Tuning a Poly61

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cineman
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Tuning a Poly61

Post by cineman » Fri Jan 04, 2008 11:54 pm

Hello all. I bought today a Korg Poly61 for 60$. It is working but it is not tuned, when i get it he was losing a semitone each octave. I opened it and tried to regulate the trimmer wich affected the VCOs and i managed to have it quite tuned but it was an hard work, and it is still not good.
I have this trimmer that affects the VCOs: trimmer LOW, MIDLE, HIGH, BEND and the Main +15volts trimmer.
Has someone a procedure, a service manual or some info for tuning it?
Thank you all.
Andrea

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Post by plus321 » Sat Jan 05, 2008 2:18 am

http://www.mik-music.org/DIY/Poly-61-Repair/

The site above seems like a good resource for Poly-61 information, but I didn't see a tuning procedure outlined.

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OriginalJambo
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Post by OriginalJambo » Sat Jan 05, 2008 3:50 am

Doesn't the Poly-61 have DCOs?

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caligari
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Post by caligari » Sat Jan 05, 2008 5:15 am

Yes it is DCOs .

The manual says nothing about needing to tune it aside form the tune knob on the front panel to match other keys you use with it.

On page 6 it does have this weird little note though.

"About Tuning:
To provide the richest most natural sounds, the Poly-61 uses an 'acoustic' tuning system. As a result, there will be a very slight 'beating' effect between notes played at an octave apart. This is not a malfunction."

So apparently, the dcos will never be in tune if they are played an octave apart? Sounds like a malfunction to me. Cheaper to detail it as a "bonus feature" than to fix the problem?

Also audio files of all the preset data which can be loaded via tape load and the manuals online. If you can't let me know and I'll dig them up and post for you.
Current Rig : Mac G5, MOTU 828 mkII, Moog Minimoog, Arp Axxe, Korg Poly 61m, Korg Poly-800, Korg Microkorg, Ensoniq ESQ 1, SQ-80, EPS sampler, DX-27, DX-7, Rx-5 drum machine, Alesis HR-16, Boss Dr-55, Synare, PAIA 9700 system, Roland MKS-100,

cineman
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Post by cineman » Sat Jan 05, 2008 2:00 pm

thank you for the repair page.
Yes, it has 2 DCO but it still have analog VCO controlled with voltage: there are trimmer to regulate how much.
I found the Poly61 Schematics: this is the analog part and voices with the trimmer i spoke about, the trimmer near the red arrows.
It is a big image, 1.2mb.

Image

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Post by rjd2 » Sat Jan 05, 2008 4:15 pm

this is just a guess(i hsould know this as i have one) but......

on the image, i see "bend", and then "low" and "high"-surely these are calibrations for the hi and low range of the pitch bend, right? i have seen that on other DCO's. i have not seen individual voice tuning pots on a DCO. (i HAVE seen pitch reference pots for the ENTIRE bank of dco 1 and dco 2 before.)

assuming that, then "middle" could possibly be RELATED to the bend, not the actual voices.

cineman-are you 100% sure that as you cycle through the 6 voices(i key at a time) that they are not the same pitch at the SAME key on the manual? check this with a tuner if your ears dont blatantly tell you otherwise. let us know.

cineman
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Post by cineman » Sat Jan 05, 2008 4:36 pm

i tought so that that trimmer regulate only the pitch bend max and min but if i touch them i can regulate all the synth tune, also without touching the bend knob. maybe there is some problem with the DCO controlling...mmm i just realized that the DCO 2 is not working :( I dont have the output from the 8255 chip...reading the help page posted.
I'm not SO expert in synth lol but im quite good in electronic...maybe i'll make a little video and see you what i have with the synth ^^

cineman
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Post by cineman » Sat Jan 05, 2008 5:14 pm

Here the complete schematics of the Korg Poly 61 im using...

Image Image Image Image

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Post by rjd2 » Sat Jan 05, 2008 5:35 pm

well, if "middle" is the middle of the pitch bend, then it WOULD change the whole pitch of the synth without the bend in effect.

if you can do that test i said, it would help to ensure that ALL voices are changing.

next, if you have already turned the trim pots, see if all the MIDDLE, LOW, AND HIGH pots affect pitch WITHOUT the bend in effect, or if its just the MIDDLE.

cineman
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Post by cineman » Sat Jan 05, 2008 8:39 pm

i have played with that the trimmers. All the trimmers have effects in every time. The pitch bend knob have another board a part with another two trimmers, wich regulate the max and min level of the pitch like you said.
The trimmer LOW MIDDLE and HIGH are a thing a part i think, not related with the pitch knob: i played with it and they do the following... the LOW work more on the tune of the Low octaves...the middle on the middle octaves and the High on the high octaves. I played with them a lot and with the help of a frequency counter software for the pc audio card i managed to have the synth a bit more tuned. But i still have the doubt that something is not working: the DCO2 im sure, is not working :( i will try to replace the 8255 controller chip... also im not SO sure that the digital control wich keep the synth in tune is working: in the 6 voices, if a play like you said the same note 6 time, i have a pair of voices a bit different than the other, not much to hear but i can see on the software.
and sorry if im not that clear :D im italian

cineman
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Post by cineman » Sat Jan 05, 2008 9:15 pm

well, thinking about it, surely the Digital control is not working, if i can tune all the synth with triggers on the CV. Maybe i dont have the Sync signal delivered to the VCOs from the CPU... now i check ^^

cineman
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Post by cineman » Sat Jan 05, 2008 11:58 pm

ok... i found that my clock board is not working correctly, i dont have clock 1 and clock2... i tried to change the capacitor on the flip flip oscillator (Q5 and Q6 on the poly61a schem) for try and i have a clock now but very little, on the Oscilloscope. If i put my fingers on the board maybe making some particolar resistance, sometimes the clock is enoght to reach the I/O chips and the DCO2 begin to work ( i can see his waveform and hear it on his chip, and the DCO2 control on the menu works on it) but only sometimes when i put my fingers... i checked every connection and tested the transistor and it seems all good... what a pain... lol

cineman
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Post by cineman » Sun Jan 06, 2008 1:25 am

BINGO !!!

i became mad all the evenining with that board... i checked every connections, components etc... but i couldnt have a good clock signal.
In the end i changed a capacitor, C16 on the poly61a schematic... is a capacitor that bring the clock signal to a transistor buffer for the logic. Normally a capacitor is difficult to broke but...ta da... all is working :D
Maybe with the year the capacitor lost their diaelectric...
Now the DCO2 is working and the TUNE is correct! the trimmer low middle and high now dont affect more the tune, because the logic do it, they work only for setting the limit of the octave...
what a madness... but hey :D im happy for the affair, i have now a poly 61 working lol

cineman
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Post by cineman » Sun Jan 06, 2008 1:41 am

lol and thank you for the help, without the page you suggested me about repair i couldnt have some of the idea i had to try :D

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Post by cineman » Sun Jan 06, 2008 1:54 am

Say hello to the new re-born Korg Poly61 :D :D :roll:

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