I have probably hijacked this thread, sorry
Just a little update, I have fired it up and tried it out a bit. I have just tried the headphone out.
There seems to be some issues for sure. It has some issues with voices being played at different velocity or not at all, but split, poly and mono mode seems to be working besides these issues. I have managed to find a service manual with item descriptions and schematics, but i think i will make an own thread about this so this one can stay on topic.
Please don't rip apart any valuables for my sake yet, I'll have to verify the culprit

A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort.