TM-255A SWR Foldback reducing power?

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by K4ZOT, Jul 23, 2019.

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  1. K4ZOT

    K4ZOT XML Subscriber QRZ Page

    I have an old TM-225A that I acquired some years ago at a very good price and had it sit on the shelf since I bought it. I now decided to use it on 2M SSB. I'm working on installing a 4 element beam. But, I may have a power output problem. I can only get 30 watts out into a dummy load with a 1.4 SWR. The power should be 40-45 watts. Just wondering what the experiences have been with other TM-255a owners. I have heard that the radio is very sensitive to SWR foldback?

    Tnx for any feedback.

  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You could try a better dummy load, one rated for VHF-UHF so it's a better match. You might also try validating your wattmeter reading to see if it's correct.

    Also, check power supply voltage...a lot of "12VDC" rigs like this only provide max power output with about 14VDC applied, and start to lose output power with lower applied voltage.
  3. K4ZOT

    K4ZOT XML Subscriber QRZ Page

    Tnx for your response - I will follow up on your suggestions.
  4. KK6VQK

    KK6VQK Ham Member QRZ Page

    check input power, sure. but yes the kenwood mobiles got some foldback protection at some point and it;s pretty touchy.
  5. AI3V

    AI3V Ham Member QRZ Page

    Your radio, like every other fixed tuned radio ever built sets the output power by assuming a 50 ohm, non reactive load, and adjusting the rf output signal voltage to provide rated power.

    If the output impedance (vswr) changes, so does the output power.

    A given vswr can be caused by a load impedance either higher than 50 ohms or lower than 50 ohms.

    The "foldback", can only reduce the output power more.

    Read this:

    Start on pdf page 25 and read "technical facts of life" and "swr - 2 kinds"

  6. KK6VQK

    KK6VQK Ham Member QRZ Page

  7. KK6VQK

    KK6VQK Ham Member QRZ Page

    ..and it's sensitive for a reason. The finals can go "pop" pretty easily.. :/

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