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Pre-show testing found that the background graphics scrolling wasn't working, most evident on the mother ship phase where the ship was stuck in the same position. The game PCB was removed for repair.
The scroll data latch was IC 8 (LS374) on the video PCB. The chip select input was pin 11, "CS-32" that was idle high throughout attract mode. Moving upstream to IC 35 (LS32) on the CPU PCB found output pin 11 idle high, input pin 12 high with poor low pulses and input pin 13 active OK. IC 35 input pin 12 was driven by IC 33 (LS138) output pin 12, also measuring high with poor low pulses. I suspected one of IC 33 or 35 was bad. A piggy back of a replacement LS138 on IC 33 had no effect so I replaced IC 35 (LS32) that also had no effect. Replacing IC 33 (LS138) fixed the game.