Vintage Synth Repair

Hi all !

I got a Matrix 6 which has suffered some rust on its computer board, 
could be that it had some rain going over it in the past... I cleaned 
the computer board and replaced the battery. The synth boots up ok, and 
all control options and presets works just fine. Display is ok as well. 
Have followed the reset procedures for all main sections as described 
on the manual, successfuly. However, once I power it up, I hear some 
faint undefined sounds, and after a few seconds, I can hear some random 
low frequency sounds all the time, on every preset I choose.

I tried to upload it with the factory presets via the cassete interface 
but still the same results. Tried to reset it again, no change.

I suspected that there was a bad IC on the computer board Gate/Env 
section, but could not figure where it is on the schematics... the 
schematics available on the web is very poor quality.

Any ideas or suggestions ? any high quality Matrix 6 diagram ?

Thanks all for your advice !