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Thinkpad T520 no backlight, need help & advice, schematics provided

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    Thinkpad T520 no backlight, need help & advice, schematics provided

    Hi, I have a Thinkpad T520, i5 with integrated graphics. Some days ago I decided to try upgrading my laptops screen to a FHD one, when I first installed the new screen it all went nice and smooth and worked perfectly. Unfortunately I decided to swap back the old screen except this time I didnt take out the battery, now neither the old or new screen backlight will light up. So ive been reading the possible causes and I was hopeful it was something simple like a blown fuse, because the computer works perfectly on external monitors and I can even do normal stuff if I point a flashlight at it, that means the only problem is theres no backlight, the led board (where the wifi and bluetooth leds are) works ok too.

    I couldnt find the specific diagram for the T520, but I found the one of the W520, these machines are very similar and its my understanding all screen options are compatible between them, so I believe at least regarding the lcd circuitry they are the same, both are led backlight btw.

    Attached to my post is the pdf schematic , on page 30 there is the motherboard 40 pin lcd connector and its components. What I understand is the fuse F6 carries a 3vcc power and is the one in charge of switching backlight on or off, F2 is for the lcd itself and the one marked as F1 carries the main 20vcc power for the backlight led. After testing continuity with a multimeter F1 is open, and it also seems to have a slightly different voltage when tested between its sides and sides with motherboard ground, also I may be doing something wrong because I cant read its ohms, it just marks 1 on all scales. F6 and F2 have continuity and zero volts across them. That means F1 is bad right? I read voltage should be zero across them.

    Now, Ive been looking for someone that sells smd components and fuses, and failed because I cant find someone who provides these without importing and consequently waiting up to a month and I have no credit card anyway, I havent had luck with local repairshops either because they want to charge me what a new computer would cost and the so called motherboard component level repairshops would just put a solder blob over it, something that may work but in that case why pay if I can do that too

    My questions are: am I correct in assuming its simply the fuse whats wrong? Do you guys think I should test further to for example see if the 20V are actually provided right before the fuse? Or maybe check the whole lvds cable and make sure it has continuity and no shorts. Im a little afraid of breaking something else if I have to do tests with power on and battery. And, considering I cant find an exact smd replacement, can I use instead one of those little car fuses? I'd solder two little wires to it and then solder it on top of the old fuse, I can find them with the same specification of 3A 32V, is this a good idea? Are glass fuses acceptable too? And does anyone know if those are fast or slow acting and if that matters? I know this sounds like a really cheap and dirty fix, but money's short and thats better than simply bridging. Unless I can trick Lenovo into fixing it for free because the computer still has almost 2 years left of warranty.

    I also attached the screen im upgrading to product spec. pdf sheet and a pic of the backside of the connector, there's nothing of help on top. Thanks in advance.
    Attached Files
    Last edited by enrique_21; 08-06-2014, 03:26 AM.

    #2
    Re: Thinkpad T520 no backlight, need help & advice, schematics provided

    When you are saying voltage across fuse - that is not a test.
    Tests are 1. power off, meter on ohms200.- probe to each side of fuse should be the same as touching the probes together.
    2. power on test each side of fuse to a nearby chassis ground.

    Read this thread https://www.badcaps.net/forum/showth...969#post471969
    and see if you can use your meter in DCamps mode to test for shorts.
    Your screen may even come on
    Please upload pictures using attachment function when ask for help on the repair
    http://www.badcaps.net/forum/showthread.php?t=39740

    Comment


      #3
      Re: Thinkpad T520 no backlight, need help & advice, schematics provided

      Ok so I tested the ohms again and it keeps showing 1 from 200ohms to 20M, multimeter manual says it'll show 1 when the measurement is above the rank, I guess this is whats happening and the fuse ohms are greater than 20M ohms. When testing other fuses the reading is the same as when touching the probes together (0.04). On the second test what should I be trying to measure? ohms or volts and am I grounding to the motherboard ground or the laptop chasis?

      About DC Amps sorry if im a bit skeptic but how safe is this assuming I dont slip the probes to other components? Am I trying to find what current is being provided? If the current provided is the normal that means the backlight would turn on but when I take out the probes that are shorting the circuit wouldnt that be like suddenly disconnecting the power to the screen and isnt that what broke the fuse in first place? So I'd have to take this measurement with the probes firmly in place and dont move them correct? I'll see if I can safely tape the probes to the fuse or get some tweezers.

      Comment


        #4
        Re: Thinkpad T520 no backlight, need help & advice, schematics provided

        what?!!??!?!
        i had a dell adapter and i cut the cable cuz it has small wires and use one to putt in there!

        it still runs perfectly

        that was t430
        https://www.badcaps.net/forum/showthread.php?t=32313
        Just cook it! It's already broken.

        Comment


          #5
          Re: Thinkpad T520 no backlight, need help & advice, schematics provided

          Ok so I got it back and running. Just one question I hope someone can clear out, when I first turned it on (on battery) after complete dissasembly including the backup battery I saw it "try" to turn on 3 consecutive times (power button lighted and died 3 times), then it showed the bad cmos checksum. After setting time and all that it boots normally at first try like before.

          Is it normal that it tried to boot up and failed or was it doing some kind of general checking? I had it dissasembled for 3 days without cmos battery and pressed the on button many times before without power to make sure it drained all circuits. Also if it matters the fuse is now wired up on the other side and is sitting on top of the cmos battery. My guess is the caps didnt have enough power and thus it didnt boot straight tue first time but I want to know your opinion guys.
          Attached Files

          Comment


            #6
            Re: Thinkpad T520 no backlight, need help & advice, schematics provided

            Hi

            Can anyone tell me or point me in the direction of how to get to the fuse in question?

            Is it accessed from behind the keyboard?

            Thanks

            Rob

            Comment


              #7
              Re: Thinkpad T520 no backlight, need help & advice, schematics provided

              Hello Everybody,

              I have a T520 without backlight here... As I am no electronic genius, is there a possibility to jump the fuse with conductive wire glue like it was done with a T400 as written here?

              http://forum.thinkpads.com/viewtopic.php?t=107279

              If yes, where I can find the fuse exactly?

              Looking forward to your answer, thanks a lot!

              Greetz,

              XStoneX

              Comment

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