Lenovo Y50-70 (4K, no touch) - Backlight issue

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  • artjvl
    New Member
    • Jan 2020
    • 3
    • The Netherlands
    #1

    Lenovo Y50-70 (4K, no touch) - Backlight issue

    Hello,

    The problem is that the backlight of my Lenovo Y50-70 (the 4K (no touch) option) stopped working. The LCD is confirmed to still work after shining a flashlight through the panel. External monitors also work. My problem started occurring after the laptop slammed shut with a little too much force, so it might be physical damage. I initially expected the ribbon connector to be the culprit, but with the LCD still functioning completely, this seems unlikely.

    R375 (i.e., the 0-ohm resistor that works as a fuse for the backlight) measures 7.5V on both sides. I know the voltage should be 20V. Is there any way to diagnose whether this is a motherboard issue or a software issue?
    Tags: None
    • artjvl
      New Member
      • Jan 2020
      • 3
      • The Netherlands
      #2
      Re: Lenovo Y50-70 (4K, no touch) - Backlight issue

      Bump!

      I have attached the motherboard schematics for convenience. It seems that the backlight is powered via pins 38-40 of the 40-pin eDP connector. Those pins are connected via the 0-ohm resistor (R375, which acts as a fuse, and which is not blown in my case!) to the CPU_B+ net. This net is then conncected, via the EMI filters PL1101, PL1105, PL1106, to the B+ net, which I expect to be the battery output. A quick Google search tells me that the battery output voltage is 7V, which is also the voltage that I measure over R375. This forum thread (https://www.badcaps.net/forum/showthread.php?t=61690) suggests 20V to be the correct voltage for the backlight to work, but I cannot figure where that 20V comes from. What are the next steps that I should take in order to figure out what the problem is?
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