Today's project is attempting to diagnose and hopefully repair a Vizio LCD TV, a small one that's maybe 22" diagonal. The thing powers up with the little lights on the invisible buttons turning on (I'd like to bitch slap whoever is designing TVs with invisible touch buttons so that you have no idea where they are) and they respond to being touched, but the backlight and the LCD don't turn on. With a flashlight on the screen I can see the LCD isn't doing anything at all, unlike a screen where just the backlight went out.
Nothing looks popped and I don't smell any burning, so visually everything looks fine. I checked all the voltages on the power supply board where it leads to the main board, and everything checks out except for the LED ON/OFF line (there's also a Dim line, sitting at 0V until shutdown). On standby that line sits at about 0.1V. On powerup it jiggles just a hair above that level. On shutdown, after several seconds it slowly ramps up to about 0.5V and then slowly ramps back down. The flat flex lines to the LED backlight come directly out of the power supply board instead of the main board (and they are seated tight). The Dim line stays right at 0V until shutdown, when after several seconds, about the time the LED On/Off line ramps up and back down, it has a split second spike up to about 1.25V and then right back to 0V.
Should the voltage signal for the LED On/Off line come from the power supply to the main board, or from the main board back to the power supply to signal the supply to turn on the backlight? And would the Dim line come into play in all this? Also, any idea why the LCD wouldn't turn on even if the backlight is inoperative? I just need some pointers for what to look into because my past success has always been more with CRT TVs.