Backlight dead on ThinkPad P51 20HH?

[cheeseandjamsandwich]

Hi all,

I have my Lenovo ThinkPad P51 20HH laptop with a dead backlight.
It's a Xeon E3-1505M v6

The picture is visible on the panel with the help of a flashlight. So it's looking like the backlight is dead i guess...?

I've tested the one fuse next to the 40-pin on the MB that i can find, and that hasn't blown, it has continuity... So i'm wondering where else it has fuses? Or what else it could be?

I sadly didn't get photos when I've had it opened. And it's busy processing photogrammetry stuff at the moment. As it's otherwise works great with an external of course. But if needed i could crack it open and get some.

The story:
The 4k panel one day developed a corrupted 1 inch strip over towards the left side, top to bottom... But if i tapped the bezel it corrected... I live with this for a few weeks i think... Eventually i needed to take the bezel off and poke the silver mesh tape covering the panels driver board at the bottom, and it would correct it. (wowzers it's hot!)
But sadly one day it wasn't playing ball... I remember that i'd been tapping the driver board and also the top cover near the hinge of the laptop, under which the LCD cable is routed, and the panel then went totally black.
So seeing the 1 inch corrupted strip, i assumed the panel was dead, so got a new one from Aliexpress. It came and looked perfect/identical. But it didn't work. Then ordered the cable, but when that came, it didn't work. I do not remember disconnecting/connecting the panel with the battery in place, but i may have... I gather that's a backlight fuse popper... But only later did i try looking to see if it was working, but without a backlight...

So i have a new panel, new cable and a display that i can see is there with the help of a flashlight.

Anyone have any ThinkPad wisdom that may help?

Thanks in advance!
Rich.

[RethoricalCheese]

Re: Backlight dead on ThinkPad P51 20HH?

Thinkpads have atleast 2 lcd fuses. And they are always near the lcd connector but might be on the other side of the board. So you would need to take out the board and just take a look.

[Vesko356]

Re: Backlight dead on ThinkPad P51 20HH?

Connect the old screen and measure the signals on the screen board test points, 19V, 3.3V backlight enable, and backlight pwm (should be 3.3V too on full brightness) signals. If some of the siignals is missing, first check fuses (19V and 3.3V) on the screen board, if the fuses are OK, but some signal is missing, then you can check the motherboard. Finding the cause of backlight fault is a pretty simple job.

[cheeseandjamsandwich]

Re: Backlight dead on ThinkPad P51 20HH?

Thinkpads have atleast 2 lcd fuses. And they are always near the lcd connector but might be on the other side of the board. So you would need to take out the board and just take a look.

F6 & F7 it looks like (thanks to SMDFlea for supplying the Schematics! https://www.badcaps.net/forum/showpo...43&postcount=2)
And yes, F6 is on the backside. I've tested F7, and it had continuity...
Thanks!

[cheeseandjamsandwich]

Re: Backlight dead on ThinkPad P51 20HH?

Connect the old screen and measure the signals on the screen board test points, 19V, 3.3V backlight enable, and backlight pwm (should be 3.3V too on full brightness) signals. If some of the siignals is missing, first check fuses (19V and 3.3V) on the screen board, if the fuses are OK, but some signal is missing, then you can check the motherboard. Finding the cause of backlight fault is a pretty simple job.

I have a few days of processing work to do and then i'll open her up and connect the old panel as suggested. The panel has marked test points i guess?
The schematics suggest it's F6 & F7 on the motherboard for the lcd/backlight fuses. I know F7 is ok, it had continuity when i had her open last time. It was right next to the connector.

I'll report back when i have a break in processing jobs and get her open.

Thanks!

[cheeseandjamsandwich]

Re: Backlight dead on ThinkPad P51 20HH?

Thinkpads have atleast 2 lcd fuses. And they are always near the lcd connector but might be on the other side of the board. So you would need to take out the board and just take a look.

i've cracked her open tonight and F6 seems to be blown... If the white SMD to the immediate left is the fuse...
It doesn't have any markings, like F7 does (P)...
Am i correct in assuming this is the fuse?

I've attached a few microscope pics.

To test on the bench, i just need the heatsinks/fans on, ram and the DC-In connector to power it up i guess?

Thanks

[mcplslg123]

Re: Backlight dead on ThinkPad P51 20HH?

Yes F6 is a fuse. You can get similar fuse at the back of screen(if you have any damaged/broken).

[cheeseandjamsandwich]

Re: Backlight dead on ThinkPad P51 20HH?

Yes F6 is a fuse. You can get similar fuse at the back of screen(if you have any damaged/broken).

That's a very handy little trick to learn!
I found 3 fuses on my old, dead panel.

But OMG aren't they so amazingly tiny!
I'm working with a USB microscope, with more lag than you can shake a stick at. So it was horrendous trying to get it in place. I got the old F6 fuse off using the big blob of solder trick, as i only have a simple soldering iron, with a smallish tip. But putting the new one into place took me ages. It ain't pretty, but it's electrically correct. Must get an optical microscope, and sharpen my tweezers...
The fuse then had continuity and no shorts to ground, so all good...
After i'd gotten her back together with some new paste and pads, and then the hilariously horrifically long initial seemingly 'dead' screen/laptop ram check (64GB!) (gets me every time... patience......) i finally saw the lovely red Lenovo logo.

Many thanks to all that helped!