Ok, so I opened up the Xperia Z Ultra yesterday since it was not showing networks - I thought it might be the antenna. Anyway, while I was inside I saw a lot of white stuff covering the mainboard processor and surrounding pins - i started to clean that gently with a soft old toothbrush. Now when I put everything back together, everything works but the backlight is off - I can see the OS underneath, digitizer works fine but no backlight. I suspect I may have damaged the pins on the mainboard where the flex cable might be connecting for the LCD display - unsure how to test that. Is there a separate backlight flex cable notch that might have been detached? Would a multimeter help to measure voltage between the pins - unsure which ones I should be testing anyway. Any ideas guys? Would appreciate all the help I can get.