Discussion about the Korg PolySix synthesizer

Another idea:

They built in three TPs that are only useful for calibrating the antilog amp.
TP4 first caught my eye. This is -10V.

With the Trident you were required to attach the multimeter ground point to
a -11V point.

All the following measurements are done on a new production voice board.

I checked what happens when connecting P17 to DB7 with a diode.

The voltage at TP2 went

16': -8.62
8': -11.22
4': -13.33

Now, TP2 is conveniently labelled "Oct/V CV" while TP3 is labelled "Hz/V CV".

Voltage at TP3 is:

16': -9.94
8': -8.75
4': +4.20

Now, if we connect the multimeter ground probe to the aforementioned TP4 we
get (more or less as expected):

16': +0.03
8': +1.23
4': +14.19

That this doesn't up a 100% with the TP3 voltages may have something to do
with the voltage on TP4 not being 10.00V.

Anyway; I think somewhere around this must be the crucial point for the
calibration.

It should also be noted that the voltage at the "OCT/V CV" TP2 is NOT
1V/Oct. It's pretty precisely 0.5V/Oct. This is also visible in the service
manual on the page for the old production key assigner.

I have the feeling we're getting closer.

Perhaps the factory used a "dummy plug" with the diode connecting P17 and
DB7 instead of the keyboard for calibrating the antilog amp. Given the fact
that they also had another dummy plug with the 56k resistor for calibrating
the reset circuit I find this not unlikely.