Discussion about the Korg PolySix synthesizer

> What does this switch actualy do?
> I see a change in freq when I switch up or down.

It puts the Polysix into "stretched tuning". By doing this the  higher
notes are tuned slightly sharp; read: the octaves are no longer perfect.
Stretched tuning comes originally from the piano and (as far as I know) has
something to do with the inharmonicities of the piano strings (some
harmonics tend to go out of tune).
The tuning procedure is done in "regular tuning". If you hold down a high
note and move the switch you'll notice that the pitch goes very slightly up
or down. If the pitch is higher you're in stretched tuning.

> I'll put then a reference signal of 440 Hz on my A-channel of the scope and
> the the B-channel will be connected to TP1 then.

This probably won't help much when you try to tune C6. I suggest using a
tuner. There are some free software tuners to be found although I can't say
anything about their quality (I'm using a hardware tuner myself).