I'm still studying this...
On the schematic for KLM-366 in the Service Manual, I'm assuming that "SW1" is the switch shown near IC18 that, when closed, jumps around R26. My question is, which position of the switch corresponds to what state?
If the switch is up ("normal" tuning?), does that mean that the switch is open or closed? Conversely, if the switch is down ("stretched" tuning?) is it open or closed?
Then, analyzing the circuit a little bit, I'm not quite sure how this switch affects the tuning of the instrument. Sure, if the switch shorts across R25, it'll change the DC voltage at the "+" input of IC18 a tiny bit...it'll be -6.9880V when shorted and -7.0012V when open...which is a difference of ~0.042 volts, which at 0.5 V/octave, is about 32 cents. But this isn't a CV for pitch. This is something else.
This signal (after differencing with the -7.0V highest "C" coming out of x6 of IC31) is fed to transistor Q3, which controls the current in octocoupler PC1, which affects (somehow) the exponential converter embodied by Q12 that converts the volt/octave scaling to Hz/volt scaling that is ultimately used by each voice.
Does anyone understand what is happening here? Clearly, this is not only implementing the stretch tuning, but must also be tied up with the pitch correction feedback to improve pitch stability. Can anyone explain how this works?
Thanks!
Chip
--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
> >>> From Johannes
> If you are satisfied with the precision set SW1 to the "Stretch
> Tuning" position (towards CN12). Now the tuning is set intentionally
> off a few cent over the whole range, you can check this with the
> tuner. This adds richness to the sound as the 6 voices start (even
> more) beating.
> >>>
>
> This is so illuminating! I was always confused by the part of the tuning procedure involving SW1 because (a) I couldn't figure out what the switch was doing, (b) I didn't know what the manual meant by "stretch tuning", and (c) the manual tells you to move SW1 "away from VR1" which is not possible since VR1 is to the ∗left∗ of this up-and-down switch. So confusing!
>
> Luckily, Johannes saves me from my ignorance. Again. Thank you!
>
> Chip
>