Lenovo V14-ADA - No Backlight

[RetroComputingGrotto]

I've got a Lenovo V14-ADA laptop with no backlight. I have changed the whole screen assembly from a known working one in our Spares and Repairs tub but to no avail as there is still no backlight on the screen.

Using a DMM I have tried to find the 20V power rail (it's a Lenovo so they tend to use 20V rather than the more normal 19V) on the screen connector but on pins 1 & 2 I'm getting 0V.



I have tested the continuity of the fuse (F2) and it is open circuit.


Is there anything I should be testing a this point or is it just a case of replacing the fuse?

Thanks.

[RetroComputingGrotto]

Re: Lenovo V14-ADA - No Backlight

I've just removed the fuse (F2) and shorted the connection out with a tiny bit of solder...

I reduced my bench power supply to just a few milliamps and when connecting the power my linear bench power supply immediately shut the amps down so there is definitely something still wrong with the motherboard causing this fault.

Using my (very) cheap thermal imaging camera I have connected to my iPhone I couldnt see anything getting warm/hot to indicate what might be at fault.

I have now removed the solder I used instead of a fuse (purely for fault-finding purposes) so I'm back to square one of what to check/look for next?


Although I've been fixing computers for many years (and studied electronics at college in the UK 20+ years ago) I've never attempted to repair a motherboard at component level until recently so an help/pointers on what to check next is greatly appreciated!

[SMDFlea]

Re: Lenovo V14-ADA - No Backlight

Is there a crack across that large capacitor or is it just flux or something. Is the motherboard model number NM-D151 ? ,if it is there`s a schematic and boardview https://www.badcaps.net/forum/showthread.php?t=99843

[RetroComputingGrotto]

Re: Lenovo V14-ADA - No Backlight

I think its just dirt (I could be wrong) - when doing a continuity on the capacitor it's not showing as being shorted.

[mcplslg123]

Re: Lenovo V14-ADA - No Backlight

Resistance to GND on F2(side showing 0 Volt). the fuse should be removed before measurement. This measurement should be repeated twice:
01.With Edp cable inserted
02.Without edp cable inserted.

[RetroComputingGrotto]

Re: Lenovo V14-ADA - No Backlight

Resistance to GND on F2(side showing 0 Volt). the fuse should be removed before measurement. This measurement should be repeated twice:
01.With Edp cable inserted
02.Without edp cable inserted.

Both with and without the screen connector connected I get infinity resistance so it doesn't look like it is shorted to ground.

I have also removed the capacitor and tested the resistance on my multimeter and I could see it slowly increasing it's resistance up to infinity so I believe that suggests that the capacitor is OK.

I have since found a donor board from another laptop that had a damaged case and no screen (screen used to replace a cracked one on another laptop) and when using the donor motherboard the screen (and backlight) work correctly.

[mcplslg123]

Re: Lenovo V14-ADA - No Backlight

What scale on meter you're using? use 200K scale if you wre using a low sclae.