1943 Kai - Midway Kaisen repair log

Received from Austria for a 1943 Kai - Midway Kaisen PCB for repair (the game is an update of 1943 with tweaked weapons, fewer levels, and a new soundtrack released in Japan only).Set was in good shape in both CPU and VIDEO board :

 

Game was playable but with severe sprites issue :

 

Objects are generated on bottom video board by a custom IC (in PLCC84 package) marked '86S105'

This is another well known unreliable and prone to failure part (which really would need to be reproduced someday) which I had to replace many times hence I was pretty sure it was bad also on this board.I didn't have any spare but the owner of the board came to help and sent me a working one taken from a Pang PCB.The part arrived some days later :

Time to remove the old one and install the spare :

This lead to a great improvement, sprites looked much better but yet not perfect as they were still slightly glitched with vertical lines through them:

Schematics show that on VIDEO board there are two identical sprite line buffer circuits where two 2K x 8-bit static RAMs lie :

On my board they were replaced :

During removal of the RAMs some traces on solder side were broken and then repaired with some wire .

I probed the two RAMs and found that pin 16 (data line D6) of the one @7D was stuck HIGH :

According to schematics this pin should be tied to pin 17 of the 74LS273 @6E and pin 9 of the 74LS257 @5D :

I checked on the board and found that connection to pin 9 of the 74LS257 @7D was missing.I promptly run a jumper wire between the two points:

This made the trick and fixed board completely.Another successful repair.