Fire Shark repair log

Got for repair a Fire Shark PCB, a vertically scrolling shooter arcade video game developed and originally published by Toaplan in 1990 (known in Japan as Same! Same! Same)

I powered up the board for the first time and it booted into game.I could coin up and play, sound was present too but screen was filled by garbage (mostly letters and numbers)

 

I noticed that, when I put my fingers on some pins of the 'BCU-2' custom ASIC, the screen was cleaned up from garbage although background graphics were totally absent.This custom IC is, indeed, the tilemaps generator in QFP160 package :

Looking at pinout of the 'BCU-2' found on Out Zone schematics (which runs on similar hardware) I figured out the pins I was touching where a bus that exchanges data with the near 6264 RAMs (8k x 8bit devices) 

 

 

 

Probing with a scope the data pins of the RAMs revealed weak signals :

This lead me to think that the 'BCU-2' was really faulty so I decided to replace it with a spare taken from a donor board.The chip came off quite easily using my hot air station :

 

The spare was soldered in :

Area was cleaned by flux residuals and chip was inspected with a microscope for possible bridges.All was good and ready for the test.I powered up the board again and graphics were fully restored :

I played some games with no futher issue so I could declare the board 100% working.Another sucessful repair accomplished.