit. It appears that the transformer is 450v based on the voltages and the B- does "float". It is connected thru the original audio output transformer. I cannot seem to figure out how to drop the secondary voltage or the output voltage from the 5y3gt to get the plate voltage down to the 200 vdc range (right now the plate voltage is 253 vdc). I bought a 325v plate and filament transformer and figure that replacement may be the best option.
     
  8. W5HRO

    W5HRO Administrator

    You just need to add a series 10 to 20 watt 500-ohm series resistor between the two 40uF caps to drop it down some. 253V isn't bad, but it could be a little less and the resistor will knock it down.

    5Y3GT_AC_Power_Supply.jpg
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  9. W6MQI

    W6MQI Member

    Do you want to stay with vacuum tubes in the power supply? You could use a MOSFET with a pot between the Gate-drain, adjust the output voltage to the desire value and your done.
     
  10. W5HRO

    W5HRO Administrator

    To do that you would want to use a high voltage Bi-Polar NPN transistor with a resistor and a 200V zener like the below or two 100V zeners in series and not a MOSFET with a pot. You would have really shitty regulation using a MOSFET and a pot. They make plenty of 300V to 350V NPN transistors that would do the job.

    simple-series-regulator.png

    That's all probably beyond his knowledge anyway like he was saying so the resistor would be the easiest thing for him to do. Those Navy BC-348 AC supplies below had the 500-ohm resistor so whoever built his just didn’t add it. It will also make the supply cleaner, less ripple.

    EP-298_2.JPG

    The other thing is did someone modify that receiver and place all of the tube filaments in parallel to ground? If it’s still stock then he wants a 14V supply and not 6.3V. I think the BC-224 like the BC-348 had the tube filaments wired in series.
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  11. W5HRO

    W5HRO Administrator

    By the way you could use a small 2N3439 high speed switching transistor in the TO-39 package for the pass transistor in the regulator circuit I posted and they are cheap, but are 350V @ 1A transistors. Then you could use two small 100V 5W zeners with axial leads in series or one 200V zener with an extra 2N3439 to provide enough current to the base on the next one. I probably should have added it to my BC-348 supply to regulate the B+. The transistor is small enough that it wouldn't have taken up hardly any extra room on the plate. Just be careful because the B+ would be on the case of the TO-39 package. A good test-point place though for touching your voltmeter to.

    TO-39 Package
    to_39.jpeg

    Adjust the value of R1 to get 12mA flowing through the 100V zeners under full load.
    1.png

    Adjust the value of R1 to get 5mA flowing through the 200V zener under full load.
    2.png
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    Attached Files:

  12. W6MQI

    W6MQI Member

    This is what I used in the Johnson Ranger LV supply.

    MOSFET voltage reg supply.jpg
     
  13. W5HRO

    W5HRO Administrator

    Yeah, that will work too with an FET, but they are way more sensitive and require added protection. Bi-Polars are just better for low impedance supply regulators. They are more durable and will just hold up better. I like the small 2N3439. All you need is one of those small round finned heatsinks slid down over it. They make some with higher voltages too. The two 100V zener circuit I posted would probably be the best option for the BC-244 receiver. How much current does the receiver draw, like around 70mA? Maybe 80mA to 100mA max when run down at 200V?

    P.S. You should always add small caps across the zeners. They will provide better overall regulation. 8uF to 10uF caps are usually big enough to do it.
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  14. Dnminbax

    Dnminbax Member

    W5HRO is correct, the solid state applications are beyond my understanding. I'm struggling with grasping tube power supplies. The tube filaments have been modified and it's not stock. The resistor installation seems like a baby step I can handle. However, part of my interest in owning a receiver like this is to learn, so I'll start researching these posts on a solid state option. I'll touch base once I try the resistor. Thank you for your patience and detailed explanations.
     
  15. Dnminbax

    Dnminbax Member

    Changed out the transformer to a Merit P2951. Still running a high voltage, 325vdc at B+. The plate voltage has now increased to 300v, I have ordered the resistor but wanted to point out that I have an unidentified choke on the B+ side per the attached schematic (the ac is wired thru the avc/mvc switch). Can I use a different choke to reduce the B+?
    I pick up stations on all bands, the audio is scratchy but not distorted, the CW and BFO work fine as does the crystal filter.
    I'm just getting too high of a plate voltage and don't want to burn the thing up.

    choke cap bc power.jpg
     
  16. W5HRO

    W5HRO Administrator

    If a choke is there then it's probably in the same place the resistor should be. If so disconnect the choke and try the resistor in it's place. Since you have changed the transformer the 500 ohm resistor may not be big enough now. Should have tried the resistor with the old transformer first to see where it was.
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  17. W6MQI

    W6MQI Member

    So its been verified the tube filaments have been switched over to 6.3vac? Took me a while to track out my BC-348 to verify the filaments were still in stock configuration.
    Yep one thing at a time otherwise your gonna start getting confused on what's what.
     
