Incorrect SYNC on Pacman

Screen filled with coloured lines, unstable, no sync (see above) – check clock signals from horizontal and vertical scan generators (74161’s at 2R,2S,3R,3S) – no signal feeding into pin 2 of horizontal generators. Follow trace back to 7474 at 3N, signal going in, nothing coming out – replace chip and screen display back to normal.

Game basically plays OK, but 4 characters on lower blue wall of maze were incorrect, even through ram tested out OK. Problem was more noticeable when screen flipped for player 2. Using the fluke I identified that the first of the bad characters was memory location $43BF. Writing to $4FBF then showed a character on screen, revealing that the memory select was wrong for these few positions. Checking the memory decode with the comparator showed a problem with pins 9 and 11. Replacing this chip solved the problem.

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