Analogue Solutions Nyborg-12 synthesizer repair service. Rather unusual for me, as I usually only repair classic vintage gear. This synthesizer had problems with the oscillators. It sounded like very slow vibrations, if anything. So I had the suspicion that the OSC didn't receive any control voltage.
And I was right in my assumption. The missing control voltage could be traced back to the A/D converter without any circuit diagrams or service manual. On closer inspection under the microscope, I was able to discover the cold solder joints. Probably due to bad connections with lead-free solder again, I simply re-soldered the IC and lo and behold, the Nyborg came to life.