W500 No backlight, Fuse and inverter OK

[kitor]

Just bought W500 (HD3650 version) with CCFL 1680x1050 LCD. It works OK, but there's no LCD backlight.

First thing I checked was F7 fuse, it's OK. 20V is also getting to the inverter board.
I connected W500 backlight to working T60 and turned both on - it works.
T60 is 15.1" model, has different inverter that happens to be pin and from-factor compatibile with W500 15.4 - I swapped them - W500 inverter in T60 works, working one from T60 in W500 - still no backlight.

I found somewhere sugestion to play with switchable graphics settings in BIOS - no luck.
Any suggestions where to look next? Is there any board schematics available?

[kitor]

Ok, for some reason BACKLIGHT_ON was broken on LCD ribbon, and even after patching it, it was still low. As a temporary fix I just bridged it on inverter to 3.3V (pin 3 and 5) and backlight works.