A challanging problem: P751TM-G keyboard light issue

Discussion in 'Sager and Clevo' started by kekstud, Feb 17, 2020.

  1. kekstud

    kekstud Newbie

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    The blue light channel has become much dimmer than it should be on both my keyboard and the bottom light bar. This results in "white" color being yellow in all 3 zones, "purple" being red and so on. It is much harder to see. Nothing has happened to the laptop physically (no liquid damage, etc.) I have updated BIOS and EC to the latest versions, did multiple resets, and still, the blue LEDs are much weaker while the others light up just fine.

    I have also disassembled the device, tried disconnecting the lightbar, then the keyborard, then vice versa, no result. I have tried to make use of the relevant schematics, but I have yet to reach this level of comprehension. However I am not afraid of solving this myself, and solder if necessary.

    As far as I am concerned this is related to both the keyboard and the lightbar:
    [​IMG]
    This is related to the keyboard only:
    [​IMG]
    And this is the lightbar related schematic.
    [​IMG]

    Any help, ideas would be appreciated. Thank you for dropping in.
     
    Last edited: Feb 17, 2020
  2. jc_denton

    jc_denton V̖̟en̰g̻̼̰̩͙ea̲n̪c̭e̼ ͍̘̤͓̟̤Is̙͔ ̤Mi̻̭̣͎͍ͅn̰ḙ

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    The ribbon cable connecting the RGB elements to the motherboard is quite fragile, and sometimes fails at the bend after being disassembled several times. I'd take a look at the cable itself and inspect it for tears.
     
  3. kekstud

    kekstud Newbie

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    I have done that, no sign of damage whatsoever, also moved it gently around but the brightness has remained constant. I also doubt that both independent cables would fail on the same color at the same time. Thank you for the recommendation however!
     
  4. jc_denton

    jc_denton V̖̟en̰g̻̼̰̩͙ea̲n̪c̭e̼ ͍̘̤͓̟̤Is̙͔ ̤Mi̻̭̣͎͍ͅn̰ḙ

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    When you had it disassembled, did you try cleaning the gold ribbon contacts with some isopropyl alcohol? How old is your machine?
     
  5. kekstud

    kekstud Newbie

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    I did clean them thoroughly! I bought the machine used, but the bios it had was from around 2017 before I updated it (version 1.05.03), so I assume the machine has been manufactured around that.
     
  6. Meaker@Sager

    Meaker@Sager Company Representative

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    First rule of diagnosis, you will measure voltages.

    Is the driving chip actually producing the right voltage on the blue channel?
     
  7. kekstud

    kekstud Newbie

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    This was a great suggestion. I checked the keyboard connectors with a multimeter. Since all channels were showing 3.99 Volts, I concluded that it must be a keyboard issue. This assumption was confirmed when I asked a friend with a similar laptop to swap keyboards. His keyboard was fine in my laptop, any my keyboard was still malfunctioning in his computer. So looks like I must search the problem in the keyboard, which I would prefer over replacing it, because it's in near mint condition.

    This still does not explain however the mystery of why the lightbar has the same issue.. manufacturing defect I suppose.
     
    Last edited: Feb 18, 2020
  8. Meaker@Sager

    Meaker@Sager Company Representative

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    It is odd the same channel has done the same thing on that as well. The voltages are within spec too.
     
  9. IllusiveMan

    IllusiveMan Notebook Consultant

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    Same problem on my P751DM2, seems it's the LEDs that cause this issue?
     
  10. kekstud

    kekstud Newbie

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    Luckily it is an issue with the keyboard and not the motherboard. I confirmed this when I tried my friend's keyboard in my computer, which works just fine.

    It is most likely a bad batch of LEDs, but since I bought the laptop used I did not see it before it started malfunctioning, so I cannot confirm if it was dimmer from the beginning. What was your impression?
     
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