5.6V is too high. I'm not sure if it is the problem, but you should be able to lower the voltage on the power supply with a plastic screwdriver. The power supply will have a small hole with the 5V trimpot behind it. Most likely on the same side as the terminals . Most boards are happy at 4.8 - 5.2 Volts. You could try lowing the voltage.

Do you know if it is a Jamma board or an original Galaga board ?