JVC LT-55CF890 backlight on but no image

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  • notallbad
    Badcaps Legend
    • Oct 2012
    • 1183
    • UK

    #1

    JVC LT-55CF890 backlight on but no image

    This is a Fire TV Edition
    The tv powers on, sound and can hear the remote response (clicks on the tv) but no images, no logo, menu or picture.

    This set only has power and main boards.

    Help appreciated!
  • notallbad
    Badcaps Legend
    • Oct 2012
    • 1183
    • UK

    #2
    Re: JVC LT-55CF890 backlight on but no image

    photos
    Attached Files
    Last edited by notallbad; 10-30-2021, 04:04 PM.

    Comment

    • diif
      Badcaps Legend
      • Feb 2014
      • 6978
      • England

      #3
      Re: JVC LT-55CF890 backlight on but no image

      Voltage on the fuse both sides ?

      Comment

      • notallbad
        Badcaps Legend
        • Oct 2012
        • 1183
        • UK

        #4
        Re: JVC LT-55CF890 backlight on but no image

        12.5v on both sides of the fuse on the (t-con) panel board fuse

        Comment

        • diif
          Badcaps Legend
          • Feb 2014
          • 6978
          • England

          #5
          Re: JVC LT-55CF890 backlight on but no image

          Check for any test voltages, check the diodes and caps for shorts.

          Comment

          • notallbad
            Badcaps Legend
            • Oct 2012
            • 1183
            • UK

            #6
            Re: JVC LT-55CF890 backlight on but no image

            the 3 diodes on the (tcon) panel board seem ok, caps dont appear to have shorts.
            Not exactly sure which are the test points in all cases for the voltages labeled but there are 1.8, 3.3, 1.05 and 12v present around the board.

            Comment

            • notallbad
              Badcaps Legend
              • Oct 2012
              • 1183
              • UK

              #7
              Re: JVC LT-55CF890 backlight on but no image

              When initial mains power applied the red led standby light comes on solid, when powered up from standby the red led flashes 18 times and then when powered off goes solid, next time its powered from stby it goes out with no flashes. Until next time its completely powered down then its back to 18 flashes.
              I guess this is normal being a smart tv?

              Comment

              • notallbad
                Badcaps Legend
                • Oct 2012
                • 1183
                • UK

                #8
                Re: JVC LT-55CF890 backlight on but no image

                power board voltages

                12v stby (stby- 12.55v, on - 12.55v)
                power pin 9 5v_stby (stby - 0v, on - 0v)
                dimming (stby - 2.75v, on - 3.4v)
                BL_on_off (stby - 0v, on - 5v)
                stby_on/off_not (stby - 0v, on - 0v)
                power pin 1 24v_vcc (stby - 24v, on - 24v)

                After a short period of time (5 mins) the stby volts change
                12v stby (stby- 8v, on)
                power pin 9 5v_stby (stby - 0v)
                dimming (stby - 0v)
                BL_on_off (stby - 0v)
                stby_on/off_not (stby - 1.4v)
                power pin 1 24v_vcc (stby - 5v)
                Last edited by notallbad; 11-01-2021, 07:27 AM.

                Comment

                • notallbad
                  Badcaps Legend
                  • Oct 2012
                  • 1183
                  • UK

                  #9
                  Re: JVC LT-55CF890 backlight on but no image

                  The power pin 9 5v stby is not connected to anything on the pcb side of the board so can be ignored.

                  So looking at the power board voltages I would say they look ok.

                  Anyone care to confirm please?

                  Comment

                  • diif
                    Badcaps Legend
                    • Feb 2014
                    • 6978
                    • England

                    #10
                    Re: JVC LT-55CF890 backlight on but no image

                    I was suggesting you check the voltages on the slim boards that connect to the panel, the rest are working if you have everything but picture.

                    Comment

                    • notallbad
                      Badcaps Legend
                      • Oct 2012
                      • 1183
                      • UK

                      #11
                      Re: JVC LT-55CF890 backlight on but no image

                      Yh sorry, I was partly supplying more detail as well as checking all basics were covered and correct.

                      I believe at the moment that the power board and backlights are ok. Suspect the firmware is ok.
                      But not yet convinced the main board cpu chip is 100%, which if it is ok it then only remains for the panel board(s) to be faulty. Bummer!

