Discussion about the Korg PolySix synthesizer

Next update:

Recapped most of the power supply (except the two big ones) - no audible
improvement.

Removed a few LEDs from the programmer board (for example, the "Hold" LED).
Consequently when pressing "Hold" now there is no change in pitch. Touching
the solder pads with the LED while "Hold" is active results again in a very
slight pitch increase.

So I tried to use low-power LEDs. This seems to have very little effect.

Power and ground distribution: Between the power supply and the arpeggiator
board (part of KLM369) I have a voltage drop of about 12mV. Between the
power supply and the connection on KLM369 I have a voltage drop of about
7mV. Which means that on KLM369 alone I have a 5mV voltage drop. This drop
seems to appear along only one of the distribution traces.
It's for me somewhat interesting to see that KLM369 is the only board not
getting a direct power feed from the power supply; KLM369 gets its power
via KLM367.
I've removed the +5V supply from KLM369 completely; this still didn't get
rid of the random "trembling".

I also wonder what the additional ground connection from KLM367 to KLM370
is good for. Attaching it to KLM367 lowers the pitch by roughly 2 cents,
attaching it to the "ground" test point of KLM366 lowers the pitch by
roughly 1 cent. Attaching it to the ground test point on the power supply
has no result, similarly attaching it to the ground connections point of
KLM369 (from which KLM370 theoretically should be getting its ground
anyway).
Continuity from power supply to KLM370 is there, also without the
additional connection. Is it just me, or is that ground connection
theoretically forming a (very small) loop?

Of course all measurements may be off a bit since we're dealing with such
small values here.