Dell XPS 15 9550 no backlight

[Karavusk]

Hello,
first of all I am a total noob with board level repair. My Dell XPS 15 9550 backlight died. I tried to replace the screen but it is still not working. I found the schematics online (only for rev 0.0, I have 1.0) but no board view which means I basically can't find anything really on the board. Not to mention that I have never really used a schematic.

Either way I managed to measure the backlight voltage on the 40pin connector. It is 9mv and supposed to be 25v. Any idea what I should try out next? No matter how much I look at the schematics I don't get any further. The only other thing I managed to do is to look at QV11 pin 1,2,5 and 6 which are also 9mv and probably supposed to be 25v too.

Here are a few pictures https://cdn.discordapp.com/attachmen...825/image0.jpg

https://cdn.discordapp.com/attachmen...355/image1.jpg

The capacitor under Q11 is the only one that looks a bit weird but besides that I found nothing. Any idea what that is used for? Any advise on anything about this is welcome, I have no idea what to do next and I definitely don't have the money to get a new board.

I hope someone can help me, thanks

[Techtiger]

Re: Dell XPS 15 9550 no backlight

Test QV11 all legs for Voltage pin4 +B 19V (not 25V Volts ) pin3 8-10V it’s a mosfet , there is a voltage divider connected to pin 3 in that way the voltage on pin 4 will switch to the drain side

[Karavusk]

Re: Dell XPS 15 9550 no backlight

Test QV11 all legs for Voltage pin4 +B 19V (not 25V Volts ) pin3 8-10V it's a mosfet , there is a voltage divider connected to pin 3 in that way the voltage on pin 4 will switch to the drain side

How do I know which pin is 3 and which one is 4? Also I have just shorted one of the pins next to it to 3 or 4, no idea for just a split second. I hope that didn't make anything worse. I definitely wasn't careful enough and got a short spark between the pins I think. Screen is still on with no backlight.

One of the pins is at around 13.53v and the other ones are still at around 9mv. The other pin (again 3 or 4) is at 13.48v.

[Karavusk]

Re: Dell XPS 15 9550 no backlight

Test QV11 all legs for Voltage pin4 +B 19V (not 25V Volts ) pin3 8-10V it’s a mosfet , there is a voltage divider connected to pin 3 in that way the voltage on pin 4 will switch to the drain side

I have also managed to measure QV12. It has 2 pins on one side and one on the other side. The left pin is at 3.38v, the right pin is at probably 0mv? Sometimes it shows 0.1mv or 0.2mv and then goes back to 0. The top(or bottom?, the one that is alone) pin shows 0.4mv (constantly).

[Techtiger]

Re: Dell XPS 15 9550 no backlight

How do I know which pin is 3 and which one is 4? Also I have just shorted one of the pins next to it to 3 or 4, no idea for just a split second. I hope that didn't make anything worse. I definitely wasn't careful enough and got a short spark between the pins I think. Screen is still on with no backlight.

One of the pins is at around 13.53v and the other ones are still at around 9mv. The other pin (again 3 or 4) is at 13.48v.

Test again with out shorting pins

[Techtiger]

Re: Dell XPS 15 9550 no backlight

What voltage do you get 2 resistance 2 caps

[Techtiger]

Re: Dell XPS 15 9550 no backlight

I have also managed to measure QV12. It has 2 pins on one side and one on the other side. The left pin is at 3.38v, the right pin is at probably 0mv? Sometimes it shows 0.1mv or 0.2mv and then goes back to 0. The top(or bottom?, the one that is alone) pin shows 0.4mv (constantly).

This voltage is correct for this Fet

[Karavusk]

Re: Dell XPS 15 9550 no backlight

Test again with out shorting pins

4 is 13.6v right now and 3 is 13.55v right now. All the other pins are still at like 8.7mv.

What voltage do you get 2 resistance 2 caps

From all sides to ground?

