It's been a hot minute since I had some time to dedicate to write about music stuffs on here. Since it's been so long, might as well make it special. Today, we will not be discussing about pedals, but we'll be talkin' synth. A very specific vintage one. It's going to be a story of rejection, rescue, and subsequent recovery, blood will be spilled, tears will be cried, and cold sweats will be had. Without further ado, let's dig in.

First, a bit of history about the Korg Polysix

The PolySix was one of the very first affordable polyphonic synthesizer on the market. Released in 1981 and commercialized for an unknown number of years (my Google-Fu failed me on this; if you know when it was discontinued, please edit the Wikipedia page for it), it made direct competition to the Roland Juno-6 that was released only a few months later, and was targeting the common mortal who couldn't afford a Sequential Prophet 5 or an Oberheim OB-X. At a price tag of $1095US in 1981 or a few dollars shy of $3900US in 2024, it was about a quarter to a fifth of the cost of one of those synths. What a bargain!

What did $1095US give you back in 1981? This. Eleven and a half kilos of pure joy.

Korg PolySix, slightly upgraded. Yes, it's still a bit dirty. You can't wipe off 43 years of grime that easily...

The specs

As you might expect for it being the affordable synthesizer option, the PolySix has some slight differences compared to its more expensive cousins. While the Oberheim OB-X used a full-featured Zilog Z80 microprocessor, the very same found on many home computers of the time, the PolySix used a pair of much less powerful (albeit capable) NEC D8048C microcontrollers: one on the KLM-367 "CPU board", and one on the KLM-366 "voice board". This makes things like tuning a bit more complicated, but it keeps costs down and that's exactly what the engineers wanted. "Affordable" microprocessors were still relatively new and everything around them was expensive and complex, while standard 74LS logic chips were cheap, common, and easy to implement.

The PolySix has a 61-keys keyboard with no velocity or aftertouch, much like other synths of the time, and it has six voices at its disposition. Compared to the more expensive competition, each voice has only one oscillator instead of two but it makes up that difference with an integrated chorus, phaser, and ensemble effect, each using their own Panasonic MN3004 512-stages bucket-brigade chip. Neat. The PolySix uses dynamic voice allocation, which mean every time you press a key a different voice is being played. This allows each note to decay properly and not end prematurely as soon as another note is played.

It can generate sawtooth, pulse-width, and pulse-width modulation waveforms, with the possibility to add a square wave sitting one or two octaves below pitch. It also has a chord mode, a hold toggle, an arpeggiator, and a unison/polyphonic toggle. The programmer can store 32 patches in volatile RAM, which requires a 3.6V battery to be installed somewhere inside the enclosure. This battery has been the cause of many destroyed PolySixes (is there a plural form of PolySix?) throughout the years. We will come back to that in a few paragraphs.

You will find the typical controls you'd expect on a synthesizer: VCO (voltage-controlled oscillator), VCF voltage-controlled filter), VCA (voltage controller amplifier), and EG (envelope generator) controls, modulation and bend (both with their respective wheels, neither having a spring return-to-center), effect type and intensity, arpeggiator settings, and a tune knob. The aforementioned tune knob allows you to alter the master tuning to be either flat or sharp by about a full tone to one and a half tone and will have an immediate effect on all voices.

The back panel has the usual ¼-inch jacks for synthesizers of the time: to/from tape, chord memory, arpeggio trigger, VCF fcM in, and headphones-level and line-level outputs. Both audio outputs are volume-controlled by the volume knob and the line-level output uses the high/low/off toggle switch as well. Also on the back you will find the permanently attached two prongs power cord, and two posts to wind the cable around.

Some more "interesting" design decisions

The PolySix is an incredible synthesizer that has a lot to offer, however it does have some interesting specificities. Some of them may even lead to destructive consequences if left unchecked for long enough.

Where's the ground?

The design of the PolySix' is modular and uses multiple boards, like most of the synthesizers in production at the time. This leads to noise possibly being injected in multiple points in the audio signal. A common ground (i.e., a common grounding point for all boards and components) is usually incorporated in those designs, but the PolySix does not have one which makes the overall signal very noisy. Each board acts like a giant antenna and the noise makes its way to the output since each ground is isolated.

The power supply board is prone to failure

The whole power supply is split in two parts. The power cable is connected to an interposer board, the KLM-425, which contains a RIFA-branded "safety" capacitor (between live and neutral, of course). The current makes it way to a noise-killer to minimize the noise coming from the power line itself, then to a transformer that provides 39.5V and 29V. The RIFA capacitors all end up cracking from thermal stress and aging, and none of the capacitors from the 80's are reliable anymore. One would argue that even new production RIFA capacitors are unreliable, but I digress. If it hasn't blown already, it definitely will. These can also release the Magic Smoke™ even with the power switch off as it bypasses the power switch itself. It is imperative to replace it by an appropriately rated safety capacitor. Any X2 capacitor with similar specs and dimensions will do.

