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Job lot LCD no.1 - Dell 1908FPc

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    Job lot LCD no.1 - Dell 1908FPc

    A couple weeks ago, i won a job lot of 8 *non-working* LCD's on a "local" auction site, for 10 euros (~12-13 US$), and finally this week, i managed to source a ride to go pick'em up

    First "victim": a Dell 1908FPc, 19" 5:4 aspect ratio, in really good shape, visually. Only a couple scrapes on the bezel.

    Behaviour: 2 seconds to black.

    It was one major b*tch to crack open, but i managed to get the back cover off. I was amazed - chock-full of Rubycon YXG and Chemicon KY, with a Chemicon KXG primary; that pretty much ruled out bad caps

    Time to break out the test-CCFL (recovered from a smashed laptop LCD ). You can see its pink-and-white wires going off to the side, in the attached pic.

    Verdict: one of the two lower CCFL's seems to be... "out of spec". Occasionally, the inverter seems to "pulse" the CCFL's around 40-50Hz i'd reckon, but i've kept the screen on for ~10mins now, and everything's still lit.

    First i tried increasing the "start-up" cap for the inverter PWM (OZ9938; Ctimer in the datasheet, on page 7 and onwards), to try to delay the regulation and/or to give more time for the shoddy CCFL to light up or something. I soldered a 3.3uF cap on top of the existing one (0805 ceramics), but no change.

    Now, i fixed a 19" HP screen a couple months ago, with a similar behaviour, and the fault ended up actually being the "cold" wire to one of the CCFL's - it had broken right where it was soldered to the tip of the lamp. Oddly enough, that had black insulation, same as this Dell. I think i'm gonna have to take the whole panel apart... Fun, fun, fun...

    For now, i'm attaching a top photo of the whole ensemble. It includes a 4-port SMSC-powered USB 2.0 hub - that's what the narrow board on the left is, and it seems to have its own DC/DC converter, which is quite impressive (considering the main PSU provides a 5v rail "too").
    Attached Files
    Last edited by Khron; 11-02-2012, 04:54 AM.
    Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

    #2
    Re: Job lot LCD no.1 - Dell 1908FPc

    Hmm, "damn"...

    I just switched the two lower CCFL's (in the lowest connector), leaving the "test" one in the same (second-lowest) connector. It "still" works fine.

    Maybe they (originals) are "ok enough" on their own, but both of them together "feel" out-of-spec to the inverter controller?
    Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

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      #3
      Re: Job lot LCD no.1 - Dell 1908FPc

      You may have to access the lamp assemblies and do close inspection.
      They can fail badly:
      http://s807.photobucket.com/albums/y...klight%20lamp/
      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

      Comment


        #4
        Re: Job lot LCD no.1 - Dell 1908FPc

        That's the plan for tomorrow (saturday). Today i've kept it on pretty much all evening, with the white-wire bottom CCFL plugged in, and my "test" one, and all was well. I think i'll do the same tomorrow with the black-wire one, just for kicks.

        But fear not, i WILL (have to) crack open the LCD itself and see what's what with those lamps, and try'em out with a laptop inverter.
        Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

        Comment


          #5
          Re: Job lot LCD no.1 - Dell 1908FPc

          You may want to get one of this for lamp testing:
          http://www.ebay.com/itm/Universal-CC...item41606f799e

          And if you have a lot of old CFL, you can use them for testing:
          http://s807.photobucket.com/albums/y...g%20old%20CFL/
          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

          Comment


            #6
            Re: Job lot LCD no.1 - Dell 1908FPc

            Thanks for the tips, but i'm pretty much set for testing I've got an inverter from an older laptop, that i've wired to be pluggable into an ATX (12v for the power stage, 5v for the logic "enable" signal), and i've got a couple CCFL's reclaimed from smashed laptop screens. That oughtta do for fault-finding, methinks
            Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

            Comment


              #7
              Re: Job lot LCD no.1 - Dell 1908FPc

              M'kay, "just to make sure", i'd rather cover all the bases. Let's call the bottom lamp connector, No.4, and the one right above it, No.3. My known-good test-CCFL is "test", and "white" / "black" are the colours of the "cold" wires from the two lower CCFL's in the Dell.


