Cynascan DS551LT4 backlights working but no video or menu

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  • freakaftr8
    Badcaps Legend
    • Oct 2012
    • 3743
    • USA

    #21
    Re: Cynascan DS551LT4 backlights working but no video or menu

    I would say that you have a shorted LCD panel on that side. Sounds like your classic symptom that's failed LCD gate driver.

    If you can access the panel boards start running down-the-line checking capacitors for shorts.
    Last edited by freakaftr8; 12-14-2017, 09:22 AM.
    Did I leave the soldering iron on?

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    • dicky96
      Sun Seeker
      • Mar 2017
      • 1825
      • Spain

      #22
      Re: Cynascan DS551LT4 backlights working but no video or menu

      OK I have got a little further with this.

      I got to the panel PCBs

      I could see short circuits across all of the larger and and most of the medium sized capacitors on the panel board that was dragging down VHDD and VCOM. I removed the first of the larger capacitors and this made no difference to the short. I measured the capacitor our of interest and it reads about 11 uF.

      I then noticed on the PCB this capacitor is marked as DUM6 (I also have DUM2 to DUM5 which also read short circuit) but this is because, looking with my jewelers loop, both ends of these capacitors go to the ground plane on the PCB!! I'm guessing DUMx stands for dummy? but wtf is this all about other than wasting capacitors????

      On a similar topic what are these funny little metal bridges with a rubbery thing inside GA205 etc (see pics) they don't seem to do anything useful either

      Anyway I then removed the first of the medium sized capacitors C204 and the short went away. This was confirmed by measuring the capacitor out of circuit, it is short.

      I now have VHDD 8.25V and VCOM 7.10V with both TCON cables attached

      YEsss! I thought.. I left the capacitor out of circuit just for now (I figure it is probably just a decoupling cap) to see what the screen does now. I didn't bother to clean the flux just yet either as you will see on the pic, I'm not being unprofessional I wanted to see if I fixed it... but all is still not good.

      The fault has changed at least, and I now have vertical lines down both sides of the screen instead of just a dark screen with backlights only. If I remove the TCON ribbons in turn then that side of the screen goes blank white and the other has the vertical lines.

      I still do not have any image. On the good working monitor they have a blue 'no signal' box on screen with no video input when there is no input so I expected to see that at least.

      One of the flex cables has a little bit of damage by the way, and using my jewelers loop I can see one track inside it is broken, but this track goes to one of the pins on the panel PCB that has no connection so I don't think that is causing a problem

      At least I am getting somewhere thanks to you guys, but what next?

      Rich
      Attached Files
      Last edited by dicky96; 12-17-2017, 12:43 PM. Reason: extra info
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      • freakaftr8
        Badcaps Legend
        • Oct 2012
        • 3743
        • USA

        #23
        Re: Cynascan DS551LT4 backlights working but no video or menu

        The more I get into this the more I understand that these drivers are Daisy chained serial. I would replace that missing capacitor with a good substitute. If the signal is lost in transition with the one that's missing it may not have the feedback circuit to carry the information on to the other drivers. This is just my theory of course. Other knowledgeable people might be able to chime in.
        Did I leave the soldering iron on?

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        • dicky96
          Sun Seeker
          • Mar 2017
          • 1825
          • Spain

          #24
          Re: Cynascan DS551LT4 backlights working but no video or menu

          Yeah no worries I can do that. If I pull one of the other ones I can measure the value. That doesn't really explain to me why I have nothing but vertical stripes on both sides of the screen though - and plain white with the relevant TCON disconnected

          Rich
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          • freakaftr8
            Badcaps Legend
            • Oct 2012
            • 3743
            • USA

            #25
            Re: Cynascan DS551LT4 backlights working but no video or menu

            Do the panel driver boards have ICs on one side only? These are attached to the panel circuit board on one side only. If so, daisy chained info needs to go from one to be passed to another, and also I believe the capacitor closes the circuit it's driving as a whole..
            Last edited by freakaftr8; 12-17-2017, 05:08 PM.
            Did I leave the soldering iron on?

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            • dicky96
              Sun Seeker
              • Mar 2017
              • 1825
              • Spain

              #26
              Re: Cynascan DS551LT4 backlights working but no video or menu

              No ICs on the panel boards just a few capacitors etc
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              • budm
                Badcaps Legend
                • Feb 2010
                • 40746
                • USA

                #27
                Re: Cynascan DS551LT4 backlights working but no video or menu

                https://www.badcaps.net/forum/attach...5&d=1513535647
                There is embedded IC on each one of those flexible brown cable between the tab bonding and the LCD glass panel, they do go bad because they run hot.
                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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                • dicky96
                  Sun Seeker
                  • Mar 2017
                  • 1825
                  • Spain

                  #28
                  Re: Cynascan DS551LT4 backlights working but no video or menu

                  Yes Bud I am well aware that there are chips on the flex cables. But it seems strange that they would all fail and I would not get some image on some part of the screen

                  On your advice I previously checked that there was data on the LVDS cable and most of the twisted pairs had some data. Now I fixed the short on the panel PCBs and have VHDD and VCOM how can I determine if all those vertical lines (and no image) on the screen are due to a faulty TCON board or some other reason?

                  BTW I honestly do not understand why this Dynascan has more than one fault. I thought fixing the short would have sorted it :-( However the damn thing was about 6000 euros new so the owner would like it fixed if possible

                  Rich
                  Last edited by dicky96; 12-18-2017, 05:11 PM.
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                  • freakaftr8
                    Badcaps Legend
                    • Oct 2012
                    • 3743
                    • USA

                    #29
                    Re: Cynascan DS551LT4 backlights working but no video or menu

                    The only real way of knowing is swapping the tcon. You said you have another one of these tv's? Also scope ready t's between the two.
                    Did I leave the soldering iron on?

                    Comment

                    • dicky96
                      Sun Seeker
                      • Mar 2017
                      • 1825
                      • Spain

                      #30
                      Re: Cynascan DS551LT4 backlights working but no video or menu

                      Unfortunately the other one is a different model DS55 LT6 and it is in daily use

                      OK thanks, I will have a look though if I can see signals coming out of the TCON board - I know there are signals getting in to it on the LVDS though of course I don't know the integrity of the data
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                      • dicky96
                        Sun Seeker
                        • Mar 2017
                        • 1825
                        • Spain

                        #31
                        Re: Cynascan DS551LT4 backlights working but no video or menu

                        I spoke with the nice guys at Dynascan and they are sending me a TCON board early in the new year. Will let you know if that fixes it.

                        Rich
                        Follow me on YouTube
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                        Learn Electronics Repair
                        https://www.youtube.com/channel/UCFX...R8UZ2vg/videos

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