Strange Power Supply Behavior on Skyworth 55U5A14G

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  • Gjackson
    Senior Member
    • Aug 2021
    • 134
    • Malta
    #1

    Strange Power Supply Behavior on Skyworth 55U5A14G

    Hi, I have a 55 inch Skyworth tv that has a visible standby light but won't power on, Usually this fault is indicative of a bad main board but I went to test the power supply just to make sure of that. The power supply output has 14 pins (see image attached), here are my readings when the main board is connected to the power supply (PSU model: L5R021 ,I cannot find much info about it as it is not popular):

    22/12v - 12v
    22/12v - 12v
    GND - GND
    GND - GND
    12V - 12V
    12V - 12V
    GND - GND
    GND - GND
    NC
    NC
    STB - 4.66V
    NC
    ADJ - 4.98V
    ENA - 0.05V

    This is already a little confusing to me, first off I do not understand how 12v is at the output, isn't the enable pin practically at 0 volts? What is ADJ? and The STB seems a little low to me.

    The following are the readings I get with the main board and backlight connector disconnected from the PSU.

    22/12v - 12v
    22/12v - 12v
    GND - GND
    GND - GND
    12V - 12V
    12V - 12V
    GND - GND
    GND - GND
    NC
    NC
    STB - 0V
    NC
    ADJ - 0V
    ENA - 0V

    How come STB is now completely gone but the 12v remains still? STB should always maintain a constant voltage even when the main board is disconnected correct?

    If I have 12v at the output always, is the missing STB when it is disconnected an issue? Could I just buy a replacement main board and it would work?

    Forgive my inexperience but this all seems a little too confusing for me, any help would be very much appreciated!!
    Attached Files
    Last edited by Gjackson; 05-21-2022, 04:23 PM.
    Tags: None
    • R_J
      Badcaps Legend
      • Jun 2012
      • 9532
      • Canada
      #2
      Re: Strange Power Supply Behavior on Skyworth 55U5A14G

      This is an always on power supply that outputs 12 volts at low current, The stb pin voltage is supplied by the main board and turns on the pfc circuit etc on the power supply so it will operate at a higher current to operate the tv/backlights etc.
      The main board will likely have a 12 volt to 5 volt regulator.
      The STB pin is the same as a PS_ON pin.
      Last edited by R_J; 05-21-2022, 05:07 PM.

        Comment

        • Gjackson
          Senior Member
          • Aug 2021
          • 134
          • Malta
          #3
          Re: Strange Power Supply Behavior on Skyworth 55U5A14G

          Thank you!

            Comment

            • dskall
              Badcaps Legend
              • Oct 2016
              • 2905
              • usa
              #4
              Re: Strange Power Supply Behavior on Skyworth 55U5A14G

              So then STB should change state when you push power button? What is ENA for?
              I assume no responsibility for any stupid suggestions I might post.

                Comment

                • lotas
                  Badcaps Legend
                  • Jan 2016
                  • 4502
                  • Russia
                  #5
                  Re: Strange Power Supply Behavior on Skyworth 55U5A14G

                  ENA - enable backlight

                    Comment

                    • Gjackson
                      Senior Member
                      • Aug 2021
                      • 134
                      • Malta
                      #6
                      Re: Strange Power Supply Behavior on Skyworth 55U5A14G

                      I see, I had them mixed up. I thought ENA was for the PSU enable not backlight, i will buy a new main board and replace it. The PSU seems fine at this stage

                        Comment

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