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Why does my power supply keep burning out?

Flexsn Member, Unconfirmed ✭✭
So far my power supply has burned out twice during a radio conversation and can no longer be switched on. I had to send it to the manufacturer for repairs.

The power supply failed during about an hour and a half of radio operation in FT8 and FT4 with 90% CQ-Calls and 10% QSOs. The power was set to 100W. The frequency was 3,6 MHz, 7MHz and 14MHz. The antenna was a Kelemendipol DP804020. I use the internally installed ATU for tuning.

Previously, the repaired power supply unit with the Flexradio 6400M was in operation without any errors for around four days of two to three hours each. I used different transmission powers between 5W and 100W. The majority of connections were made in FT8 and FT4. But I also carried out a few SSB-QSOs, some with longer speaking parts.

The radio was not damaged and continues to function perfectly.

The power supply is located outside of a cabinet and is additionally cooled throughout with a fan.

The power supply is a Microset PT135.

Technical data: current 35 A (maximum current for a 50:50 cycle)
Continuous current: 20 A (time test 4h 17.5A).

Bst 73

Best Answers

  • VE7ATJ_Don
    VE7ATJ_Don Member ✭✭✭
    Answer ✓

    H Holger... in looking at the Microset website, they now have a 135B, which specifically says it has been designed for digital, full-time use so maybe there have been improvements to the circuitry/cooling to help out. Also, I note that the specifications state continuous output at a max of 22A, so running @ 20A is pretty tough on the power supply and running awfully close to its max. I imagine it was getting quite hot.

    Digital use is very hard on a radio and a power supply -- especially running at full power. And running at full power, the Flex will pull 23A (according to the hardware manual).

    A couple of suggestions:

    a) run at lower power -- possibly 50 or 70W maximum. Remember, FT-8 is designed for low-signal situations. I doesn't really need a lot of power to make connections/qsos.

    b) get a bigger power supply -- maybe look for one that can handle higher current on a continuous basis.

    just some thoughts.

  • Gord-VA7GP
    Gord-VA7GP Member ✭✭
    Answer ✓

    Hello, Holger:

    I think Don's comments are the best for you - lower, or larger. If it's burned twice, I think it will do so again if you repair it again.

    I am not much different than you, but my luck has been completely different. I have never tried FT4, FT8 nor any of these "continuous" modes. Perhaps my RTTY (and contests) may be similar, when I run 100W.

    My power-supply is also rated 35A intermittent, but 25A continuous. I wonder if this subtle detail for the continuous rating is important? I don't block my power-supply airflow, but I also do not use a fan on it. It sits beneath my desk, near my feet. My power supply happens to be an Astron RS-35A, also linear like yours, and in-use nearly continuously for 30+ years. I am impressed that it keeps going!

    I wish you better luck, whether you choose to replace or try another repair.

  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin
    Answer ✓

    My 2 cents

    I have a 30A Powerworx PS (the ones we sell).

    Over the weekend I was doing some destructive testing and the finals shorted out due to heat (I won't mention what radio it was or who made it) and the PS crowbarred when the current went through the roof.

    The good news is once I unplugged the radio, it worked just fine.

    When shopping for a new PS, make sure it has crowbar protection.



  • Neil D Friedman N3DF
    Neil D Friedman N3DF Member ✭✭✭✭

    I have made many thousands of FT8 full-power QSOs with the Powerwerx 25 amp (continuous) switching supply that Flex sells. I leave it on 24/7 to keep the GPS powered.

  • Neil D Friedman N3DF
    Neil D Friedman N3DF Member ✭✭✭✭

  • Flexsn
    Flexsn Member, Unconfirmed ✭✭
    edited December 2023

    Good Morning Don, Neil and Gord,

    thank you very much for your quick answers. These confirm my suspicion that the power supply is too weak.

    For FT8/4 I will reduce the power to max. 75W. But for SSB I should be able to use the 100W.

    The defective power supply was repaired after the first defect and after the second defect my online retailer sent it back to the manufacturer. I was sent a completely new power supply.

    The power supply does not have a built-in fan for cooling. To be on the safe side, I put a PC fan right next to it that runs all the time.

    Thank you very much for your help. I wish you a nice Advent season and a Merry Christmas.

    Bst 73 from Germany


  • VE7ATJ_Don
    VE7ATJ_Don Member ✭✭✭

    and Frohe Weihnachten to you, Holger!

    Maybe Der Weihnachtsmann will bring you a new power supply :-)

    Glad to be of help. Have a great holiday season!

  • Flexsn
    Flexsn Member, Unconfirmed ✭✭

    Dear Mike, dear Don,

    thank you very much for your informations and your help. My pocketmoney for the hobby is currently not enough for a new power adapter. So I will reduce the output.

    Dear Don, I laughed a lot at your message in “Denglish”.

    Dear Mike, your interesting videos on YouTube have helped me a lot.

    Thank you very much and Merry Christmas. Maybe we'll have a QSO together. My call sign is DL2DHM.Bst 73


  • Flexsn
    Flexsn Member, Unconfirmed ✭✭
    edited January 11

    Hello Yls and OMs,

    I wish you and your family a happy new year.

    I used the days off at Christmas to do a few tests.

    I measured the current strength at 5W, 50W and 100W transmission powers in FT8. I came to the following results:

    80m: SWR 1:1,1: 6,5A; 15A; 18,5A

    40m: SWR 1:1: 10A, 22A; 26A (60W) and 28A (70W)

    20m: SWR 1:1: 7A; 16A; 20A

    I was amazed at the very high current at 7.2Mhz. I could only measure up to 70W because the power supply with an ammeter could only deliver a power of up to 30A.

    My antenna is a Kelemen dipole for 80m, 40m and 20m. I also use the internal tuner.

    Bst 73

    Holger (DL2DHM)

  • WX7Y
    WX7Y Member ✭✭✭

    I would do this test without the Antenna tuner, a 50 Ohm dummy load and a external Watt meter to get a accurate test of what the radio is doing. What kind of Amp Meter are you using? A clamp on Amp Meter JUST CLAMPED on the Red Positive Power wire may give the most accurate reading.

    Not sure of your station, you may be getting RF back in the shack and getting into your Amp meter which may give you a false reading, this is why you want to use a Dummy load. It is hard for people to help when all the facts are not known and as much of the local station setup can be duplicated for comparison as possible.

  • Flexsn
    Flexsn Member, Unconfirmed ✭✭

    Good evening Bret,

    thank you very much for your answer. You're right. I will take the measurements again with the correct equipment.

    For my first measurements I used a power supply with a built-in ammeter. I connected my Flexradio directly to this power supply. If I have any news, I'll get back to you. Have a nice weekend and see you soon.

    Bst 73


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