High SWR only when amp (SB-221) is keyed

Discussion in 'Amateur Radio Amplifiers' started by K4OW, Jun 15, 2021.

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  1. K4OW

    K4OW Platinum Subscriber Platinum Subscriber QRZ Page

    I have a SB-221, only when keyed, it throws back high SWR 2:1+ readings on multiple meters and radios on all bands. I’ve been trouble shooting anything I can think of and can’t find anything obviously wrong. The grid and plate voltages read normal. With the amp turned off or on standby the SWR readings are near perfect. The higher the frequency the worse it gets when keyed. Any ideas where to start looking is appreciated. Thank you

    the real K4OW.
    Jack

    (Someone has been pirating my call sign.)
     
  2. N1EBC

    N1EBC Ham Member QRZ Page

    Ok Start with a dummy load. This may seem like over kill but it should suss out simple stuff.

    Put the dummy load on your radio key up. SWR should be flat.
    Now your put the dummy load on each jumper wire you use all should show a flat swr.
    Then connect up the amp to the radio and put the dummy load on the amp and see what you get.
    If ok there put the dummy load at the end of your feedline and see what happens.

    Somewhere in there you should be able to hone in on the issue.
    nosaj
    N1EBC
     
  3. N7WR

    N7WR Premium Subscriber QRZ Page

    My bet is a bad connector on one of the jumper wires--radio to amp is one possibility. I've had that happen myself
     
  4. K4OW

    K4OW Platinum Subscriber Platinum Subscriber QRZ Page

    I’ve used several jumpers and have tested and replaced the bad ones. These jumpers are fine and work perfectly on my other amps and radios.
     
  5. N7WR

    N7WR Premium Subscriber QRZ Page

    OK I was just suggesting to look there--obviously you have
     
  6. KD9IQO

    KD9IQO Ham Member QRZ Page

    The Heath SB series amplifiers were designed to work with vacuum tube transmitters. If you are using a solid-state rig, then you will need to modify the tuned-input circuits. Attached, please find the values that I used after a total modernization of my SB-220.
     

    Attached Files:

    N8VIL likes this.
  7. K4OW

    K4OW Platinum Subscriber Platinum Subscriber QRZ Page

    I appreciate the suggestion. Thank you.
     
  8. K4OW

    K4OW Platinum Subscriber Platinum Subscriber QRZ Page

    I’m familiar. I’ve been using FT990, FT1000MP, FT991, FTDX3000 and FTDX10 with ARB-704 buffers with 5 different Heathkit Amps and have never had this problem. I currently have another SB-221 that runs fine. It’s only with the one amp. Thanks for the info. I’m certain that it’s either in the tuning circuit or relay or output circuit. Just can’t see which is causing this issue. Thanks for the reply.
     
  9. KD9IQO

    KD9IQO Ham Member QRZ Page

    Is the high SWR between the transceiver and the amplifier or between the amplifier and the load?
     
  10. W4HWD

    W4HWD Premium Subscriber QRZ Page

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