[sdiy] diagnosing polysix output noise (fwd from AH)
2026-03-27 by Jimmy Moore
Forwarding here because I feel like this is the better audience to help diagnose korg polysix issues. Quick recap:I have already replaced the cpu board and psu with a synthronicss kit.. There's still noise, which sounds like this:
After signal tracing, The audio gets corrupted from pins 13 and 14 of IC3 - the BBD chip (schematic markup here)
however!
I also see noise showing up on clock input of the same chip (pin 2). Any time I hear noise, I see it on that pin as well. It is worth noting this noise does not appear on any of the other clock or audio pins of the other BBD chips (IC1 and IC2). This leads me to think the problem is localized to this area.
Basic Question #1: How likely is it that the BBD chip is actually fine and what I'm hearing is caused by passing noise through the clock input?
Assuming that's possible, I can trace the noise farther upstream all the way back to IC23, an LM324.
Basic questions #2: Could a corroded resistor cause or contribute to this problem?
Basic question #3: It makes sense that a bad chip would cause noise on its output, but is it possible it could spit out noise through its inputs?
IC23 is the first thing control signals see coming over from the KLM-367 control board (Other than the nearby resistors). This noise issue existed before I replaced the controller board with the one from synthronics so I highly doubt the issue is coming from the there....unless the CPU chip is messed up? I do remember it having corrosion on its pins as well. But the synth plays and behaves as I'd expect.
however!
I also see noise showing up on clock input of the same chip (pin 2). Any time I hear noise, I see it on that pin as well. It is worth noting this noise does not appear on any of the other clock or audio pins of the other BBD chips (IC1 and IC2). This leads me to think the problem is localized to this area.
Basic Question #1: How likely is it that the BBD chip is actually fine and what I'm hearing is caused by passing noise through the clock input?
Assuming that's possible, I can trace the noise farther upstream all the way back to IC23, an LM324.
While investigating I found several resistors with the same kind of green corrosion on thier leads in this portion of the circuit. The above picture has the corroded resistors highlighted in red,with suspicious resistors highlighted in orange. These are resistors R133, R172, and R199-204. All showed the same kind of visible corrosion as seen near the battery, but on a completely different board. Makes me wonder where else this may have migrated to. Replacing these resistors did put a dent in the persistence and frequency at which the noise appears, but it's not gone.
Basic questions #2: Could a corroded resistor cause or contribute to this problem?
Basic question #3: It makes sense that a bad chip would cause noise on its output, but is it possible it could spit out noise through its inputs?
IC23 is the first thing control signals see coming over from the KLM-367 control board (Other than the nearby resistors). This noise issue existed before I replaced the controller board with the one from synthronics so I highly doubt the issue is coming from the there....unless the CPU chip is messed up? I do remember it having corrosion on its pins as well. But the synth plays and behaves as I'd expect.
I'll read back through some messages to see if I can rule more things out or address any questions. thanks!
Show quoted textHide quoted text
---------- Forwarded message ---------
From: Jimmy Moore <jamoore84@gmail.com>
Date: Wed, Mar 25, 2026 at 10:22 PM
Subject: Re: [AH] diagnosing polysix output noise - was Korg Polysix: CPU board replacement options?
To: Florian Anwander <fanwander@mnet-online.de>
Cc: AH send <analogue@hyperreal.org>
From: Jimmy Moore <jamoore84@gmail.com>
Date: Wed, Mar 25, 2026 at 10:22 PM
Subject: Re: [AH] diagnosing polysix output noise - was Korg Polysix: CPU board replacement options?
To: Florian Anwander <fanwander@mnet-online.de>
Cc: AH send <analogue@hyperreal.org>
Thanks to everyone who reached out with tips and well wishing! The audio signal path gets corrupted from pins 13 and 14 of IC3 - the BBD chip (schematic markup here)
however!
I also see noise showing up on clock input of the same chip (pin 2). Any time I hear noise, I see it on that pin as well. It is worth noting this noise does not appear on any of the other clock or audio pins of the other BBD chips (IC1 and IC2). This leads me to think the problem is localized to this area.
Basic Question #1: How likely is it that the BBD chip is actually fine and what I'm hearing is caused by passing noise through the clock input?
Assuming that's possible, I can trace the noise farther upstream all the way back to IC23, an LM324.
Basic questions #2: Could a corroded resistor cause or contribute to this problem?
Basic question #3: It makes sense that a bad chip would cause noise on its output, but is it possible it could spit out noise through its inputs?
IC23 is the first thing control signals see coming over from the KLM-367 control board (Other than the nearby resistors). This noise issue existed before I replaced the controller board with the one from synthronics so I highly doubt the issue is coming from the there....unless the CPU chip is messed up? I do remember it having corrosion on its pins as well. But the synth plays and behaves as I'd expect.
however!
I also see noise showing up on clock input of the same chip (pin 2). Any time I hear noise, I see it on that pin as well. It is worth noting this noise does not appear on any of the other clock or audio pins of the other BBD chips (IC1 and IC2). This leads me to think the problem is localized to this area.
Basic Question #1: How likely is it that the BBD chip is actually fine and what I'm hearing is caused by passing noise through the clock input?
Assuming that's possible, I can trace the noise farther upstream all the way back to IC23, an LM324.
While investigating I found several resistors with the same kind of green corrosion on thier leads in this portion of the circuit. The above picture has the corroded resistors highlighted in red,with suspicious resistors highlighted in orange. These are resistors R133, R172, and R199-204. All showed the same kind of visible corrosion as seen near the battery, but on a completely different board. Makes me wonder where else this may have migrated to. Replacing these resistors did put a dent in the persistence and frequency at which the noise appears, but it's not gone.
Basic questions #2: Could a corroded resistor cause or contribute to this problem?
Basic question #3: It makes sense that a bad chip would cause noise on its output, but is it possible it could spit out noise through its inputs?
IC23 is the first thing control signals see coming over from the KLM-367 control board (Other than the nearby resistors). This noise issue existed before I replaced the controller board with the one from synthronics so I highly doubt the issue is coming from the there....unless the CPU chip is messed up? I do remember it having corrosion on its pins as well. But the synth plays and behaves as I'd expect.
I'll read back through some messages to see if I can rule more things out or address any questions. thanks!
On Mon, Mar 16, 2026 at 3:31 AM Florian Anwander <fanwander@mnet-online.de> wrote:
the signal tracker aside
I assume it is not related to a certain voice. So assumingly it is the summing amp on the mainboard or something on the chorus board. Does it depend on the on/off-setting of the effect section? If not then there are only a few parts that remain as suspects.
And just as remark: many people forget, that there is a VCA on the effect board that is controlled by the resonance setting. So, if the noise depends on the resonance setting, you can narrow down the source quite well.
Florian
Am 15.03.26 um 19:22 schrieb Jimmy Moore:
d'oh, definitely should have thought of the signal tracer sooner! Thanks for the reminder.
On Sun, Mar 15, 2026 at 7:57 AM Bob Grieb <rlgrieb1@verizon.net> wrote:
Hello,
A transistor or op amp that has failed can cause the sort of
noise you are hearing. One way to track down the source is with a
"signal tracer", which is just an amplifier and speaker. Usually capacitor
coupled input, since the point you are probing may have DC voltage on it.
A small cap, something like 0.1uF should be usable. You would connect
the ground lead of the amplifier input cable to ground at the Polysix.
The signal input lead would feed through the cap and into the amplifier.
Then you can touch various points in the circuit to see which ones have the
noise and which don't.
Another technique that helps for this type of problem is circuit
cooling spray.
You just spray various semiconductor devices in the signal path to see
if the
noise changes when you cool any of them. Just one second of spray on each
device is plenty.
Bob Grieb
On 3/15/2026 9:16 AM, Jimmy Moore wrote:
> Thanks for everyone's earlier feedback. I ended up getting a
> replacement processor board and power supply from synthronics.de
> <http://synthronics.de> which has helped me sleep a lot better
>
>
> With those failure modes addressed, there's now a noise issue to
> solve. Here's a short recording from the bench speaker
>
> https://drive.google.com/file/d/14MpNs5ZL6Ze7kJ5bgBbiDW5y5Eahphe3/view?usp=drivesdk
> <https://drive.googlecom/file/d/14MpNs5ZL6Ze7kJ5bgBbiDW5y5Eahphe3/view?usp=drivesdk>
>
> This does not seem to be affected by any panel controls, system
> settings, or bank presets.
>
> The noise is most prominent when playing notes, but there's also that
> lowish rumble when silent. more prominent on the phones output if that
> helps.
>
>
> Is this a telltale sign of anything in particular?
>
>
>
>
>
> On Fri, Feb 27, 2026, 5:36 PM Jimmy Moore <jamoore84@gmail.com> wrote:
>
> You can get a new case, too! 😂
>
> https://reverb.com/item/93243657
>
> On Fri, Feb 27, 2026, 3:44 PM Tim Parkhurst
> <tim.parkhurst@gmail.com> wrote:
>
> So, besides the knobs and the end cheeks, everything on the
> PolySix needs to be replaced.
>
> Got it.
>
>
> Tim (embraceable, replaceable me) Servo
> ---
> "Imagination is more important than knowledge." - Albert Einstein
>
>
>
>
>
> On Fri, Feb 27, 2026 at 2:09 PM DJ Maytag (DJMaytag)
> <djmaytag@djmaytag.com> wrote:
>
> It certainly is the easiest, but it is an expensive board.
> I did one a few years ago, with the Kiwisix PSU upgrade,
> and then had to get a replacement panel board from
> Synthronics.de <http://Synthronics.de> since the one panel
> got some sort of acid/gas all over the bottom side of it.
> I think it’s worth springing for the SynthGraphics.com
> <http://SynthGraphics.com> overlay for all the added Kiwi
> functions. IIRC it’s about $129, and is well worth it if
> your top panel is scratched up (it cover almost all the
> panel surface).
>
>
>
>> On Feb 27, 2026, at 10:00 AM, Jimmy Moore
>> <jamoore84@gmail.com> wrote:
>>
>> *tl;dr*: is kiwi the best option?
>>
>> I got a korg polysix for $100 recently and it is
>> suffering from the usual creeping corrosion. Several
>> list members have battled this in the past with some
>> common takeaways:
>>
>> Tom Wilshire
>>
>> The Polysix has a lot of reliability problems, but
>> aside from the keyed and the panels switches, *most
>> of them stem from corrosion on the main board.*
>>
>> Brianw
>>
>> A local musician brought me a Polysix that had had
>> repairs made to 'fix' a previous battery leak. That
>> old repair had failed. We gave the internet advice a
>> chance, and soaked the board in vinegar to neutralize
>> the acid, but that effort did not pan out. The
>> batteries are still available, but that doesn't help
>> if the CPU board is less than 100%
>> *The solution was a Kiwitechnics Kiwisix board, and
>> the added benefit is that this old Polysix now has MIDI.*
>>
>>
>> and KennyBalys
>>
>> I have repaired this board annually with success :)
>> One could say, I derive a lot of self esteem from my
>> repairs, in doing them correct. The board itself,
>> visually, looks like a very nicely repaired
>> board that, once upon a time, had some damage. It
>> looks nice enough to
>> sell, if I had a lack of moral fibre (its going to
>> re-cycling)
>> This problem keeps on re-occuring. Corrosion
>> appearing inches away from
>> where the original damage occured. I can repair it
>> again now, it will
>> likely be off again next year.
>> Perhaps some of the PC boards from that era were more
>> fibrous than
>> others?
>> *As of now, I will no longer attempt repairs on the
>> Polysix CPU board.
>> Replacement only.*
>>
>>
>> I am aware of the Kiwi main board + PSU mods. I have
>> also heard of people having good experience with
>> Synthronics.de as recently as 2022, but I can't find them
>> online.
>>
>> Are there any other options to consider?
>
--
This email has been checked for viruses by Avast antivirus software.
www.avast.com