Put your finger on different IC's - especially PIAs - carefully - see if any are getting abnormally hot. In addtion to U51, you might want to also check U54...that one is involved with the lamp matrix.

To test the blanking circuit for proper operation, check U20 pin 2 or U43 pin 3 at power-on, using a logic probe. You should get an initial LO for a few seconds, followed by a continuous HI after the game has booted.