Check brightness and contrast adjustments first. Probably the contrast.
Make sure the monitor's edge connector foil fingers are clean and making a good ground contact.
Also, over on the game power supply board, replace electrolytic capacitor C9. Originally it was a 1 uf @ 50 volts, but because of frequent failures, Arachnid issued a bulletin requesting the capacitor be upgraded to 2.2 uf @ 50 volts. Be sure to note the polarity before removing it. Many of the power supply boards did not have any silkscreened polarity markings on it for this capacitor!