Loading the factory patches brought the synth back to live. At least with 4 voices. Then I noticed, that I could not edit any sound. The problem was solved by replacing a dead comperator IC (pcb top, the socketed one). Besides that the fader dust cover were worn out and needed a replacement, I figuered out that the two dead voices had a problem in two dead IR3R01 envelope generator ICs. Well, the problem is to find one. A working one. At a decent price. These parts are obsolete and I had to figure out a replacement.