Rainbow Islands (original and Extra version) double repair log

Some days ago I received for repair a couple of Rainbow Islands PCBs, they were the original and Extra version.The game needs no introduction, it's the Taito masterpiece (released in 1987) that built a generation for video games.

The Extra version PCB was the first one I troubleshooted :

The board played absolutely fine but the sound was faint and corrupted :

Inspecting the board I noticed an electrolytic capacitor was installed in series between one output of the 'TL074' OP-AMP and the input of the potentiometer :

This was clearly a poor attempt to workaround the fault, the board indeed became totally mute if I removed this kludge.Using my audio probe I figured out the sound was correctly generated by the analog output of the 'YM3012' DAC:

But then it got worse before reaching the main amplfier.In the middle there are two OP-AMPs (a quad 'TL074' and a dual 'uPC4556') responsible of pre-amplifying and mixing the signals :

The 'TL074' is a well known to be an unreliable part, I have encountered tons of bad ones in all these years of repairs so I went straight to remove and replace it :

This improved things a little, now I could hear clear sound but it was still faint also at highest volume sting.So the other OP-AMP was most likely faulty too :

Removed and replaced with an equivalent part:

 

This brought back a lound and clear sound and fixed board completely :

 

The Rainbow Islands original version PCB was in good shape :

But it booted to an 'OBJECT RAM ERROR" message :

The object/sprite RAMs are four 2K x 8-bit devices (addressed by the near PGA sprite generator 'PC090' which process their data)

Probing the RAMs with a scope revealed unhealthy data signals on the one @IC50, here is below a comparison with a good signal where you can see the correct data transaction on the left snaphot :

I pulled the chip, it failed the out of circuit testing :

Fitted a socket and a good RAM chip :

Board booted into game and it was fully playable with no other issue.

End of (double) repair job.