I bought this game declared 100% working for my collection but as soon as I fired it up I noticed something strange:

One of the mountain layer looked very strange and there was also some stripes of mountains in the sky

This game has no schematics and it is a particular complex and populated hardware with lots of TTLS and rams.

The game is on a 3 layers pcb so I had to find a way to test the components in a confortable way:

The upper board with the connector has the cpu and sound section, therefore I started to test the bottom pcbs and by shorting some signals I finally found the circuit dedicated to the last parallax layer which is near eproms 19 and 20.

All the signals looked good but the 2114 srams has some dead signals coming to 4 addresses.

Tracing back I found a Texas instruments TTL  74ls157@9C   whose outputs were all totally dead

Changing it fixed completely the background layer: