Keep holding volume up for about 20 seconds (this should be a lot faster with bootloader locked) then release volume up. Wait for another 10 seconds to be safe. (If you hear it connecting to computer it's not actually done booting to eRecovery yet) Now, click on Volume down twice, then power. Device should now shut down. If there are no images to be seen, then either way as a DIY repair all you can do is to temporarily replace the LCD screen and check what happens and go from there. If still no good it is on the systemboard. You would need to find the schematics for the systemboard to locate the backlight power supply circuit. 16vW.