[Karavusk]

Re: Dell XPS 15 9550 no backlight

What voltage do you get 2 resistance 2 caps

Here are all the measurements to ground https://puu.sh/EHeG6/96f5696cc6.jpg

Actually the 0v is more like 0.4mv, measured multiple times at that place just to make sure.

edit: Right resistor is 382.6Ohm, left one is 99.2kOhm

edit2: accidentally swapped the ohm values

[Techtiger]

Re: Dell XPS 15 9550 no backlight

can you confirm the measurements you have given QV11 with the battrey insert ? if so remove the battrey and test again on pin 3 4 voltage ?, RV120 Resistor or QV11 mosfet is bad test those components with Multimeter with out power QV11 is a P-channel Fet RV120 is a 100K Resistor , if you dont have QV11 replacment you can short pin 4 to 5/6 this is the last option , do a multimeter continuity test from QV11 pin 5/6 to LVDS pin 40 39 38 you should get the Buzzer sound

[Karavusk]

Re: Dell XPS 15 9550 no backlight

can you confirm the measurements you have given QV11 with the battrey insert ? if so remove the battrey and test again on pin 3 4 voltage ?, RV120 Resistor or QV11 mosfet is bad test those components with Multimeter with out power QV11 is a P-channel Fet RV120 is a 100K Resistor , if you dont have QV11 replacment you can short pin 4 to 5/6 this is the last option , do a multimeter continuity test from QV11 pin 5/6 to LVDS pin 40 39 38 you should get the Buzzer sound

I don't quite understand what you want me to do. Should I test with or without the battery? Also the left resistor is at 99.3k Ohm. That is pretty much what RV120 is supposed to be and I am fairly sure that this is RV120. I measured points with 0 Ohm resistance between them and found this https://puu.sh/EHfVc/1ecd75c6b8.jpg (all measured without power). From what I can tell the 378 Ohm resistor is supposed to be RV 119 which is supposed to be 1m Ohm?

https://puu.sh/EH4PO/88f0016834.png

https://puu.sh/EHoGY/edef2df9a6.jpg

Maybe I have the wrong schematics (my schematics is rev 0.0 while my board is rev 1.0) but it would make sense to me.

edit: I shorted pin 4 and 5 and the backlight actually turned on!!! so what now? Replace the right resistor with 1m ohm?

[Karavusk]

Re: Dell XPS 15 9550 no backlight

Ok I did it, it works now the capacitor CV168 was defect. This is what gave me the 378 Ohm. A guy took it off the board for me and now it works. Should I replace that or can I leave it off?

[Xenoaeuriix]

Re: Dell XPS 15 9550 no backlight

Ok I did it, it works now the capacitor CV168 was defect. This is what gave me the 378 Ohm. A guy took it off the board for me and now it works. Should I replace that or can I leave it off?

May I know how the capacitor looks like and where it is located. I am a total noob here. I got a replacement lcd thinking it was the lcd but when I got replacemed same issue as your still got no backlight. Mine is different do its using 30 pins edp but same model xps 15 9550. Thank you in advance.

[mon2]

Re: Dell XPS 15 9550 no backlight

What are the white silk screen markings on the logic board?

Your unit works ok other than the backlight? Working ok with an external monitor?

Perhaps you can see the lcd graphics using a phone in torch / flash light mode on the front of the display to confirm.

[mon2]

Re: Dell XPS 15 9550 no backlight

Found a similar(?) bv file that may help.

[Xenoaeuriix]

Re: Dell XPS 15 9550 no backlight

What are the white silk screen markings on the logic board

I am sorry, I am not really sure what you meant by silk screen marking. Super noob here.
__________
Your unit works ok other than the backlight? Working ok with an external monitor?

Yes everething works, I can see it booting up to my desktop and can navigate with the screen it is still there like silhouette. You just to look closely on the lcd.
__________
Perhaps you can see the lcd graphics using a phone in torch / flash light mode on the front of the display to confirm.

Yes thank you that is confirmed, you better see it with flashlight and plugging it to the monitor looks okay too.

[mon2]

Re: Dell XPS 15 9550 no backlight

Share any white text markings on your logic board. This will help to locate the exact schematics /boardview files for your unit.

[Xenoaeuriix]

Re: Dell XPS 15 9550 no backlight

Share any white text markings on your logic board. This will help to locate the exact schematics /boardview files for your unit.

Thanks for the response. Sorry for responding late here. I ended up sending the board back to the seller. the board is pretty toasted there are obvious signs of the previous repair. I did try to test out the panel on a different Laptop and confirmed that the LCD is working okay. The part where you can solder the backlight is already jumpered. Thanks for the help