TM-281A turns off when mic keyed.

Discussion in 'General Technical Questions and Answers' started by K5PEB, Apr 8, 2020.

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  1. K5PEB

    K5PEB Ham Member QRZ Page

    I have an old Kenwood TM-281A 144Mhz radio that I replaced with a Yaesu FTM-100DR in my truck. Well, with all this spare time now, I decided to use an old pc power supply to provide power to my 281A. The antenna, the only thing I have extra lying around is an Arrow II antenna that I primarily use for my HT working satellites. The coax connecting the radio and antenna is approximately 4 ft long. I do have a Diamond X30A antenna and coax ordered but decided to give it a go without for the meantime. I can receive all day long, great reception inside the house, but the second I hit the mic key to transmit the radio shuts off. I have to turn the power supply off then back on again and the radio will come back on and keep receiving with no problem. Until I hit the mic key again to transmit.

    I took the radio back out to my truck, used the truck antenna and truck power and was able to transmit just fine. I then brought the power supply out and used it instead of my truck power but used the truck antenna, still no problems at all with transmitting. However, when I connect the Arrow II antenna and try transmitting, the radio shuts off again.

    Anyone have any ideas why using the Arrow II antenna would cause the radio to shut off?


  2. W9GB

    W9GB Ham Member QRZ Page

    Paul -

    Two possible “root causes” : Insufficient Current (amps) from 12 VDC =or= Common-Mode.

    It is common to find PC SMPS that fail regulation or current capacity == and fail (shut-off).
    I have an HP 700-414 desktop, in for repair, that only had a failed power supply (shuts off)

    Switch-Mode Power Supplies (SMPS) do NOT like stray RF .... especially Common Mode.
    Describing your temporary antenna system setup, suggests that is possible cause.

    A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing
    Revision 7 Jan 2019
    by Jim Brown K9YC
  3. VU2NAN

    VU2NAN Ham Member QRZ Page

    Hi OM Paul,

    Maybe there's a problem with the Arrow II antenna as well as with the PC power supply.

    High SWR with the Arrow II antenna may be the cause of the TM-281A shutting down. At the same time the high current drawn from the PC power supply also causes it to trip.

    At the truck too, the TM-281 shuts down due to high SWR with the Arrow II antenna.


  4. KC3SWL

    KC3SWL XML Subscriber QRZ Page

    Should get or borrow a vhf SWR meter to check the antenna. Also the TM281 needs every amp it can get or it will shut down. So both antenna and low voltage amperage power supply will do this. I have one. I had same thing happen.
  5. K8JD

    K8JD Ham Member QRZ Page

    RF source (ant on 4 ft coax) near some power supply supplies makes them puke !
    A "computer' power suply is built without much strong, RF field, protecftion in mind.
  6. KC3SWL

    KC3SWL XML Subscriber QRZ Page

    Yeah I didn't see that 4 ft of coax........ that would saturate quickly
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That's very likely the problem.

    Get the antenna 20 feet away from the radio, or shielded effectively from the radio such as a mobile installation would be with a lot of sheet metal between the rig and the antenna, and the problem is likely to disappear.

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