Vintage Synth Repair

The ARP Pro Soloist oscillator control continued:

When looking at the schematic it becomes clear that the octave switching is
actually happening in two places:

- In the DAC there is just a "correction voltage" generated. The output of
the DAC actually "restarts" for every octave. So the CV coming directly out
of the DAC is not continuous for the whole keyboard.

- The second octave switching happens in the Frequency to Voltage
convertor. This one comes AFTER the Master oscillator but (surprisingly)
controls it with a kind of feedback loop.

Given the nature of the error I would look there, specifically Z1, Z3 and
Z4. The follow through Z22 up to the differential amplifier at Z23.

Please keep us updated. I've always been fascinated by the way those two
ARP synths were designed; and this gives me a good excuse to learn a bit
more about this design.