LCD Backlight Not Powered (Dell Inspiron 7559)

merudo

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Feb 20, 2018
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My Dell 7559 no longer powers up the screen backlight.

I tried two different LCD LED screens and both have the same issue (If I shine a light at the screen I can see some text)

The problem started a few days ago. The screen became a lot darker, then a bit later it became pitch black.

I'm not sure what the problem could be. I think it could be a fuse on the motherboard, the lcd cable, the laptop hinge LCD switch, or something else entirely.

Any idea on what the problem could be?
 

Pentium4User

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Jun 17, 2016
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I would say fuse, since they don't use inverters anymore, but first check out the cable see if there is an visual damage to the pins or cable it self, if not the I'm pretty sure it the fuse.

 

merudo

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Feb 20, 2018
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I can't seem to find the fuse anywhere - the only fuse I found was for the keyboard backlight (FS1).

Maybe there is no such fuse for my 7559? In which case, how come I don't get LCD backlight? (HDMI external monitor works great, and the LCD screen shows the text but without a backlight to make it visible).
 

Pentium4User

Commendable
Jun 17, 2016
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Sorry I haven't responded I been busy lately, I'm pretty sure most laptops now days have fusees, but also the only other way you can track down the problem is voltage testing the pins that provide power to the backlight and seeing if it is providing enough power or is not even having power. This processes tho is more difficult since it not as easy to get schematics for this board and without it very near impossible unless you have loads of time. And are you sure the connector to the board and screen and cable are good?
 

merudo

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Feb 20, 2018
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I've found the schematics (available here: http://s000.tinyupload.com/?file_id=02860262227962922751) and it seems mosfet Q7 is broken. I'll try to replace the piece today and give you an update.

EDIT: Replacing the mosfet fixed the problem!!!
 

Pentium4User

Commendable
Jun 17, 2016
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Alright well done, best answer chosen.

 
Jun 10, 2019
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I've found the schematics (available here: http://s000.tinyupload.com/?file_id=02860262227962922751) and it seems mosfet Q7 is broken. I'll try to replace the piece today and give you an update.

EDIT: Replacing the mosfet fixed the problem!!!
Greetings Merudo, my laptop dell inspiron 15 7559 it happened the same as yours, I saw your comments on this page and another, but even with that I could not resolve, because I do not know what the MOSFET transistor exactly on the motherboard , if you have some images available and some way to guide what I should do, I would thank you brother.
 
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