Discussion about the Korg PolySix synthesizer

Another update:

After swapping out all new MC4458L's for the old opamps from the damaged
board I did not notice a strong improvement but perhaps a bit.
However, very careful listening to the signal showed that there actually
was a pattern; it wasn't random (at least not fully). Long story short:
It's neither the LFO nor the PWM LFO, it's the arpeggiator!
Right now it appears as if the instability is caused possibly by a
combination of the LFO and the arpeggiator; or more precisely: the two LEDs
(with the arpeggiator clearly being the main culprit).
What I'm not sure of is: Is this the fault of the power supply; or are the
power-hungry LEDs to blame (or are possibly the caps near the LEDs dried
out); or is the whole power distribution problematic? I've removed the
arpeggiator LED for now on one machine and have a much more stable tuning;
the slight pulse is gone.
This behaviour was also verified on a second Polysix with both old board a
clone board.

If you want to try this for yourself here's the setup: After the Polysix is
sufficiently warmed up connect it to a precision tuner. Turn arpeggio speed
way down. Play a single note and use the master tune knob to get a precise
reading. Now turn arpeggio speed to max (so the LED is always on). On my
machine this results in a very slight pitch increase, roughly 1 cent. With
a strobe tuner it is clearly visible since the strobe starts to move; and
it stops again when turning the arpeggio rate down.
I find it easier to hear with a square wave.

I'd be interested to hear if other Polysix owners can reproduce this.
You'll need a very precise tuner for this, though.