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Emerson LF391EM4 A no power

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    #1

    Emerson LF391EM4 A no power

    So I have an emerson LCD TV that belongs to my son. It had no power at all, so I popped the back off it and started checking voltage coming into the power supply board. Found the fuse was blown. So started checking all the other components to figure out what blew the fuse. I found several resistors, a mosfet and a varisistor that was bad. I desoldered each component to check if they were bad. Replaced the bad components and now the board has power on the hot side, but nothing is going across to the cold side. I have checked the different power leads. I found a service manual online that shows the 5V, 3.3V and 21V terminals. This is only the second TV that I have worked on, so not sure what the check next. I found several posts that seem to lead to the same problem but nothing seems to work out. I am getting 170V DC out of the rectifier on the hot side. I have not found anything stating that is correct voltage. so anyone have any idea what I can check next? I have attached pictures of the power supply board, the ID sticker for the board and the label for the TV showing the model.
    Attached Files
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    #2
    Re: Emerson LF391EM4 A no power

    What component board reference numbers did you check and put back/replace.
    Willing to help but I'm no expert.

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      #3
      Re: Emerson LF391EM4 A no power

      checked all diodes and resistors on hot side, changed F602, R601, SA601, R611, Q601

      Comment

        #4
        Re: Emerson LF391EM4 A no power

        Can you post the schematic which could be helpful.
        What voltages do you have on the 3 legs of Q601, Q602, Q621

        If Q601 blew I would also check D608, R611, D609A, R609, D607A
        Last edited by dick_barton; 01-29-2017, 01:48 PM.
        Willing to help but I'm no expert.

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          #5
          Re: Emerson LF391EM4 A no power

          here is the schematics. Do I check the three legs to ground or between each leg?
          Attached Files

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            #6
            Re: Emerson LF391EM4 A no power

            To test the transistors / mosfet use you meter in diode mode and test between all 3 legs.
            Eg Mosfet Q601 test D-G, D-S, G-S then reverse meter leads and reverse
            Transistor Q602, Q621 test Pins 1-2, Pins 1-3, Pins 2-3 then reverse meter leads and repeat
            Last edited by dick_barton; 01-29-2017, 04:53 PM.
            Willing to help but I'm no expert.

            Comment

              #7
              Re: Emerson LF391EM4 A no power

              A FET you check between the legs for a short. If there is anything shorted to the gate, you want to check the gate drive circuit too.

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                #8
                Re: Emerson LF391EM4 A no power

                On Q601, G-D climbs to 2.0 then opens, D-S is .34, G-S is .48. On Q602, 1-2 is .095, 2-3 is .89 and 1-3 is .28. On Q621, 1-2 is 1.66, 2-3 is .77, and 1-3 is 1.48. not sure what they should be, so at least I am not baised. Just to make sure, I check all the legs without power applied right? Thanks for your help.

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                  #9
                  Re: Emerson LF391EM4 A no power

                  I would remove Q601 and test as follows
                  https://www.youtube.com/watch?v=RkWy1EirEu8
                  Willing to help but I'm no expert.

                  Comment

                    #10
                    Re: Emerson LF391EM4 A no power

                    The mosfet checked out just like in the video. This one I replaced. So I assume I need to remove D608, R611, D609A, R609, D607A and check them.

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