Oberheim Matrix 6 Synthesizer Repair Service. The problem with this device was that although it turned on, there was no sound and the display either showed nothing or only strange characters. The reason for this was, among other things, a lot of bad soldered connections around the power supply unit and on the display board.
But even after refreshing the soldered joints, there were still problems. I found out that many electrolytic capacitors had a bad ESR value and / or significantly deviated from the manufacturer's specifications in terms of capacity. I then renewed them. The last problem was sound problems. These could be remedied by re-playing the factory sounds.