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(SOLVED) - ONN 100012588 No Power, Then Rebooting After Main Board Change

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    #1

    (SOLVED) - ONN 100012588 No Power, Then Rebooting After Main Board Change

    ONN 100012588
    Issue: No Power, Then Rebooting After Main Board Change

    Short answer: replace main board and power board
    Technical answer: replace main board and Q306 & Q307 on power board.

    Long answer:

    TV wouldn't turn on. Power switch was fine which means the main board isn't processing the power on signal. Replaced main board. TV turns on now but randomly reboots. Bad main board? EEPROM? Tried to pull main board and got a shock touching the corner of the board. That's weird. C334 (450V 10uf) capacitor is still holding a high charge. That's not right. Took my time discharging it and the rest of the board. C334 had good ESR, so why isn't the board discharging the voltage itself? Poked around, poked around, found that Q306 and Q307 are blown (MBRB20100CT DIODE SCHOTTKY 100V 20A) were blown. Weird again. TV turns on, shows a picture, but the power board is still bad? First for me.

    Replaced power board and all is well now. When benching the TV, do NOT connect to the network as it will not allow you use it as a regular tv afterwards unless you register it.
    Tags: None
    #2
    Re: (SOLVED) - ONN 100012588 No Power, Then Rebooting After Main Board Change

    that should read "Tried to pull POWER board and got a shock touching"

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      #3
      Re: (SOLVED) - ONN 100012588 No Power, Then Rebooting After Main Board Change

      Be very careful where you touch it, it can be DEADLY, so where did you touch it that it shocked you? Always be mindful where you are touching with your hands, I only touch the edges of the board with my hands, to try to eliminate shock and possible static electricity messing something up.
      Last edited by nomoresonys; 07-17-2022, 07:44 AM.

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        #4
        Re: (SOLVED) - ONN 100012588 No Power, Then Rebooting After Main Board Change

        capacitor discharging with resistor, can be done with capacitor in circuit as well: https://www.youtube.com/watch?v=2TcVZ2opuNc

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          #5
          Re: (SOLVED) - ONN 100012588 No Power, Then Rebooting After Main Board Change

          a little more on it, capacitors can hold a charge for 24 hours without power applied, if they have resistors across them in circuit they will discharge very quickly tho, can test them with multimeter set to vdc to see voltage they are at, best to test them to make sure or just discharge them with resistor: https://www.youtube.com/watch?v=mftW52E6_fk
          Last edited by nomoresonys; 07-17-2022, 07:47 AM.

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            #6
            Re: (SOLVED) - ONN 100012588 No Power, Then Rebooting After Main Board Change

            Originally posted by nomoresonys View Post
            a little more on it, capacitors can hold a charge for 24 hours without power applied, if they have resistors across them in circuit they will discharge very quickly tho, can test them with multimeter set to vdc to see voltage they are at, best to test them to make sure or just discharge them with resistor: https://www.youtube.com/watch?v=mftW52E6_fk
            Yes, you are correct on all 3 posts. I practice discharging the main filter capacitor or at least check for an errant voltage upon powerdown. What surprised me was the 450v corner capacitor on the power board still had a charge. Strangely enough had I not received the shock, I probably wouldn't have figured out anything was wrong because like I said the TV turned on.

            In the 10 years I've been repairing TVs, I've never had one turn on and yet there's an issue with the power board.

            Comment

              #7
              Re: (SOLVED) - ONN 100012588 No Power, Then Rebooting After Main Board Change

              yes that is strange, usually don't get much showing up with bad powerboard.

              Comment

                #8
                Re: (SOLVED) - ONN 100012588 No Power, Then Rebooting After Main Board Change

                Can you elaborate a bit more on the process that led to conclusion of q306 and q307 being blown?

                I am currently working on a similar TV - ONN 65 inch that originally wouldn't show picture - replaced all backlight strips and now all backlight LEDs are lighting uniformly and read the same voltage on my backlight tester. Now I get a single flash from the backlight when pressing the power button.

                This TV also shocked the hell out of me when attempting to disconnect the backlights from the PSU. It's very consistent that each time a power attempt has been made, the backlight connector has plenty of voltage even hours after being unplugged.

                My PSU - https://imgur.com/a/zG61ERH

                Comment

                  #9
                  Re: (SOLVED) - ONN 100012588 No Power, Then Rebooting After Main Board Change

                  Sorry, I guess I couldnt be bothered to read the datasheet.

                  Q306 & Q307 are schottkey barrier diodes. They are tested just like any other diode - They will conduct in one polarity, and insulate in the opposite polarity.
                  The only caveat for the MBRB20100CT is that there are 2 diodes in one package. Since the board is designed so the anodes are connected, the whole package functions as a single diode. The package would need to be removed from the board to test each diode in the package separately.

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