  18. Dnminbax

    Dnminbax Member

    Yes, filaments changed over.
    W5HRO - Agreed should have been diligent in my use of what I had..... That dawned on me AFTER I switched it out. I know better......

    Just another lesson, it has me reading about chokes in a power supply and calculating resistance. I guess I could go back to the original setup....I can always use more soldering practice.
     
  19. W6MQI

    W6MQI Member

    I would put the old transformer back in and start over.
     
  20. W5HRO

    W5HRO Administrator

    None of his numbers are adding up. The old supply was a full-wave CT supply. His original post said he was seeing 253V to the tubes and that the transformer was 450V based which had to be incorrect. Now that we think it's a choke in the middle then you would take the 253V divided by 0.45 which is 562V. Then the voltage drop of the 5Y3 tube would be around 60V. So, 562 + 60 = 622V and that would be about right. Most of the old tube supplies for those receivers used around 600V transformers and if capable of a bit more than the 70mA to 75mA needed to run the receiver they would run hotter and with the voltage drop of the 5Y3 and multiplying that by 0.45 with a choke would still give you the plate voltage. If you used the 500-ohm resistor instead of the choke then the plate voltage to the tubes would be less.

    He then said he got a new 325V transformer. It's probably a 650V CT transformer and that's why it's too hot now, but the math doesn't come out. 650V - 60 = 590V. Then 590V x 0.45 = 265V so how the heck is he getting 300V to the tubes?. Even if the transformer puts out more than 75mA it should be lower than 300V unless it puts out well over 100mA. About 20V higher max would be the norm, but 35V higher? My next thought is that choke is not a choke and it's a 14V or higher filament transformer. I don't see any other transformer in his pic on that metal plate so what was powering up the tube filaments? Maybe he just measured across each of the tubes and saw around 6V, but that's exactly what you would see. It doesn't mean the filaments are not connected in series, in parallel with the other tubes connected in series. If that is a filament transformer instead of a choke then the voltage math with the new transformer would make more sense and it would be a lot higher like it is because it's all capacitor input. Even with a choke its still considered a capacitor input supply, but with the choke in the middle you go by the average calculation.

    It's either that or the original transformer had the filament winding to power up the tubes and the numbers he posted like the 325V are just wrong and it’s even higher than a 650V transformer unless, he went to a full-wave bridge supply, but I’m assuming he’s still using the 5Y3 and the full-wave center-tap B- setup.

    Anyway, something is just not adding up with what he has told us. I've been doing the simple supply math in my head since the early 1970's and something isn't right. If he wants around 200V or just slightly above on the tubes then he wants like a 525V max transformer with a center-tap using the 5Y3. If not then he has to use a 500-ohm resistor instead of a choke to drop it down using a 600V transformer. A 650V or higher transformer is simply too hot and it would require a much bigger resistor to drop it down.
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  21. Dnminbax

    Dnminbax Member

    Perfect, even tho I may be wearing y'all out. I've looked at scores of postings and videos and never recd so much insight.
    I need to go back thru and validate my information. The solution is only as good as the data. I may not be properly measuring or communicating the voltages. Shoot, I didn't realize that the heater voltage was ac in lieu of dc until I studied further.
    Thank you
     
  22. W6MQI

    W6MQI Member

    Shot in the dark, but is that a 117v primary transformer if it is that may account for the higher output voltage running 120+Vac line voltage I'm sitting around 123.5Vac right now sometimes its even higher.
    I would still go back to the original transformer and when you get the resistor set it up and check back here with the new numbers.
     
  23. W5HRO

    W5HRO Administrator

    That's also a possibility. My line voltage here runs hot around 130V with the occasional surge up to 140V and sometimes higher. Poor regulation down here in the South Bay. It's the reason I bought the APC unit for my vintage McIntosh Hi-Fi stuff and leave it in buck mode so it always stays down around 110V max.
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  24. W5HRO

    W5HRO Administrator

    I went back and looked at his supply pics and found the old data for that Peerless R-196-A power transformer and it does have the 6.3V winding so that answers that question and I was right about it being a 600V transformer. It's a 300-0-300 so it's 600V with the "0" center-tap being the B- reference. It's also only a 50mA transformer so it was a bit light for the job. If the 6.3V winding is connected to the filaments then that means someone did convert it for 6.3V which is fine and was common practice back then.

    Untitled.png

    What he wants is another transformer around 520V minimum (260-0-260) with at least a 80mA to 100mA current limit. 520 - 60 X 0.45 = 207V would be perfect. Most people run those receivers around 210V anyway. I think only 200V is a bit low, but will still work. Hammond makes some small 550V transformers with the 5V and 6.3V windings, but it would require the resistor where the choke is like I said before.
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  25. Dnminbax

    Dnminbax Member

    Wow
    I looked all over the internet and couldn't find the transformer spec. So I am now thinking I should replace the Peerless as you outline above.
    I'm gonna head that direction and let you know where I end up.
    Thanks for the help.