Besides checking voltages, unplug the edge connector but leave the two molex connectors plugged in, and then power it back up. The two molexes are power and video. See if you get a good picture (even though you can't play).
I recently had a Galaga board that gave a good picture with the edge connector off, but an image like yours when the edge connector was on. Turns out that a resistor pack near the ribbon cable had bent over and some of the pins had broken off. Can't recall which board and am too lazy to go find it now...