I received an NES cart with some extremely corroded pins. After cleaning them and also using some high grit sandpaper, it would run, but there were major graphics problems.
On a lark I opened up the cartridge and started checking continuity. Wouldn't you know it one of the traces near one of the pins was totally corroded through.
1 bodge wire later and it works like a charm.
The game is 'Zanac' by the way.



