It's the RGB/VGA selects (pins 6&7) that need to be grounded a certain way, you put in a switch so that you can either ground both for normal VGA, none for TV mode, and just pin 7 for semi-VGA mode - this is for booting non-vga games in vga mode.
So put in a switch, and then have it on TV (none grounded) then a few seconds after you here the sound of the Dreamcast swirl switch to sem-VGA (ground pin 7) then the game should work. It takes practice and sometimes some games are really buggy with it but most of the time it works great.
You'll need to open up your VGA box and figure which wire on the box side is pin 7 on the DC side and then put in the three position switch.
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