I used to connect my xbox 360 through a DVI adapter on an HDMI cable to a monitor, which worked fine for ages, but then one day it suddenly stopped working. I tried a couple different cables and adapters, they didn't work either. The xbox did output fine through scart to an old SD TV through the VGA cable. It will also output SD fine through SCART to a new 1080 TV.
However, if you flick the switch on the VGA cable to HD, the screen goes black, with 'no video signal', although sound carries on playing. And if you put an HDMI cable in, there's no sound or video, and it looks like the xbox isn't booting properly, and won't connect with the controller, with the green light going round in circles, although the TV does react to the signal, but again displays 'no video signal'.
I've tried resetting the display options, I've tried 3 different HDMI cables. There's no setting to change on the TV to allow HD sources or whatever, I've been through the instructions and menus with a fine tooth comb.
I'm guessing the xbox must have a fault? Any suggestions gratefully received