Act Fancer - Data East 1989 (repair log)

This board was given to me as faulty in a deal.

Upon power up game was stuck on a garbage screen:

I quickly noticed work RAMs were shiny ones (known to have a high failure rate) but I dumped the 3 program ROMs first: they matched the Japanese rev.1 romset in MAME. Probing the RAMs didn't revealed anything weird but by using the oscilloscope I could "see" main CPU was sending data cyclically. Cleary no valid code was executed and CPU was stuck in a loop. I pulled both work RAMs and both tested bad on my programmer. After replacement game booted mostly fine, with sound, texts and background but sprites were replaced by big misplaced blocks of garbage:

Again I pulled and dumped the 8 sprites ROMs but they all matched. RAMs didn't show anything weird either so I started probing the TTL around and noticed one of the inputs of the LS10 @J4 was stuck low. Being a triple NAND gate it meant the associated output was also stuck (high this time = NAND). I tracked the signal to the Q output of one of the latches of the LS74 @J1. All inputs were correct with activity where it should be but outputs Q and /Q were just stuck. Piggybacking a new LS74 chip on top of the suspected faulty one cleared the issue. I replaced it and sprites were fully back:

Game fixed.