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Korg Polysix problem Virtual Instrument Plugins
Old 30th December 2010
Here for the gear

Korg Polysix problem


Resolving a problem with my newly bought Korg Polysix. Everything on the synthesizer works as it should, but the the following thing bothers me. When I play the instrument sounds in isolation, ie. only one note is played, synth sounds as if the cutoff on the filter is slightly suppressed (though physically is not). Conversely, when I play chords (just need a two-tone at the same time), filter cutoff stops to suppress and the tone sounds as it should.

It is evident that the cutoff of the filter with separately played tones improperly suppresses the tone, whilst at any at least two notes are played simultaneously, the cutoff is working properly and do not suppress the sound. (But by talking about the filter, it do not necessarily mean the bad behavior of the filter must necessarily be associated only with the wrong settin or adjustion in the filter).

I ordered a MIDI interface for the synth, so it will go to the service soon anyway. We have already tried to find the reason for the problem described above in the service but we have not solve it. Me and the service man agreed that so far I ask the problem here on the forum and then according to any reaction and advise from you we will proceed.

I kindly ask other Korg Polysix owners and aslo owners and servicemen of other old analog synthesizers, if you know where inside the Korg the problem might lie and how to resolve it.

Here are the sound examples. Please pay attention to the bass sound.

Example No. 1 Normal saw, no suboscilator. In the beginning I play separate tones. Bass sounds like it was slightly suppressed by the cutoff of the filter (but it is not suppressed physically). Once you add another tone (the one in higher octave in this case), bass starts to sound as normal bass saw should sound.

Pedro Zakatchku - Ukázka 1 on SoundCloud - Create, record and share your sounds for free

Example No. 2 Same setup, but I use arpeggiator. This time the demonstration starts with the bass as it sounds, if an extra octave tone is played at the same time ( by means of the chord memory function), after switching to normal mode (the only one tone is played ) it is obvious that the filter turns to suppress the cutoff slightly.

Pedro Zakatchku - Ukázka 2 on SoundCloud - Create, record and share your sounds for free

I would like to highlight that the problem described above is not caused by the wrong value on the KBD track. The value on the KBD track was at zero during recording of the examples.

Thank you very much in advance for your response.
Old 30th December 2010
Lives for gear
grumphh's Avatar

I am not a tech <-- just a disclaimer.

But could it be that just the first voice is out of calibration/defect?

If the Polysix somehow always assigns "voice 1" on the board to the first key press (or possibly the highest key press) you would get an effect similar to what you describe and have recorded.

If you do the same thing as in the example clips - but only press a key maybe a fifth above the original dull note instead of several octaves above, would you get two bright notes or one dull and one bright?
Old 30th December 2010
Here for the gear

When I press lets say the fifth key above the original dull note, the both notes get immediately bright.
Old 30th December 2010
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Oli's Avatar

I can't tell you exactly what the issue is, but a calibration couldn't hurt. I guess your tech would be looking at that anyway, though.

You could try the Yahoo user group. There are some well informed folk in there.
Old 30th December 2010
Lives for gear
Kenton's Avatar

I don't think it's a single voice that's the problem - in that case 5 notes would sound the same and the 6th would be different.

It sounds more like some sort of crosstalk problem with the filter control voltages.
If you open up the synth, is there any sign of leakage around the battery? THAT might cause a weird problem like this and battery leakage is a well known issue with the Polysix.
Old 30th December 2010
Gear Addict
Steck's Avatar

Many moons ago I owned a Polysix. I believe that somewhere on one of the circuit boards that are visible when you hinge the front open are tiny LEDS that show which particular voice is playing when you press a key. (Pity they didn't put it on the front panel, as eye candy).

I would suggest you consider opening the polysix up and look at the voice assignment LEDs whenever you are experimenting, to see if it's one particular voice that has the cut-off effect you describe. If so, I'd guess that that particular voice needs to be looked at. If not, then as another poster says, some kind of general calibration and checkout would seem in order.

Either way, I'd say that, if you're not a tech with tools to measure DC offset and such, you're probably going to want to get the 'board to someone who can go through the service manual and align everything, replace the memory back-up coin cell if needed [make sure your patches are backed up], and so forth.


Old 31st December 2010
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Acid Hazard's Avatar
Do all 6 voices share 1 common VCF? I would think they don't, and if not, the VCF cutoff freq could be out of whack on voice 1. And it may just be being glossed over when the other voices kick in.
Old 3rd January 2011
Gear Maniac
I had an SSM filter chip go on mine. Could well be that!

Originally Posted by Acid Hazard View Post
Do all 6 voices share 1 common VCF? I would think they don't, and if not, the VCF cutoff freq could be out of whack on voice 1. And it may just be being glossed over when the other voices kick in.
Old 3rd January 2011
Lives for gear
Kenton's Avatar

Originally Posted by Acid Hazard View Post
Do all 6 voices share 1 common VCF? I would think they don't, and if not, the VCF cutoff freq could be out of whack on voice 1. And it may just be being glossed over when the other voices kick in.
On the Polysix each voice has it's own VCF.

The suggestion about checking voices by looking at the LED's on the voice board is a good one.
Another trick is to use the HOLD and CHORD MEMORY to isolate voices - if you hold one note and engage CHORD MEMORY mode, the selected voice will be the only one that sounds thereafter. Normally notes will cycle round the unused voices 1-2-3-4-5-6.

A bad SSM chip is a possibility but more likely is something in the filter control circuit. It's a long time since I looked at the schematic...

*** Having just looked at the schematic - there are three 4558 (?) op amps which buffer the CV for the filter cutoff (IC20,21,22). I'd bet one of those has gone flakey. That's what it sounds like to me - but further investigation is required to isolate which voice(s) are affected.
Alternatively it could be the 14051 analog demultiplexer (IC23) - either way it should be fairly straightforward to fix for a good tech. ***
Old 3rd January 2011
Jose Ramón Alvarado Villa
Don Solaris's Avatar
Originally Posted by pedrozakatchku View Post

Resolving a problem with my newly bought Korg Polysix.
Google for Korg Polysix Service Manual

Read from first to last page.

I'm sure you will find the calibration procedure.

Also give it to someone to replace all the electrolyte capacitors in the unit, prior to calibration (else, there's no much point calibrating it). You can let the technician calibrate the unit.

Before you buy a 30 year old analog instrument, keep in mind that:
  • No instrument was manufactured to last forever.
  • You need to maintain it.(!)
  • It will cost you.
Old 30th July 2016
Lives for gear
autoy's Avatar
Sorry for resurrecting this thread but yesterday I had this very same problem and found the solution. The problem resides in the klm-368 effects board. The VCA adjustments are made there with four trim pots at the top right of the board, two for the main out and two for the headphones out. Each pair adjusts an extra low pass filter with an offset parameter and a gain stage. If you fiddle with these in order to boost the output signal you will find combinations that suppress the main filter output depending on the gain. In my case I saw filter tracking and main filter completely messed up opening and closing abruptly depending on the wave selected and number of notes played. These interact somehow with the gain staging in the VCA of the effects board and renders the sound unusable.

Of course properly following a calibration of these trim pots makes the problem go away. So for future fellow Polysix refurbishers, please take a look at these before deciding to go for busted opamps or capacitors.
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