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Dell Inspiron 15 3558 - no backlight

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    #1

    Dell Inspiron 15 3558 - no backlight

    Hi,
    I'm trying to fix friend's Dell, it had liquid damage in the past, but as far as I know it was fixed in professional service. It was working fine for quite some time, except for dead battery, but recently there was blackout in the building and after power was restored, laptop was broken. When I got it, it was posting, but OS didn't boot, also backlight was gone. After some tinkering I switched off secureboot and it started booting normally, but backlight issue persisted. I opened it up and started looking for issues. Checked both fuses on display and all fuses on motherboard which I found and everything looks ok. After some measurements all I found is that BL_ENABLE signal in display is 0.0V (but not shorted to ground), BL_POWER was ok, at around 19V. Display model is NT156WHM-N12. I traced it back to notebook motherboard, but I ended up tracing it to NPCE285PA0DX-GP IC (pin BLON_OUT) and it was 0.0V all the time. I also measured L_BKLT_EN and it was 3.3V. Does it mean that there is software issue or NPCE285PA0DX-GP is bad, or it can be something else? After I found out that BLON_OUT is 0.0V, I don't really have idea on what to check next.

    Motherboard model is: Iris HSW/BDW 14216-1 Rev:A00
    Notebook doesn't have dGPU
    Screen model is NT156WHM-N12

    Regards,
    Milosz
    Tags: None
    #2
    Voltage on pin20/21 of edp connector LCD1
    D5202/D5201-voltage on all pins
    U5201-voltage on all pins

    Give us these measurements. the measurements should be done after pressing power button.

    Comment

      #3
      Hello,
      Thank you for your answer

      Here are measurements:

      D5202
      1 (BKLT_CTRL) - 3.17V
      2 (eDP_BKLT_CTRL) - 0V
      3 (EC_BRIGHTNESS) - 3.37V

      D5201
      1 (LCDVDD_EN) - 3.2V
      2 (EDP_VDD_EN) - 3.37V
      3 (LCD_TST_EN) - 0.00V

      U5201
      1 (EN/LCDVDD_EN) - 3.2V
      2 (GND) - 0.00V
      3 (VOUT/LCDVDD) - 3.36V
      4 (VIN#4) - 3.37V
      5 (VIN#5) - 3.37V

      LCD connector:
      PIN 20 - 3.17V
      PIN 21 - 0.00V

      I'm not sure if I mapped correctly pins of D5202/D5201, I assumed that physical pinout matches pinout on schematics.

      Regards,
      Milosz

      Comment

        #4
        D5202 measurement seems suspect as its pin1 & 2 are perhaps connected together with a pcb jumper trace. You can check continuity with RN5201,pin7 to D5202 and the pin which beeps is pin1.

        U5201 is working fine. What is the LID status? Is it located on power switch or mb?

        Comment

          #5
          Hi,
          So it looks like that jumper (R5222) is missing:

          Click image for larger version Name: 20240314_202453.jpg Views: 20 Size: 919.0 KB ID: 3235060

          I traced R5222 pads and it looks like pins are swapped, so it should be like this:

          D5202
          1 (BKLT_CTRL) - 3.17V
          2 (eDP_BKLT_CTRL) - 3.37V
          3 (EC_BRIGHTNESS) - 0V


          I also measured LID_CLOSE#_C signal and its 0.00V

          Regards,
          Milosz

          Comment

            #6
            Small update - I did some more software-side testing. I started with bios update, but it didn't change anything. But I also noticed that power supply is detected as 0W power, which cause system to run very very slow. I tried another power supply (both genuine), but issue peristed. Also for unknown reason internal display disappears when Windows starts, there is no way to switch to internal display, I can only use external monitor via HDMI. So internal display works fine in BIOS, but in Windows it's unavailable. I did drivers update with dell tool, but still only external display is visible.

            Regards,
            Milosz

            Comment

              #7
              PS_ID is very important and there is something wrong here and probably this is the real issue.
              measure voltage on both sides of PR4205 and check its 33 ohms. often this resistor goes bad.
              Voltage on PQ4201-all pins?

              Comment

                #8
                Hi,
                PR4205 looks fine (33Ω, 3.31V on both sides), but PR4206 seems to be missing
                PQ4201 voltages:
                1 (PS_ID_R2) - 3.31V
                2 (PSID_DISABLE#_R_C) - 5.15V
                3 (PSID) - 3.31V
                no significant changes after pressing power button

                Regards,
                Miłosz

                Comment

                  #9
                  One more thing - I attached logic analyzer to PS_ID_R pin of DC1 connector and to PSID_EC line and in both cases all I got was 10 reset signals:
                  Click image for larger version Name: Screenshot_20240318_214440.png Views: 150 Size: 33.9 KB ID: 3238050
                  Click image for larger version Name: Screenshot_20240318_214821.png Views: 116 Size: 8.5 KB ID: 3238051

                  I tried two different power adapters and in both cases it was the same.
                  I also checked continuity between small middle hole in power socket and DC1 PS_ID_R pin and it seems to be fine. What else can I check here?

                  Comment

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