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Samsung LN40D550 Power supply short circuit

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    #1

    Samsung LN40D550 Power supply short circuit

    Hello Everyone.
    I have been attempting to repair a Samsung LN40D550 board that is blowing fuses. I have founde one of the power MOSFETS (10NM60N) shorted out. It's in parallel with another of identical type. I read the forum on https://www.badcaps.net/forum/showthread.php?t=22919 and one of the guys there removed the MOSFET and powered on the TV and had no problems. As a troubleshooting test, I removed it and tried the same. It blew the fuse and the remaining MOSFET as well. Now I'm not sure if it was because the circuit is different and needed the second one to function or something else is causing the MOSFETs to fail. The board model is BN44-00440B. Any help would be greatly appreciated. Thank you.
    Last edited by Zorix; 09-19-2013, 08:44 PM.
    Tags: None
    #2
    Re: Samsung LN40D550 Power supply short circuit

    After a little bit more testing, I found a 5 Watt resistor of unknown ohms showing no resistance at all. It's connected to both MOSFETS from the source pin to the negative side of the rectifier. So far this could be another symtom of another issue or part of the cause. Sadly, I can't find any information about it. The part is labeled with: MPR-5W R070JSMT. Any ideas?

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      #3
      Re: Samsung LN40D550 Power supply short circuit

      I think this is your resistor (see attachment). If so, it's resistance reading may be normal.
      Attached Files
      LG Plasma Mal-Discharge Correction Service

      Comment

        #4
        Re: Samsung LN40D550 Power supply short circuit

        That could be right. My multimeter might not be able to read such low values accurately. Any ideas what else could be causing the shorts?

        Comment

          #5
          Re: Samsung LN40D550 Power supply short circuit

          If the MOSFETs blew, it likely took out their driver IC. The driver IC controls the FETs, if it fails it can cause the signal to stay turned on making the FETs short.
          Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
          For service manual, schematic, boardview (board view), datasheet, cad - use our search.

          Comment

            #6
            Re: Samsung LN40D550 Power supply short circuit

            Alright thank you. Thats what I was wondering. It's a surface mount 8 pin chip it looks like. However, I cannot seem to read the writing on it. I wish I had a nice camera to take a close up of it. I have really good sight but I can't make out anything on it. Are these standard ICs or are they all different? Would be really nice if I had a schematic for it but haven't found one yet. Thanks a lot for your help guys!

            Comment

              #7
              Re: Samsung LN40D550 Power supply short circuit

              They are all different. I can ask a friend if a schematic is available, or a service tip perhaps (containing component values.) ShopJimmy may also have high res photos for you to see the IC.
              Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
              For service manual, schematic, boardview (board view), datasheet, cad - use our search.

              Comment

                #8
                Re: Samsung LN40D550 Power supply short circuit

                Originally posted by Zorix View Post
                It's a surface mount 8 pin chip it looks like. However, I cannot seem to read the writing on it.
                Walmart has magnifying glass with a insert that's even more magnified and it has a light in it. For like 5 bucks!! Super handy to read those tiny little letters on those super small components...
                Attached Files
                Proud owner of dozens of broken TV's and many,many,many boards.


                Our website and passion:http://hollenhundshepherds.com/

                Comment

                  #9
                  Re: Samsung LN40D550 Power supply short circuit

                  Thanks Martino, I'm going to have to look for one of those. That's really handy for sure. I was just using the magnifer on my clip thing that I use to hold boards and wires and such. Using a few types of flash lights only produced glare and not much detail. I did make out what looked like FAN on it near the middle but that's about it.

                  Comment

                    #10
                    Re: Samsung LN40D550 Power supply short circuit

                    I was able to finally read off the chip model. I found it to be a Fairchild Semiconductor FAN7930B. Is there a way to test this chip to know if it's faulty? Thank you.

                    Comment

                      #11
                      Re: Samsung LN40D550 Power supply short circuit

                      I would not even try to find out if the IC is good or bad, I would automatically replace the blown MOSFETS and the IC at the same time instead of taking a chance of blowing up the new MOSFETs.
                      Never stop learning
                      Basic LCD TV and Monitor troubleshooting guides.
                      http://www.badcaps.net/forum/showthr...956#post305956

                      Voltage Regulator (LDO) testing:
                      http://www.badcaps.net/forum/showthr...999#post300999

                      Inverter testing using old CFL:
                      http://www.badcaps.net/forum/showthr...er+testing+cfl

                      Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                      http://s807.photobucket.com/user/budm/library/

                      TV Factory reset codes listing:
                      http://www.badcaps.net/forum/showthread.php?t=24809

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                        #12
                        Re: Samsung LN40D550 Power supply short circuit

                        Thanks guys. I was able to replace both mosfets and the driver chip and everything came back up again.

                        Comment

                          #13
                          Re: Samsung LN40D550 Power supply short circuit

                          Great to hear, thanks for the repair feedback.
                          Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
                          For service manual, schematic, boardview (board view), datasheet, cad - use our search.

                          Comment

                            #14
                            Re: Samsung LN40D550 Power supply short circuit

                            Does anyone have the Power Supply schematic for the model LN40D550 LCD TV? If yes, will you kindly post it on this thread.

                            Comment

                              #15
                              Re: Samsung LN40D550 Power supply short circuit

                              The value is 100 ohms

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