I'm beginning to think you have a problem with the DM module, which is the large metal box on the left side of the TV when seen from the back. It also has the HD tuner. This module has been the source of many oddball problems on this set.
You say you get sound when you power the set up, so obviously you're receiveing a station.
What I want you to try is to unplug connector VA on the signal board. The signal board is the large board on the right as you're viewing it from the front access hole. The VA connector should be toward the front edge of the board, pretty much in the middle, and be perpendicular to the front of the board. Once you've unplugged that, turen the set on and see if you get video from the source you are tuned to. You won't get any on screen menu or graphics, but at least you should get a picture, assuming the signal isn't coming from the set's HD tuner, which is in the suspect DM module.
If you don't have success with that, put connector VA back where it should be. Turn the set on, and carefully press MENU - 0 - 1 - 5 - 9 on the remote. What that's supposed to do is bring up a green crosshatch grid on screen. This grid is generated in the convergence module while the normal menu graphics come from the DM module. If that works, press the HOME button on the remote, and turn it off.
Try that, let me know how it goes, and thanks for the Accept, I really appreciate it!