LED 104 is your flashlamp power, check fuse 107 and connector J128.
This shouldnt cause the other problems your describing. Are Leds 100, 101, 102 and 103 lit? 101 is 5v dc, 102 18vdc, 103 is 12 VDC unregulated and 100 is 12 v regulated. Then report back
Also when you said LED 104 is off, did you have the coin door open? If so shut it and tell us if it comes on as the coin door opens the high voltage soliniod power which includes the Flash lamps. As such if you open the coin door LED104 will go out once the capacitor discharges.
