LA-8554P No backlight

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  • pc1staid
    Senior Member
    • Apr 2014
    • 55
    • England
    #1

    LA-8554P No backlight

    Hello fellow techs...
    I have a HP Spectre XT come in for no display.
    Diags have shown there is no backlight, connected to a monitor- OK, checked cabling- OK, removed screen and shone a torch through- display evident, changed screen as I have seen on a few forums, it could be the screen itself- display evident. Just no backlight.
    I have managed to find a schematic for the board but I am unable to find where the backlight fuse is.
    Would anyone have an idea as to where it may be located please?
    Thanks in advance.
    Tags: None
    • emestee
      Badcaps Veteran
      • Dec 2019
      • 228
      • Bulgaria
      #2
      Re: LA-8554P No backlight

      Why do you assume it's the fuse?

      https://cdn.badcaps-static.com/pdfs/...5f57ec8880.pdf

      This board should have similar layout. See sheet 33 for backlight control.

        Comment

        • pc1staid
          Senior Member
          • Apr 2014
          • 55
          • England
          #3
          Re: LA-8554P No backlight

          Thank you @emestee. I am using this schematic (attached) page 24.
          I was refering to where it was located on the mainboard.

          Schematic moved to new thread - https://www.badcaps.net/forum/showthread.php?t=124843

          .
          Last edited by SMDFlea; 10-18-2023, 04:11 AM.

            Comment

            • arber1232
              Badcaps Veteran
              • Dec 2019
              • 368
              • Albania
              #4
              Re: LA-8554P No backlight

              Originally posted by pc1staid
              Hello fellow techs...
              I have a HP Spectre XT come in for no display.
              Diags have shown there is no backlight, connected to a monitor- OK, checked cabling- OK, removed screen and shone a torch through- display evident, changed screen as I have seen on a few forums, it could be the screen itself- display evident. Just no backlight.
              I have managed to find a schematic for the board but I am unable to find where the backlight fuse is.
              Would anyone have an idea as to where it may be located please?
              Thanks in advance.
              Attached Files

                Comment

                • pc1staid
                  Senior Member
                  • Apr 2014
                  • 55
                  • England
                  #5
                  Re: LA-8554P No backlight

                  thanks arber1232. The board has very little screen print on it and I have tried to pinpoint the location of where the feed comes from on the board I have been unsuccessful. I am getting no 19v at Pins 1&2. I need to find either the fuse or where if comes from on the board.

                    Comment

                    • emestee
                      Badcaps Veteran
                      • Dec 2019
                      • 228
                      • Bulgaria
                      #6
                      Re: LA-8554P No backlight

                      Originally posted by pc1staid
                      thanks arber1232. The board has very little screen print on it and I have tried to pinpoint the location of where the feed comes from on the board I have been unsuccessful. I am getting no 19v at Pins 1&2. I need to find either the fuse or where if comes from on the board.
                      On sheet 24 of your schematics in the upper left there's the power control scheme. You would be looking for 3 transistors not far from the LED connector. Two of them are 8-legged, and the last one would have a thick leg on top and three thin ones at the bottom.

                        Comment

                        • pc1staid
                          Senior Member
                          • Apr 2014
                          • 55
                          • England
                          #7
                          Re: LA-8554P No backlight

                          Originally posted by emestee
                          On sheet 24 of your schematics in the upper left there's the power control scheme. You would be looking for 3 transistors not far from the LED connector. Two of them are 8-legged, and the last one would have a thick leg on top and three thin ones at the bottom.
                          Now thats what I thought, yet i couldnt find them. I was looking for 3 components and I was hopeing a fuse...

                          I have found what I beleive to be Q6 and Q7. No Q6a/b and its a 6 legger, Q7 is a 3 legger. looking at the schematic Q6a/b looks like 2 components were as on the board its a single component, a dual channel diode. so I think thats what was throwing me.

                          Thanks emestee, you have pointed me where i need to be. I will post back if i need and further assistance.

                            Comment

                            • pingaroon
                              Senior Member
                              • Nov 2019
                              • 121
                              • uk
                              #8
                              Hi, I realise this is 4 years after this thread, but I have just come across a board with the same problem, no backlight. Did you manage to solve your issue and locate which component(s) had the fault?

                                Comment

                                • pc_okay
                                  Badcaps Veteran
                                  • May 2022
                                  • 241
                                  • FRANCE
                                  #9
                                  Hello,

                                  You have several things to check: (take a look at page 24 of the schematic)
                                  • carrefully check voltage BKOFF# signal (with laptop powered on, lid open and screen attached)
                                  • carrefully check voltage DISPOFF# signal (with laptop powered, lid open and screen attached)
                                  • carrefully check voltage +LCDVDD (with laptop powered, lid open and screen attached)
                                  • check mofset Q6A and Q7 (check resistantance without any power (no DC/ no battery!), in both side : Gate <-> source / Drain <> Source)
                                  And post your result.

                                  Good luck.

                                    Comment

                                    • pingaroon
                                      Senior Member
                                      • Nov 2019
                                      • 121
                                      • uk
                                      #10
                                      Thanks for your reply. I'll get those readings and post them back here.

                                        Comment

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