No backlight / dim screen

[JJudd]

Hello!

I manage several hundred chromebooks and repair them as needed, and I've got one that I've seen twice now and not found a good solution for. 

My issue is that we have a Chromebook 11 G3 that has a dark screen, very faintly you can see what should be displayed. Not a case of the brightness being turned down, but rather the LED backlighting on the screen appears not to be working.

I've now tried replacing, screen, motherboard and display cable in an attempt to fix this issue. The screens were brand new, as were the replacement screen cables, though the mother boards were second hand. HDMI is still functional in all cases. 

I have called HP support and not gotten any information other than "replace the motherboard". 

As I have now seen this twice in the past month and all of those devices were purchased at the same time I have concerns that this may be more frequent. 

I guess my question would be if there is a schematic for testing the boards or information on fuses that might have blown and could be jumpered or replaced. I would also love to know if anyone else is running into this issue. I would also be thankful for any resources on testing procedures for components like the screen and output on the eDP connector. I have a little background in electronics and don't mind testing and/or soldering.

Also, I have already tried powerwashing/resetting the device to no avail, stating that as I know almost all of the support responses get that procedure pasted in as a fix. It is not for this issue.

[NOMStech]

I believe I have the same issue with an HP G3 Chromebook. I have replaced the LCD and the motherboard. However not cabling. Hopefully someone has a solution.

[JJudd]

I may have found the source of at least some of the failures I've been having.  I had another one come in today that was behaving the same way, dim screen, etc. However, this time there was one position the screen could be in that would turn on the back light.

Suspecting a loose cable I popped it open and verified the ends were connected, but it turned out that there was a problem with the cable. If it bent wrong it would short out.  I replaced the part with one salvaged from another chromebook that had been used for parts, and the back light came on and there was much joy.

This joy was short lived. Short being the key word. After assembling the chromebook again, there was no back light. While moving it around a little, I noticed that the corner, near the hinge where the cable for the LCD passes through was quite hot. I quickly pulled the bezel back off, and it started smoking!

Now in a much bigger hurry, I quickly tore into the base to disconnect the cable.

I'm thinking that I saw two versions of the cause of the issue. One where the cable was not completely shorting, but still having issues with broken wires, and a second where the broken wire lead to it making contact with the metal shielding.

My first instinct was to look for any pinches in the routing, I can make mistakes with the best of them... Like the crab from the honda commercial says, no pinch...

So, ultimately, I think it is this cable that is causing these issues. Shorts in this cable could account for both the screen and the motherboard going belly up. Anyone else running into this issue? I know we are a little more exposed to the issue because we have several hundred of these devices deployed, but I'd like to know if this is a bit more common than one would expect.

In a possibly related note, looks like prices on that cable are going up, often to ~$40. I wonder if they've been going out and the supply on them is getting much smaller.

[mgrele]

I have a large number of this model with this problem. I also service other brands that have the same symptom. However, on the other brands I have a 100% fix rate, it is always a loose/damaged cable or a screen issue. On the HP I can fix a very small number of these by reseating the cable, and another 5-10% by replacing the cable. I suspect that the problem with most of them is the backlight fuse, but I haven't been able to identify the location of the fuse on the logic board since I don't have a schematic. Further evidence of a fuse problem is a couple of user reports that there was a small flash before the screen want dark. I don't know why this HP model is more prone to fuse blowing than others, but there it is.

[mililani]

Sorry about reviving the dead.  I deal with large # of these chromebooks at a school district.  I have some handy tools to help diagnose and fix this issue.  In my experience, this issue could be due to these  3 things:

1) The system board

2) The display cable

3) The LCD panel (there's a logic board under the display that can fail)

I usually test the system board with a good known display/cable first.  If that powers the display fine, then I know it's the cable or display.  If it doesn't, I know it's the system board.  If it's the display/cable, I remove the display and take it apart, and test the cable with a known/good display.  If that works, I know it's the display. 

Usually, though, in my experience, it's typically the system board.  Bad display cables often cause glitchy screens that turn on and off when the display is moved back or forward.  A bad LCD panel logic board usually manifests itself as a screen that turns on with backlight but with no display.  However, I have seen the odd one offs here and there.