Problems with Dreamcast internal VGA/RGB output

I have added an internal VGA mod to my Dreamcast but I am experiencing some problems. I have just a black screen through both the VGA socket and AV socket using RGB scart but there is still audio through both sockets. I have tried using a composite cable and I get a picture, but it has a wide black band across the bottom and the image is not fully visible at the top of the display. I have performed this mod many times before and never experienced this outcome. It must be something to do with the RGB signal?

 

Anyone? Is this too obvious and nobody can be bothered to answer?

 

This probably goes without saying, but did you verify the console was working properly with the stock cables before attempting the mod?

 

Try a 2nd TV?

 

Sounds like either bad TV or bad console. Not much more to go on at this point.