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    Asus k52jr no backlight

    Hello everyone,

    I am not an expert on hardware, but I do my best and I'm here to learn so please be patient, every little step is my first step in this.

    I have trouble with my laptop, motherboard model k52jr, LCD led screen model lp156wh2. I have isolated the problem to the backlight, computer works perfectly with external monitor plugged in, the native screen works except for the backlight and I can see the desktop if I shine light on screen.
    I did some research and find out about multimeters, how they works, what they do, so I bought one at a fair price and started doing some tests.

    I downloaded the specs for my screen, but I was unable to take measurements on the 40 pins on the back of the screen, everything is simply too small, so I had no idea which pin receives what. I checked the screen plug on the motherboard instead, but I counted only 30 pins on it. Anyway, a couple of them reported 19.4 volts (I connected one end of the multimeter to the ground), so I guess the motherboard is doing its job.
    I checked voltages on the back of the screen then, i found an F1 and an F2 fuse. They passed continuity but when the screen was plugged in they measured 19.4 volts instead of the 32 written on the board. I check VLED and VLED_OUT and they both get 19.1-19.3v.
    At this point I don't know how to isolate the problem a little more and get closer to the solution, I am aware that i probably made it simpler than it is, but that's precisely why I'm asking for your help.

    I suspect it might be the led strip itself, since everything seems powered on, but i have no way to test them directly.

    Any kind of insight is very welcome, I attached some pictures if you wanna take a look.
    Thanks a lot guys


    Attached Files

    #2
    Re: Asus k52jr no backlight

    19 volts on back of the screen is ok. For the LEDs to turn on you need a couple more pins to be 3.3v, specifically LED_EN and PWM. Pins 36 and 35 on the back of the LCD connector. See here for reference: http://s599.photobucket.com/user/Chr...l?t=1332836621 You are most likely to find that you are missing LED_EN.

    A bad screen or a bad LCD cable can cause this. I have a similar Asus laptop in for servicing where the screen shorted out LED_EN and melted the wire in the LVDS cable, so this one it's going to need both a new screen and a new cable. Yours might be luckier.
    Originally posted by PeteS in CA
    Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
    A working TV? How boring!

    Comment


      #3
      Re: Asus k52jr no backlight

      First, thank you so much for the suggestion!

      So, I got a thinner probe and took those measuments. I don't know how can you do it for a living guys, I had a hell of a time to measure those tiny pins and I couldn't have done with some help to take the readings and write them down. I double checked them just to be sure.

      This is what I got:

      35 ==> 3.3
      36 ==> 3.2
      37 ==> 0
      38 ==> 19.2
      39 ==> 19.2
      40 ==> 19.2

      So i got luckier than the Asus you repaired and I think I can assume that whatever is not doing its job lies on the screen, but correct me me if I'm wrong, I might not being considering things I don't know.

      A big part of the problem is that i know nothing about the circumstances that lead to the backlight failure, a friend took this laptop to a technician for a keyboard issue and it came back with the backlight problem. So she came to me to recover her datas and left the laptop as a present. I don't really need it to get fixed imediately, I think this is a great chance to start experimenting and learning without paying any real consequences if I fuck up big time, however it goes I want to learn a big deal in the end.

      Any other suggestion is greatly appreciated, I'd like to concentrate in finding a way to test those leds since they seem to be the next best candidate for backlight failure.
      I found this page which describes an reletively approchable way to test the led circuitry, which gets close to what I want to do, but it's not really that.

      I'd like to feed energy directly into the led strip itself, so first I need to know which are the points where i should stick my pos and neg, which voltage and amperage I need to get even a glimpse of light from them.

      Comment


        #4
        Re: Asus k52jr no backlight

        UPDATE:

        I took it one step further and measure voltages directly on the led strip socket and I got those values (image attached):



        1==> 19.1
        2==> 19.1
        3 to 9==> 0

        Can I safely assume that the led strip itself is not working anymore?
        If yes, what may have caused the problem in the first place and how can I be sure I won't have the same problem once I replace them?
        Attached Files

        Comment


          #5
          Re: Asus k52jr no backlight

          Update 2

          I took out the led strip trying to check voltages on the leds themselves and something strange happened.

          I connected the back probe on the ground as usual, when i put my red probe on the side of one led it lit up, and other leds every 5 or 6 too. The more I moved away from the plug, the more bright they became, but never as bright as they should be. When i moved to the last led i got the highest value of 16.4v, while the first was 0.8. I check
          every led and they all work.

          At this point I'm lost.

          Comment


            #6
            Re: Asus k52jr no backlight

            After couple years a lots of learning I took back my old screen and gave it a look.
            The ribbon cable was damaged. I have no idea where because it's not visible, but that's it.
            Thank you all for the help!

            Comment


              #7
              Re: Asus k52jr no backlight

              Originally posted by Th3_uN1Qu3 View Post
              19 volts on back of the screen is ok. For the LEDs to turn on you need a couple more pins to be 3.3v, specifically LED_EN and PWM. Pins 36 and 35 on the back of the LCD connector. See here for reference: http://s599.photobucket.com/user/Chr...l?t=1332836621 You are most likely to find that you are missing LED_EN.
              This help me resurrect a Dell Inspiron 3000 Series Screen that would not give backlight only picture. On the screen itself on the bottom raised the plastic paper and on the test points LED_EN and PWM was missing just like Th3_uN1Qu3 mentioned. Those 2 are very important for Backlight. So i took a jump from a test point called Acc that had 3.3V and jumped LED_EN and PWM together and now I have full backlight working. One thing to note at first I only jumped LED_EN with the 3.3v and i didn't get any backlight. Only when i jumped both LED_EN and PWM i was able to get full backlight working. Thanks a lot uNiQu3 for pointing this out.

              Comment

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