Fortunately with those older boards, schematics are readily available as they came in the game manuals. Grab the schematic and start backtracing from the problem point (in this case, no video - also check sync) until you find the problem (in this case, that would be a place that has video on the "input" but not the "output"). If you're not familiar with electronics, it's kinda tough, and you'll need some equipment you might not have (logic probe, true RMS meter is handy, o'scope is handy, etc.).