SB-200 Looking for the T/R relay? Harbach sold out.

Discussion in 'Amateur Radio Amplifiers' started by KD0SGX, May 10, 2020.

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  1. W1QJ

    W1QJ Ham Member QRZ Page

    Somebody read this thread and saw where I had a good used one and scooped it up in a heartbeat.
     
  2. KD0SGX

    KD0SGX Ham Member QRZ Page

    Makes sense, mine's working right now I just want to source a good replacement that's new that all my replacement parts would be clean and shiny and look like new. I'm sure you understand. plus that I don't have to worry about next week when the relay stops working because it worked perfect when it was removed but now it doesn't because it's 40 years old.
     
    W1TRY likes this.
  3. W1TRY

    W1TRY XML Subscriber QRZ Page

    I believe Greg @W9GB gave you a great place to start and do the research and cross reference to find what you need. It'll take a little sleuthing and some patience, but that's part of the fun. I believe that there is a new equivalent relay in current Potter & Brumfield offerings. I don't have the amplifier to be able to help, I'm just a cheerleader...
     
    KD2ACO likes this.
  4. WB4YVO

    WB4YVO Ham Member QRZ Page

    would the relay from the SB-220 work?
     
  5. W1QJ

    W1QJ Ham Member QRZ Page

    Yes, with the right resistor.
     
  6. WB4YVO

    WB4YVO Ham Member QRZ Page

    Ok What is the right resistor? Or how do you figure it out..

    Skip
     
  7. W1QJ

    W1QJ Ham Member QRZ Page

    You have to figure the operating impedance of the original relay. To do that you have to measure the current the relay draws at the voltage across it as supplied by the sb200 amp. Using ohms law you can calculate hot impedance of the coil. Then calculate the total resistance of the original relay coil in parallel with the resistor across it using ohms law. You’ll end up with single resistance of that parallel network. Now get whatever relay you want to replace the original, like an sb220 relay which will work. Do the same thing as before. Put the same voltage across the relay as it will see in the amp and once again measure the current drawn by the relay. Use laws law to calculate the operating impedance of the coil. Using that value now calculate the value resistor you need in parallel to equal the net value you calculated with the original relay. Now, when the relay is closed that net current drawn through the relay/ resistor combo should equal the original combination resulting in the proper idle current of about 90 ma for two good tubes. It’s that simple.
     
    NQ1B and W1TRY like this.
  8. W9GB

    W9GB Ham Member QRZ Page

    W1TRY likes this.
  9. W1QJ

    W1QJ Ham Member QRZ Page

    Replacement relays are fine in most cases but it needs to be pointed out that not just ANY replacement will be proper for the SB-200. It depends on the actual relay coil impedance to set the idle current. If it is not the same impedance relay it has to have a resistor to make the match. Please be aware of this. CTR probably does not an exact relay that would be "drop in" unless it is the same.
     
    W9GB likes this.
  10. W9GB

    W9GB Ham Member QRZ Page

    Lou,
    Agree.
    The relay’s coil resistance is important in the SB-200, as commented and noted here.
     

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