10 Jan 2018

Sky Soldiers - SNK 1988 (repair log)

After a first visual inspection I noticed someone previously worked on the board: 2 capacitors have been replaced and 2 resistors have been reflowed. Was it part of a previous repair or an attempt to repair the current problem? I will never know.
Anyway, I also found a cut trace I patched.

I powered the game, it was stuck on the initial screen (the one before the game starts its RAM/ROM tests):

I probed the main CPU, reset signal was toggling between high and low: again game was stuck in a reset loop, watchog was barking, no valid code was executed. With the help of my logic probe I found 4 data lines were behaving weirdly. I identified few chips on the data bus: two 8 bit RAMs (LSB/MSB), 4 ROMs, few TTLs (mainly LS245 Hitachi brand, i.e. rock solid) and two weird SIL custom chips used for inputs (29 pins). ROMs turned out to be good. I pulled the RAM connected to the 4 suspicious data lines but it was OK too. The remaining TTLs on the data bus were all Hitachi brand so I was pretty confident they were good too. I was left with the two big SIL29 custom chips marked "ALPHA INPUT 87". I pulled them, few pins were broken on each. As often I tried to run the game without them looking for any change: the game booted! Obviously I couldn't play it because of the missing controls. Quite weird the game doesn't boot with the chips in place with missing pins but plays fine with them removed. I repaired the broken pins with resistor legs and game was fully functional:

Game fixed.

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