Sony VAIO VPCF13M1E CCFL Backlight Control

Out of blue the display turned "dark". After looking into the issue I concluded that the inverter, ccfl, display and the display cable are all ok so I turned my attention to the motherboard. I came across the fact that at CN5 connector the inverter enable signal (pin 3) is 0.14V and the brightness adjustment signal (pin 4) stays high at 3.3V disregarding the brightness settings in Windows. On a sound system the enable signal should be about 3.3V while the brightness adjustment signal would vary with the brightness settings. The display off signal gets through though (sort of) as the brightness adjustment signal switches on and off with the display off signal as it should.
On the schematics of M932 (MBX-235)motherboard apparently these signals go via a quad AND gate chipset (see U89 that is 74LVC08APW). I wonder if that chipset went bad so I'm contemplating the idea of replacing it. Nevertheless, before starting this tedious job I though I would check with you guys if any of you encountered a similar behavior of the backlight control signals and replacing that chipset solved the issue.
P.S. I asked Sony support but so far they gave me only the usual crap (reinstall drivers, reinstall OS to factory, etc.). Once you get those guys out of their comfort zone you are toasted.

 

Never mind guys. Just in case someone will face a similar issue in the future, I went ahead with replacing the 74LVC08APW chipset with a brand new one. It worked.