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Samsung 740n no backlight after cap replacement

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    #1

    Samsung 740n no backlight after cap replacement

    hey guys, this is my first post in this forum so i hope im doing everything right

    so i hava a Samsung Syncmaster 740n that stopped working properly (blue led was on but no picture)
    i did a little reserch and found out that the three 820mf 25v capacitors on the power board were bad, it took a while but i eventualy found a store that sells 820mf 25v caps, i bought them and replaced the caps.
    when i tested it out (before reassembling the plastic casing) i could see that the lcd was working, however the backlight was not.

    what do think the problem is? and how can i fix it?

    - when i reassembeled the screen to the board i accidentaly connected bouth of the pink/white cables to the place where the black/green should be (see image - http://img683.*************/img683/4687/asdfkk.jpg ), and turned it on for like 4 seconds, could it burn out the backlight bulb?
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    #2
    Re: Samsung 740n no backlight after cap replacement

    Originally posted by shaisudri View Post
    what do think the problem is? and how can i fix it?
    1) Post pictures of your boards using the manage attachments button so the picture is hosted here.

    We want clear focused pictures with the following composition as shown in

    https://www.badcaps.net/forum/showpo...94&postcount=1

    2) Some boards have a picofuse near the inverter area. If it is bad, the backlight will not come on. A good fuse will measure less than 1.0 ohms.
    --- begin sig file ---

    If you are new to this forum, we can help a lot more if you please post clear focused pictures (max resolution 2000x2000 and 2MB) of your boards using the manage attachments button so they are hosted here. Information and picture clarity compositions should look like this post.

    We respectfully ask that you make some time and effort to read some of the guides available for basic troubleshooting. After you have read through them, then ask clarification questions or report your findings.

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      #3
      Re: Samsung 740n no backlight after cap replacement

      i found the white/pink & black/green image on google, its just to show which cables im talking about.
      but here are the pics of my board (sorry lol .. as i said im new here )
      i dont have a multimeter right now but i'll try to get one today to check the fuse

      any other pictures needed?
      Attached Files

      Comment

        #4
        Re: Samsung 740n no backlight after cap replacement

        Were the backlights working before you replaced the caps?

        You seem to have installed Nichicon KZE caps, those should be good. In general, replacing only a few caps is a recipe for continuing problems. Congratulations.

        CN2 connects the power supply to the signal card. Pins 3, 4, and 5 are ground. What are the voltages on the other pins using any of the ground pins as reference?

        There is a 16 pin IC in the inverter area, U201. What is the part number on that IC?

        PlainBill
        For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

        Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

        Comment

          #5
          Re: Samsung 740n no backlight after cap replacement

          Originally posted by PlainBill View Post
          Were the backlights working before you replaced the caps?

          You seem to have installed Nichicon KZE caps, those should be good. In general, replacing only a few caps is a recipe for continuing problems. Congratulations.

          CN2 connects the power supply to the signal card. Pins 3, 4, and 5 are ground. What are the voltages on the other pins using any of the ground pins as reference?

          There is a 16 pin IC in the inverter area, U201. What is the part number on that IC?

          PlainBill
          i dont think the backlight worked before i replaced them.

          sorry, im a total amateur... i dont really know how to check CN2 and i dont have a multimeter at the moment.

          "BIT3195G F343R G0606" is written on the IC

          Comment

            #6
            Re: Samsung 740n no backlight after cap replacement

            Hmmm .....

            Should check F301 3A pico fuse, replace if opened. If still doesn't work, check U202 and U203, AM4502C.

            Comment

              #7
              Re: Samsung 740n no backlight after cap replacement

              Originally posted by gammo
              Checked my F301 and it read 3.1 ohms. So it means that it needs to be replaced, right?
              What reading do you get when you touch the two probes together?

              Comment

                #8
                Re: Samsung 740n no backlight after cap replacement

                I think those new caps are fake.
                KZE made by United Chemi-con, their vent mark must be Y shape.
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                  #9
                  Re: Samsung 740n no backlight after cap replacement

                  ^ lol
                  I was just about to write the same thing. dodgy sleeve + wrong vent

                  Seeing how old the thread is, and when shaisudri last posted, I don't think it's relevant anymore.

