T520 Hall sensor or System Board fault, no backlight and keyboard with working sensor

[segacleop]

My T520 boots with no backlight and keyboard is non functional if i connect a working lid Hall sensor. I bought the laptop used and found that the sensor had been cut out. I have a sensor from another t520 that i tried, and when this is connected the laptop boots but nothing is shown on the display. During this episode an external monitor through vga is detected and works fine but properties don't show the inbuilt display. Has anyone come across this issue before and if so how did you remedy this ?

[RealBlackStuff]

Welcome to the forum.
Your problem is a blown fuse near the motherboard's LCD connector.
To get an idea what to look for, see this (from a similar T400): http://forum.thinkpads.com/viewtopic.ph ... 93#p731793

[segacleop]

Welcome to the forum.
Your problem is a blown fuse near the motherboard's LCD connector.
To get an idea what to look for, see this (from a similar T400): http://forum.thinkpads.com/viewtopic.ph ... 93#p731793

Thanks RealBlackStuff, this is a good point to start at, next thing is from the other thread hunting for the fuses will take some time. I was wondering if anyone has done this on a 520 before ? Googled and nothing is out there just more requests like mine ?

[RealBlackStuff]

If you still can't see the fuse(s) after removing palmrest, keyboard and KB-bezel, you'll have to take the whole thing apart.
Get the HMM from the link on top of this page.

[segacleop]

If you still can't see the fuse(s) after removing palmrest, keyboard and KB-bezel, you'll have to take the whole thing apart.
Get the HMM from the link on top of this page.

On the bottom side of the board, this is what i found. I have attached photos of the board below. Also i found a post on badcaps about a slightly similar fix for a T520 here by enrique_21 At that post he attached the schematics for a w520 (pg 30 shows the lcd connector). He explained the functions of the fuses as
F6 - carries a 3vcc power and is the one in charge of switching backlight on or off
F2 - is for the lcd itself
F1 - carries the main 20vcc power for the backlight led
The forum post photos are locked behind a login. He ended up doing a messy fix on f1 for his case. I have tried to test for resistance across the fuses four fuses and all show continuity except F2. I have never done soldering on such small components so i might end up using the conductive paint and shorting but i am worried since it is labelled 500MA/32V. I didn't try to measure voltage at each pad when in power since my multimeter's probes are too big, and i am worried of shorting some other component. Any suggestions will be welcome and i hope this can be used as a starting point for someone out there with the same issue and better skilled.
my Board
Enrique_21's fix
W520 schematics LCD connector on Pg 30

[RealBlackStuff]

Get the correct replacement fuse (order 2 or 3), then solder it piggy-back style on top of the old one.