dvi to video registers but no output.

using dvi to vga adaptor, when plugging in it changes the screen to blue briefly then you get all the options in display prefs, and the 2nd monitor/tv displays as expected.

But then when you want to use dvi to video the laptop screen changes to blue briefly again as expected, but the 2nd screen is receiving no signal, and nothing happens when when clicking on "detect displays". my local mac dealer says it is the video card (part of the mother-board @ £500) and they didnt even test anything, and the warranty ran out 1 month ago.

i only found out this problem a week ago as it was the first time i needed to use these outputs.

could anyone give me any advice or ideas on things to try.

cheers.