                  @gammo: a fuse should read close to zero when it's ok, and open (0L on most digital multimeters) when open/failed

                  Comment

                    #10
                    Re: Samsung 740n no backlight after cap replacement

                    A good fuse will have just about the same resistance reading as when the two probes are touched together. A bad fuse will read overlimit (OL or 1 on the left side of the screen). You could check the reading on your fuse again, but it appears the fuse is probably OK. If it was open, you would get overlimit (open). As another option, if possible, is to measure the voltage on both sides of the fuse. If you get the same voltage reading on both sides, then the fuse is allowing the voltage to pass through.
                    Last edited by jayoung; 12-07-2011, 11:55 AM.

                    Comment

                      #11
                      Re: Samsung 740n no backlight after cap replacement

                      Here is my question and the subsequent follow up answers that I received earlier:

                      My 740N comes on but I see nothing but the blue power light. When I tilt the monitor at an angle, I am able to see that there is something there, just no backlight. Following PlainBill's recommendation, I checked the voltages at CN2 and got the following:

                      Ground to 13V - 16V
                      Ground to 5.1V - 5V
                      Ground to ON/OFF - 2V
                      Ground to A-DIM - 0V
                      Ground to B-DIM - 0V

                      I have already replaced the three 830 uF capacitors on the inverter circuit. When I checked my F301 fuse, it read 3.1 ohms. jayoung asked what reading I got when I touched the two probes together - the reading was fluctuating between 0.9 and 2.4 ohms.

                      Comment

                        #12
                        Re: Samsung 740n no backlight after cap replacement

                        Originally posted by gammo View Post
                        When I checked my F301 fuse, it read 3.1 ohms. jayoung asked what reading I got when I touched the two probes together - the reading was fluctuating between 0.9 and 2.4 ohms.
                        Either your probes are shot (internal wiring is fraying) and/or your multimeter battery is dying (causes inaccurate readings).
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                          #13
                          Re: Samsung 740n no backlight after cap replacement

                          Originally posted by retiredcaps View Post
                          Either your probes are shot (internal wiring is fraying) and/or your multimeter battery is dying (causes inaccurate readings).
                          Good point. I changed the probes and got a more stable reading:

                          Touching the probes - 0.4 ohms
                          F301 value - 0.4 ohms

                          Comment

                            #14
                            Re: Samsung 740n no backlight after cap replacement

                            Originally posted by jayoung View Post
                            As another option, if possible, is to measure the voltage on both sides of the fuse. If you get the same voltage reading on both sides, then the fuse is allowing the voltage to pass through.
                            The voltage reading on either side of the fuse is -6V.

                            Comment

                              #15
                              Re: Samsung 740n no backlight after cap replacement

                              Originally posted by gammo View Post
                              The voltage reading on either side of the fuse is -6V.
                              6 volts is very low.
                              You should check the 2 MOSFETs for being shorted. One looks to be U203 and I can't read the other
                              Whatever I do, I consider it a success, if in the end I am breathing, seeing, feeling and hearing!

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                                #16
                                Re: Samsung 740n no backlight after cap replacement

                                After an exhaustive search, I finally found the U202 and U203 MOSFETs. Unfortunately, my DMM does not have a diode test mode. How else can I check if there is a short?

                                Comment

                                  #17
                                  Re: Samsung 740n no backlight after cap replacement

                                  Originally posted by gammo View Post
                                  Unfortunately, my DMM does not have a diode test mode. How else can I check if there is a short?
                                  Measure resistance between source-gate, source-drain, drain-gate. Any reading under 30 Ohms suggests a short and should be measured out of circuit to verify.
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                                  If you are new to this forum, we can help a lot more if you please post clear focused pictures (max resolution 2000x2000 and 2MB) of your boards using the manage attachments button so they are hosted here. Information and picture clarity compositions should look like this post.

                                  We respectfully ask that you make some time and effort to read some of the guides available for basic troubleshooting. After you have read through them, then ask clarification questions or report your findings.

                                  Please do not post inline and offsite as they slow down the loading of pages.

                                  --- end sig file ---

                                  Comment

                                    #18
                                    Re: Samsung 740n no backlight after cap replacement

                                    Had same problem. It was f301.

                                    Everything ok now.

                                    Comment

                                      #19
                                      Re: Samsung 740n no backlight after cap replacement

                                      Yes man it's working you are best...

                                      Comment

                                        #20
                                        F301

                                        Sorry for questio, but where is that F301 fuse?

                                        Edit: found it on another thread
                                        Last edited by HaxLi; 04-09-2013, 04:58 PM.

                                        Comment

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