HDTV picture on one TV, but not the other HDTV?

Have you tried to reset the display settings? I'm not sure if that's what you mean by setting defauly/optimal or a console reset.

This is very strange. One thing it could be is a display setting on the xbox which isn't supported by your living room TV. It's unlikely but if your living room tv is one of the earlier HDTVs it's a possibility. There's a few things you can try which will narrow down the source of the problem:

1. Try using your laptop on your living room tv using the HDMI cable and see if you get the same problems as with the xbox. If you still get no signal then you can narrow it down to the TV itself.

2. Plug the xbox back into your bedroom tv and see if you still get a picture. If not then there's something wrong with the xbox.

3. If you can get the xbox to display in the bedroom, write down the current visual settings. Then go through the tech specs of both tvs and see if there's anything that either tv supports that the other doesn't.

Hope this helps!

[EDIT] Oh, forgot to mention something else it could be. I've had trouble with my PS3 in the past getting it to talk to the tv over HDMI, giving the "no signal" error. It's usually fixed by restarting the console several times, eventually it works. Try this first before everything else, could save you some time and effort.

To Miss Portia: Yes, I've tried resetting the display settings, to no avail.

To Braunman: Thanks for the suggestions, unfortunately no solutions yet. I'm going to try with a new HDMI cable. It's as if the XBOX display settings won't update. But we'll see.

Let us know if the new cable helps :)