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Is there a VR trimpot for overall tuning of the entire Polysix keyboard

Is there a VR trimpot for overall tuning of the entire Polysix keyboard

2017-04-24 by t_tangent2003@yahoo.co.uk

Hi again,


On my Polysix most of the voices are quite closely tuned to each other

already, but all 6 voices are offset compared to the Master Tune knob,

so I am trying to find out if there is one VR trimpot that can tune all the

voices together up or down by about half a semitone so that
the Master Tuning knob can be centered to the middle position, rather

than having to keep it at about 4 o'clock to get A4 at 440Hz. I did read

the Kiwisix manual's section on tuning, and the Polysix Service Manual
but sadly it looks as though the only way to do this is individual tuning
of each voice, and after reading about some other Polysix users difficulties

with tuning, I dont really want to risk retuning the individual voices if I dont

need to.


Thanks for any advice.


Tim


Re: [PolySix] Is there a VR trimpot for overall tuning of the entire Polysix keyboard

2017-04-24 by Bob Grieb

The PolySix has a linear DAC for note selection, and a single exponential
converter for all six voices to convert the linear DAC voltage to an exponential one.
If all six voices are off, then it could be that the exponential converter is a little off.
But it's somewhat tricky to adjust, so maybe better to just leave it as is, or adjust the
voice tunings instead. Hard to say which is off without measuring voltages for various keys.

Bob

Re: Is there a VR trimpot for overall tuning of the entire Polysix keyboard

2017-04-25 by t_tangent2003@yahoo.co.uk

Hi Bob,

Thanks for the help and detail on how it actually works. A shame its not as easy as simply adjusting a single trimpot and that's that, so I may then just leave the overall tuning for a little while unless it gets any worse, and meantime I'll just use the Master Tuning knob if needed. Thanks again. :)

Tim

Re: [PolySix] Re: Is there a VR trimpot for overall tuning of the entire Polysix keyboard

2017-04-25 by Florian Anwander

Hi

Am 25.04.2017 um 03:36 schrieb t_tangent2003@yahoo.co.uk [PolySix]:
> Thanks for the help and detail on how it actually works. A shame its
> not as easy as simply adjusting a single trimpot and that's that, so I
> may then just leave the overall tuning for a little while unless it
> gets any worse, and meantime I'll just use the Master Tuning knob if
> needed. Thanks again. :)
Just a short thought without havuing looked into the schematics: could
it be, that your pitchbender is misadjusted?

Florian

--
http://www.florian-anwander.de

Re: [PolySix] Re: Is there a VR trimpot for overall tuning of the entire Polysix keyboard

2017-04-25 by Bob Grieb

Looks like the bender pot is a special one with an extra connection from ground to
the center of the resistive element. Probably that was done to insure that if you set it in
the middle, you really get 0V of bend. Since it has that extra tap in the center, you won't be
able to simply add a resistor to pull the wheel voltage to correct your offset.

Re: [PolySix] Re: Is there a VR trimpot for overall tuning of the entire Polysix keyboard

2017-04-25 by Florian Anwander

Hi,

I now had a short look at the schematics. It might be quite simple to
add such a trimmer.
Have a look at the service manual
http://www.florian-anwander.de/korg_polysix/polysix_service_manual.pdf
On the PDF-page 9 you find the bender and the tune potentiometer in the
lower right quarter of the page close to the vertical dividing lines.
Tune is VR2 which is connected by R18 (1.8MOhm) to the summing line. You
may duplicate those two parts: a 50k trimmer between +5V and -5V with
the center pin connected via a 1 - 2MOhm resistor with to the summing line.
The most easy way should be to solder the trimmer with its two outer
pins on the solder side of the left panel pcb KLM-369 at the same
location as the tune potentiometer. The 2MOhm resistor from the center
pin of the trimmer can be wired "free flying" to the right side of R18.
A mechanically quite likely suitable trimmer should be a Piher "PT 15 NV
50K" (example:
https://www.conrad.de/de/trimmer-linear-025-w-50-k-270-piher-pt-15-nv-50k-1-st-432040.html)

Florian

Am 25.04.2017 um 14:23 schrieb t_tangent2003@yahoo.co.uk [PolySix]:
>
> Hi Florian,
>
> The bender itself is set to middle position so that should be fine. I
> will have a look at schematics and see if I can find any VR and look
> if that might need adjusting. Thanks for the idea :)
>
> Tim
>

--
http://www.florian-anwander.de

Re: [PolySix] Re: Is there a VR trimpot for overall tuning of the entire Polysix keyboard

2017-04-25 by Malte Rogacki

Completely different idea: Is the knob for the Tune Potentiometer
positioned correctly?
Since the potentiometer shaft is knurled (and not a "D" type shaft) it's
easy to push on the knob one or more "ridges" to the left or right; in this
case the apparent middle position as indicated by the knob would not be the
middle position in reality.
I've come across this occasionally (though not in this scenario).

Re: Is there a VR trimpot for overall tuning of the entire Polysix keyboard

2017-04-26 by t_tangent2003@yahoo.co.uk

I had just replied to Florian's message to say I would try out his idea when I have time, but then I saw Bob had also commented, so I have deleted my last reply and will wait a little to see if this thread develops any and if this mod can be confirmed. But thank you both for your replies and efforts and for looking into this. It would be nice if a solution could be found but if not then I guess the standard method of tuning will have to suffice :)

Thanks also Malte, I just checked again to make sure but yes the Tuning Pot is definitely in the middle position. Was worth a try though and it would have been great if it was just that, but sadly not :)

Anyway, thanks again all of you,

Tim