Not so much a circuit, just a really quite handy tool for checking signals!

All you need to do is remove one of the red, green or blue wires from your JAMMA harness. You then connect that wire to a 470ohm resistor... And... That's it!!!

You can then use this video probe like a logic probe. When you put the end of the resistor onto component legs you'll get a visual representation of what's going on with the voltage/ signal from that point.

One important thing to note though is that this will only work if the sync signal coming off the board is good as that's what stabilizes the display. Another thing to note is that if you touch certain things like the clock line for the sync signal then your monitor will go a bit funny! I suppose it can't be very good for its health (judging by the noise it makes!) so probably best to use this technique as a last resort if you don't have a proper logic probe to hand.