Turns out I don't have a 1A, I have another 1-slot. However, the DAC is still principally the same. At the very least, you may be able to diagnose it without any other equipment.
If you have a multimeter, test the resistance of the 220 ohm resistor near the JAMMA edge, which is part of the DAC. There will be three of them, as part of a large bank of resistors, one per R/G/B channel. For the MV-1A, these are surface mount resistors on the bottom of the PCB, in neat little groups. The 220 and 470 ohm resistors are the ones of interest. The '273 D latches are on the top-side, and will be near at least one 74LS06. You can move the two '273s into each other's places and see if the problem moves as well.
Using an LED and a resistor, you can see if the '273's input lines are toggling, as they should be on the color test screen. It's a makeshift logic probe.
This is how the D latches are arranged, and the corresponding input pins for each signal as on my MV-1FZ, with the JAMMA edge facing down. The MV-1A is quite similar so this may help you:
On a picture of the MV-1A I found it looks like there is a 74LS05 on the right side of the latches, rather than the rest. I can't make any more claims until I've actually used an MV-1A board.
