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Samsung PN58C550 58" Plasma PSU BN44-00331A

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    Samsung PN58C550 58" Plasma PSU BN44-00331A

    I was given this TV because it wouldn't power up. Tested for standby voltage and confirmed nothing was there. The fuse was on the load side was dead, so I replaced it with another 10A 250V slow blow fuse. Plugged in the TV and then the neutral fuse blew. Me being an idiot though, well maybe the fuses were just old and bad. Replaced it and plugged in again, and this time they both blew.

    So then I watched this video and started testing continuity along the primary side of the power supply https://www.youtube.com/watch?v=cWN71R8R94E

    On the load side I make it all the way to the second lkq1af-5v-tv8 power relay and then I don't get continuity any further. (Red square on the front red circle on the back picture)

    On the neutral side I make it all the way to what I think is an MOV before I no longer have continuity from the neutral side of the plug. I believe this is to be expected and using the other side of the MOV I am able to test forward. Is that right? (Purple square on the front and purple circle on the back picture)

    Is my problem the lkq1af-5v-tv8 and can I just replace it, or is that likely more a symptom, and I'll need to replace it and keep testing? Or am I way off entirely?

    Any guidance or things to google would be greatly appreciated.
    Attached Files

    #2
    Re: Samsung PN58C550 58" Plasma PSU BN44-00331A

    Anyone have any thoughts on if it's worth ordering the power relay and replacing it to see if that solves the problem? Shipping from Mouser is $7 so if I need to test other components and order replacements as well I figure better to do it once instead of one by one. Apologies if there isn't enough information for you all to point me in the right direction. Still new at this.

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      #3
      Re: Samsung PN58C550 58" Plasma PSU BN44-00331A

      The relay will NOT be the cause of the short causing the fuse to blow. The relay is just a controlled switch. Check the two bridge rectifiers for shorted diodes, then check if there is a short across the +&- of the bridge rectifiers.

      Comment


        #4
        Re: Samsung PN58C550 58" Plasma PSU BN44-00331A

        Originally posted by R_J View Post
        The relay will NOT be the cause of the short causing the fuse to blow. The relay is just a controlled switch. Check the two bridge rectifiers for shorted diodes, then check if there is a short across the +&- of the bridge rectifiers.
        Thank you R_J. Set my meter to diode mode and put my negative lead on the positive of the rectifier and positive lead on the closest AC leg and get 0.41v. Then when I reverse my multi meter leads I get 0.73v. same thing when I try on the negative leg of the rectifier. Same is true for both rectifiers. When I test the +&- bridge there is no short.

        From what I'm finding on testing these I should have gotten 0L when I swapped the leads instead of the 0.73v. What does that mean? Or did I mess something up?

        Comment


          #5
          Re: Samsung PN58C550 58" Plasma PSU BN44-00331A

          I suspect the bridge rectifiers are ok but you likely have a shorted modfet in either the main or pfc circuits.
          There are also two MOV's VX801S and VX803S, what is the resistance across each and do they look cracked at all?
          Last edited by R_J; 06-29-2022, 12:12 PM.

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            #6
            Re: Samsung PN58C550 58" Plasma PSU BN44-00331A

            Originally posted by R_J View Post
            I suspect the bridge rectifiers are ok but you likely have a shorted modfet in either the main or pfc circuits.
            There are also two MOV's VX801S and VX803S, what is the resistance across each and do they look cracked at all?
            Sorry for the delay R_J. The two mosfets QS802 and QS804 part FQA 24N50 are both definitely bad. The other two QP802 and QP804 part 20N60C3 are okay.

            It looks like there are actually 3 MOVs on this board. VX801S and VX803S both measure 2M ohms. However VX804S I got nothing out of, so I take it that's dead too.

            Attached picture has the working parts in green and the dead parts in red for visual reference.
            Attached Files

            Comment


              #7
              Re: Samsung PN58C550 58" Plasma PSU BN44-00331A

              Anything else I should check before I go ahead and order new mosfetss and the MOV?

              Comment


                #8
                Re: Samsung PN58C550 58" Plasma PSU BN44-00331A

                Originally posted by nortnil View Post
                Anything else I should check before I go ahead and order new mosfetss and the MOV?
                What do you mean "However VX804S I got nothing out of" what where you expecting to get out of it? If the are not shorted or cracked, they are very likely good and your problem is with the mosfets and possibly there gate drive circuits. Check the two resistors and one diode in the gate circuits of both mosfets, Each gate is driven by a drive transformer so the drive ic should be ok.

                VX801S and VX803S both measure 2M ohms
                when checking these two you are measuring other components in the circuit, not so with vx804s
                Last edited by R_J; 07-04-2022, 03:37 PM.

                Comment


                  #9
                  Re: Samsung PN58C550 58" Plasma PSU BN44-00331A

                  Hey thanks for getting back to me so quickly. I saw online a quick test for MOVs is to see if they have a high resistance. The other two did give me a high resistance, but I got 0L when I tried to measure VX804S. Visually it looks okay.

                  The 24N50 mosfets I pulled out of the board to double check and they are both definitely shorted. I wasn't sure what else in the circuit around them I should test to see if anything else failed with them. Any tips there?

                  I also pulled one of the 20N60C3 just to double check, and it was fine.

                  Comment


                    #10
                    Re: Samsung PN58C550 58" Plasma PSU BN44-00331A

                    You might want to check the diodes in the power supply secondary like DS811 etc. check that they are not shorted, they will likely read low in diode test if they are schottky diodes.
                    Also check across the VS and ground on both the Ysus and Zsus boards to make sure they are not shorted.
                    Do a search with your model and you will see a few other threads about this model and problem.
                    Last edited by R_J; 07-04-2022, 03:54 PM.

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                      #11
                      Re: Samsung PN58C550 58" Plasma PSU BN44-00331A

                      Okay, those diodes look good. Y and Z boards didn't have any shorts across VS and ground and both of their fuses are still good. Anything else I should rule out before I get my parts order in? Thanks again for all the help!

                      Comment


                        #12
                        Re: Samsung PN58C550 58" Plasma PSU BN44-00331A

                        R_J we got it working! Thanks again for your help. I just replaced the two mosfets and good to go!!

                        Additional context for anyone that finds this later, the relay lkq1af-5v-tv8 wasn't actually shorted. I was just being an idiot and didn't test it out of circuit when it seemed broken. Once I did that I realized it was fine.

                        The MOV VX804S was also not physically damaged, but because I got no resistance reading out of it I thought it was bad and ordered a replacement. The replacement gave me the same result. I've since learned that's not a valid way to test an MOV and that they are almost impossible to fail without visibly being damaged like R_J pointed out.

                        Lastly in another thread I learned about a dim bulb tester as a good way to keep from blowing more components if you do have a short somewhere. The lightbulb eats the extra current so to speak and alerts you to a short. Google how to build one. After replacing the mosfets the bulb stayed off and I got standby voltage so figured I was good to test with the TV. I used the bulb tester while trying to power the TV and just got the clicking noise like it couldn't start. Turns out using the bulb tester while trying to power the TV won't work. Removed it and the TV fired right up.

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