              No.3 - test - OK (tested this configuration for ~6h yesterday, without a hitch)
              No.4 - white OK

              No.3 - test - OK (albeit with a few "waves" in the light intensity about a second after turn-on...)
              No.4 - black - OK (... and a bit more squealing / buzzing from the inverter, than the previous case)

              No.3 - white - FAIL
              No.4 - test - FAIL

              No.3 - black - FAIL
              No.4 - test - FAIL


              PS: M'kay, small facepalm-moment - what i thought was the start-up delay cap was actually on the other side of the PWM IC. I moved it to the proper spot. Now, the internal lamps DO stay lit for the extra time they should, but they still go out. I gotta probe the voltage / current sense lines, maybe i can "tweak" them so the inverter thinks those lamps are (still) ok...

              One more update:
              Fortunately, the signals going to the Vsen and Isen pins of the OZ9938 go through a couple wire jumpers, so they can be easily measured with the inverter mounted on the LCD. During the extended (~5sec instead of ~2sec) start-up time, Vsen reaches ~0.8v, and Isen only gets up to about 0.12v. For normal regulation, Vsen should reach the 3v threshold (although the overvoltage-limit should be set lower than that), and Isen should be around 0.7v.

              I'm thinking of increasing that start-up delay cap even further...

              EDIT: Nope, increasing the start-up cap doesn't help much. Lamps stay lit for the further-extended time, but they still "die" when that time runs out. Vsen manages to only reach 0.86v...
              Last edited by Khron; 11-03-2012, 05:27 AM. Reason: Updates
              Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

              Comment


                #8
                Re: Job lot LCD no.1 - Dell 1908FPc

                I just tested both those bottom CCFL's (separately) with a laptop inverter. Both of them lit up just fine, no strange noises out of the inverter.

                So, conclusion (so far):

                1) CCFL's are (pretty) fine
                2) inverter's ok
                3) the two above don't get along together anymore
                Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

                Comment


                  #9
                  Re: Job lot LCD no.1 - Dell 1908FPc

                  I decided to scope the signals that go to the Vsen and Isen inputs of the inverter PWM chip.

                  The bottom (clearer) trace is the Isen signal (DC coupled, 1v/div, the 0v level is the line right below the valleys).

                  The top (fuzzier) trace is the Vsen signal (AC coupled, 20mv/div, on top of about 1v DC).

                  Switching frequency seems to be about 60KHz.

                  First picture = test setup
                  Second picture = signal shapes with my test-lamp hooked up instead of the black-wire CCFL, 20us/div timebase
                  Third picture = same thing, with all four "internal" CCFL's hooked up, before the striking time elapses

                  Hmm... Seems in the last case, it's "stuck" in the lamp-striking mode (higher frequency, low current).
                  Attached Files
                  Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

                  Comment


                    #10
                    Re: Job lot LCD no.1 - Dell 1908FPc

                    Did a couple more tests with the Samsung LCD out of a 19" Acer i've just about given up on.

                    Still getting the inverter shutting down (despite increasing the "timer" cap), with just about any lamp pair combination

                    I increased the timebase though, and it kinda looks like it's going into some sort of "hiccup"-mode at about 1kHz...

                    Top trace = Vsen, DC-coupled, 0.5V/div (0v = middle of the screen)
                    Bottom trace = Isen, DC-coupled, 0.2V/div (0v = the bottom of the displayed "valleys")
                    Attached Files
                    Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

                    Comment


                      #11
                      Re: Job lot LCD no.1 - Dell 1908FPc

                      Originally posted by Khron666 View Post
                      Did a couple more tests with the Samsung LCD out of a 19" Acer i've just about given up on.

                      Still getting the inverter shutting down (despite increasing the "timer" cap), with just about any lamp pair combination

                      I increased the timebase though, and it kinda looks like it's going into some sort of "hiccup"-mode at about 1kHz...

                      Top trace = Vsen, DC-coupled, 0.5V/div (0v = middle of the screen)
                      Bottom trace = Isen, DC-coupled, 0.2V/div (0v = the bottom of the displayed "valleys")
                      Hello,
                      I have same problem, 5 seg. light and turn off power source, 1 CCFL is OK and second CCFL es bad, I replaced but same problem.


                      Thanks for any helps

                      Comment


                        #12
                        Re: Job lot LCD no.1 - Dell 1908FPc

                        Can you update your profile with Country and mains voltage.
                        Is that 5 Seconds?
                        Have you got a known source (Running PC with a picture) connected
                        Please upload pictures using attachment function when ask for help on the repair
                        http://www.badcaps.net/forum/showthread.php?t=39740

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