iMac to HDMI TV.... Help

[picnic4u]

Ok, I thought I had this sorted but need some help.
I have a 2007 iMac 2.8 GHz Intel Core 2 Duo connect so I know I need mini DVI out. My TV is a Panasonic TH-42PZ plasma TV with HDMI. So I have got the Mini DVI to HDMI and a HDMI lead. Now when I connect this the Mac Flashes blue and come up with the menu for the TV and says the other display is a Panasonic TV.. great... Now the Problem... TH TV screen just stays black. I tried two different HDMI leads , tried all the various 1080i,p 720, 50hz, 60hz combinations , tried both HDMI inputs in the tv. Nothing I know I still need another lead for the sound but why is this not working , The mac clearly knows it's connected to the TV? I saw on the product spec for the model it says the output support from the mini-DVI output port with support for DVI, VGA, S-video, and composite video connections via adapter. It doesn't mention HDMI but it appears lots of others are connecting ok .... am I missing something?
PLEASE help

 

[SueG]

You need to select the source to hdmi on your tv remote

 

[picnic4u]

You mean the channel where the hdmi input is? .... come on you got to try better than that.. of course i checked I was on the correct input. My blu ray is plugged in there so I put a disc in so I knew i was on the right channel then Un plugged the HDMI and plugged in the imac ..... black screen

 

[Slow Programmer]

When you try and move the mouse pointer off the right side of your desktop does it disappear or stay visible?

 

[picnic4u]

dissappears

 

[Slow Programmer]

I am running a similar set-up on a 2009 iMac and have only had a few problems. Overall it has worked well. I am not sure if it is helpful, but if i set the resolution to 1080,720 etc I will have a black screen on the TV. Actually, if I set the resolution to anything except 1360 x 768 I have a black screen as that is the only setting that will work with my TV. It also takes as long as 15 seconds for the black screen to change when i set it from an unusable resolution to the usable resolution.

Hopefully some of this will be helpful.