A challanging problem: P751TM-G keyboard light issue

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:

This is related to the keyboard only:

And this is the lightbar related schematic.


Any help, ideas would be appreciated. Thank you for dropping in.
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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.

 

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!

 

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

 

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.

 

First rule of diagnosis, you will measure voltages.

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

 

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.

 

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

 

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

 

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?

 

I was using mainly white backlit color and it was great. Then changed to some other color like green or red, and did BIOS update altogether with CPU upgrade. Not much after, a week or so, when I tried white color it was no longer same white, I though it is something wrong with the BIOS, I did not measured voltages or anything. Don't have time to dig into it, but I will check if I can swap the LEDs, new keyboard which I need I cannot find anywhere.

 

I have found a new, possibly original keyboard on AliExpress for about $50 that I'm thinking about buying. It is available in multiple layouts. I will send you a link if you are interested.

 

Yes.

Simply sad such a powerful machine to have such issue.

 

I agree. I looked at the keyboard thoroughly, and it looks way too sealed for it to be disassembled without damage/reassembled with the same stability.
Since my account is probably too recent I cannot yet send you a message, so please send me one to see if I can reply.

 

Its built in lots of sandwiched layers and cant really be taken apart.