Ghosts’n Goblins repair log #2

Got for repair from USA this faulty original Ghosts’n Goblins PCB, a two stack made of a CPU board :

And a VIDEO board:

Both boards were very dirty especially on solder side:

Some areas were corroded too:

Before applying power I washed both boards and neutralized the corrosion with some vinegar.On smoke test the game booted up, it was playable with sound too but with noticeable graphical issues :

I polished the contacts inside the connectors of the ribbon cables and the big vertical bars disappeared but backgrounds were kinda pixelated :

The circuit which generates the tiles lies on VIDEO board and it’s labeled as “SCROLL” in schematics (last three pages of PDF).Checking it with a logic probe I quickly found a 74LS273 @6A with a dead output:

The scope confirmed the output (pin 15) was floating while input (pin 14) was toggling:

The signal from this dead output is called ‘SVFLIP’ in snippets of above schematics and it’s routed to the inputs of a 74LS86 @5B which outputs some address lines for the backgounds ROMs:

Probing the 74LS86 @5B with my HP10529A logic comparator revealed troubles on output pin 3 and 6:

I removed it along with the 74LS273 @6A :

Both chips failed the out-of-circuit testing :

I installed sockets with good chips :

Backgounds were so restored and board 100% fixed.End of job.