ok, i will try to help?
when i have had these no video problems with dreamcasts in the past...
almost everytime it was from using the wrong screws on the back of the console.
the screws need to be the shorter ones in the back of the console, if not they will make contact with the mobo.
this might be the problem?
open the console up, and remove all of the screws on the back, especially those by the video port.
then try powering on the dc.