                      Comment

                      • comptek
                        Badcaps Veteran
                        • Aug 2013
                        • 606
                        • uk

                        #12
                        Re: JVC LT-55CF890 backlight on but no image

                        Check screen slim boards for anything getting hot caps or dc-dc converter etc

                        Comment

                        • notallbad
                          Badcaps Legend
                          • Oct 2012
                          • 1183
                          • UK

                          #13
                          Re: JVC LT-55CF890 backlight on but no image

                          Shall confirm with photos later but although there is hvin_12v and hvin_24v at the connector theres also a hvin_12v on one of the slim panel boards that does not appear to be there and noticed a chip labelled 8100vb (3 pin - 2 top, 1 bottom) quite hot, think its a diode. Theres a small i.c, 6 pin i think in close proximity that could be a dc-dc converter.
                          I have a temp gun so shall check around the boards on those small smd caps.
                          But again will confirm later.
                          Thanks for the help!
                          Last edited by notallbad; 11-03-2021, 10:10 AM.

                          Comment

                          • notallbad
                            Badcaps Legend
                            • Oct 2012
                            • 1183
                            • UK

                            #14
                            Re: JVC LT-55CF890 backlight on but no image

                            It would seem the slim panel board is hotter in the upper portion as per zoomed in photo below, around 41 degrees whereas its around 24 to 30 degrees elsewhere.
                            Attached Files
                            Last edited by notallbad; 11-03-2021, 07:04 PM.

                            Comment

                            • notallbad
                              Badcaps Legend
                              • Oct 2012
                              • 1183
                              • UK

                              #15
                              Re: JVC LT-55CF890 backlight on but no image

                              Theres an smd G64 6 pin ic, mosfet.

                              12.5v on pin 4 Source
                              12.4v on pin 3 Gate
                              0v on pins 1, 2, 5 & 6.

                              Drain is connected to the hvin_12v test point that reads 0v.

                              How can I bypass or otherwise check its the mosfet at fault?
                              thanks

                              Comment

                              • diif
                                Badcaps Legend
                                • Feb 2014
                                • 6978
                                • England

                                #16
                                Re: JVC LT-55CF890 backlight on but no image

                                Is it hot ? You can check mosfets in diode mode but it's not in the photo of the hotter area, have you checked the components there ?

                                Comment

                                • notallbad
                                  Badcaps Legend
                                  • Oct 2012
                                  • 1183
                                  • UK

                                  #17
                                  Re: JVC LT-55CF890 backlight on but no image

                                  Theres an ic SM4190 which I'm assuming is a DC-DC converter, but cant find any datasheet for it.

                                  But seems its not generating any of the required voltages. i.e -14v, -7.4v, -3.3v, -4.15v, 9v or 32v.

                                  I have panel slim boards from a 49" tv which are very similar but no longer connected to the panel but when powered up gives me voltages for comparison.

                                  The other day I did get a perfect picture, I did not do anything to cause it that I know of, but had to power it off and now cant get it back since.

                                  Comment

                                  • dskall
                                    Badcaps Legend
                                    • Oct 2016
                                    • 2905
                                    • usa

                                    #18
                                    Re: JVC LT-55CF890 backlight on but no image

                                    SM4190 appears to be a processing chip. What voltages are you getting at your ldvs input on the paired lines going into main processor? You should read about 1.2v if data is coming from mainboard.
                                    I assume no responsibility for any stupid suggestions I might post.

                                    Comment

                                    • notallbad
                                      Badcaps Legend
                                      • Oct 2012
                                      • 1183
                                      • UK

                                      #19
                                      Re: JVC LT-55CF890 backlight on but no image

                                      These are the voltage comparisons from the connected panel boards and the spare disconnected from panel board.
                                      Attached Files

                                      Comment

                                      • dskall
                                        Badcaps Legend
                                        • Oct 2016
                                        • 2905
                                        • usa

                                        #20
                                        Re: JVC LT-55CF890 backlight on but no image

                                        Still looks like a processing chip, not voltage chip.
                                        I assume no responsibility for any stupid suggestions I might post.

                                        Comment

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