Now that I have all components available, I was able to build up the three VCOs, attaching the small boards with the according connectors to the main board:
Meanwhile I had also finished the VCF part. As you can see I'm not using the built-in power unit, but will be using my own external power supply:
With those parts done, I could do some first test. I've connected the main board to my power supply and connected things to my oscilloscope:
Happily all seems fine so far. The correct waveforms are visible for the three VCOs and also at the output of the VCF. In the following images the upper yellow waveform is the VCO output and the lower blue waveform is the same signal passed through the VCF.
VCO 1, square waveform:
VCO 2, triangle waveform:
VCO 3, saw waveform:
So far thus no soldering errors, all seems to be working as it should.