The board originally failed to get into the game, instead resetting just after drawing the ‘MISSILE COMMAND’ logo. Test mode faired a little better, managing to report RAM OK, MAP OK and ROM OK before resetting. Ram and Rom checked out OK using the Fluke. Measuring various CPU signals revealed that the interrupt seemed very random, checking back to the flip-flop used at E7 (7474) was giving an incorrect signal. Replace the chip and the game fired into life.

Missile Command trails

The aiming cross left a trail, and the red arrows and cities had the same flashing coloured box around them (see picture). Some experimenting with mame revealed that when the cities were drawn it read what was behind them first, as did the moving cross so it could be assumed that the reading of video ram was wrong. However, the video ram had already checked out OK, so it had to be something that was only used for this feature. Missile Command has a couple of hardware blitters that read pixels from separate video banks, and the bank causing the problem returned 2 bits via P7 (74153), R7  (7475) and R6 (74125). P7 checked out OK, but R7 and R6 were not so easy to test – replacing both chips solved the problem.

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