So, you are using a HDMI cable to connect the mobo's HDMI output to a HDMI input of the TV.
1) Is this cable the same cable that is known to work? Do you use this same cable to connect to the smaller 21'' monitor's HDMI input and it works?
2) Try all four HDMI inputs of the TV
3) Make sure turning on the TV first before
booting up the PC (so that the PC has a higher chance to properly detect the turned on TV.
4) When you connect to, say, HDMI 1 input of the TV, rename
the input from 'HDMI 1' to 'HDMI 1 – PC' or 'HDMI 1 – Computer' or 'PC'...etc
5) Your TV manual (page 28 - see pic 1 below) suggests to use 1/8'' mini plug stereo connector for audio signal, even when you are using HDMI.
This is a little odd because HDMI is supposed to carry audio as well. But as the manual does specify this, I would suggest you to try it. It should be plugged into 'AUDIO' circled red in the pic 2 below.
6) With current technology, PC is still having a hard time to have full compatibility with HDTV, especially via HDMI.
So, try to get away from HDMI as much as possible. Try to use a DVI-to-HDMI cable to connect a DVI output of the mobo to a HDMI input of the TV.
And, it is for this reason why the HDTV provides a RGB input for PC connection, to allow maximum compatibility. You will have to use this method as the last resort.Edited by windfire - 8/17/11 at 9:05am