The two voltage lines from the transformer end up on the KLM-376A-PS board, or the "power supply board". This board rectifies the AC from the transformer and derives the +5V, +15V, -5V, and -15V DC rails that are sent to every other boards. The overall design of the KLM-376A-PS board is inefficient and the components it uses are prone to failure. Many PolySix synths were destroyed beyond repair due to faulty voltage regulation, sending way more voltage to components than what they can comfortably run with, frying them in the process. Replacing all the damaged chips and components across every board of the synthesizer is tedious and expensive.

Another issue with the original KLM-376A-PS is that it generates A LOT of heat. Way more than I'm comfortable with. The board itself is attached to two thick metal mounting brackets, and these brackets are screwed on the back plate of the keyboard enclosure with a thin metal plate between the brackets and the synthesizer case. Originally, there would have been some sort of thermal paste or thermal transfer compound between the enclosure and the metal plate, but 43 years old thermal paste does not transfer heat very well, and that is if any is left after drying up for a few decades. If you have a PolySix and you decide to replace the power supply board or change the thermal paste, I strongly recommend wearing gloves and a mask. Some thermal pastes from the 70's and 80's contained beryllium, which is extremely toxic and can cause berylliosis at very low dose after a single exposure, and you do not want to breathe that in.

The Varta 3.6V NiCd battery

The sole mention of the Varta brand name is enough to give veteran electronic technicians nightmares (Tadiran is a close second). The type of nickel-cadmium battery present in the PolySix was used in all sorts of electronic devices and computers up until the early 90's before being replaced by the much more stable CR2032 button cell battery that is still used today. Sadly, nickel-cadmium batteries were the cause of so much electronics destruction and the PolySix is just another statistic. After a few years, these batteries start to leak, and the battery juice likes to eat through copper traces, component pins, and even the fiberglass of the PCB itself. Sometimes, batteries that were sealed tighter than others build up pressure and explode, covering a large area with their corrosive contents and destroying everything it touches over the months and years the blast remains unnoticed.

In the case of the PolySix, the KLM-367 board has one of those batteries soldered in, and is the typical casualty when the battery eventually leaks. If the leak is caught in time, it can be as simple as removing the battery and cleaning any of the corrosive liquids with 99% isopropyl alcohol. The battery is only required for the programmer's memory and using any preset without a battery installed will result in audio garbage since the memory will be corrupted, but the synthesizer can still be used in manual mode.

The easy fix is unfortunately the exception. Unless you or the previous owner(s) knew about the risks inherent to that specific type of battery, it most likely remained in the synthesizer for years slowly leaking and wreaking havoc on the insides. Destroyed traces can be bodged back together and faulty chips can be replaced, but that is only possible when the damage is minimal, and it may not always be a possibility depending on the extent of the corrosion. Finding a replacement or an alternative board is often the better, cost-effective option.

How did I get a PolySix?

Back during my summer vacation, I had to dispose of items the garbage collection and recycling collection do not pick up. We have those eco-centers, which are basically "self-serve recycling and disposal sites" you can go to. The process is simple: you register with a photo ID, say what you are dropping for recycling or disposal, and the employees direct you to the stations you need to go at. There's one for electronics, small appliances, and batteries, one for Styrofoam and larger appliances, one for anything with refrigerant, one for cardboard, etc.

I was dropping some batteries and broken power strips at the electronics station and saw, in a large garbage bin, half of a keyboard sticking out rejected like, well, garbage. It seemed to be in okay shape and looked like some sort of old Korg with wooden sides, but I didn't think much about it. It was in a garbage bin for a reason, right? As I was going to the other stations I needed to stop at, I was wondering who in their right mind would throw that away. A keyboard with wooden sides is definitely older than the 90's and would be somewhat valuable, I think. At this point I still had no idea exactly what that keyboard was, but I saw some woodgrain and I was intrigued. My "obsessed music gear collector" side wanted to bring it back home because I could most likely fix it, but my "rational thinking" side said it was in the trash for a reason. So, of course I doubled back and asked the staff if I could take it, and here we are...

Now what?

First thing first, I decided to seal it in some garbage bags for a day or two just to make sure there weren't any surprises hiding inside. Thankfully, there were none. After some time to fully dry off since I picked up on a rainy day and it was outside, it was time for a proper damage assertion.

The outside

This synthesizer saw things. Things, I tell you. The exterior is in rough condition. The metal case is scratched and some of the paint is chipping with some rust here and there. However, all the markings are present, the knobs are all there, and the buttons and switches all work. The particle wood sides have some damage on the two bottom back corners, but the rest of the wooden enclosure is still solid. It is missing its power cord, which is unfortunate and was probably done at the eco-center, but the plastic posts used to wind it for storage are both undamaged.

Some weird stains on front panel and on the keys, scratches, and rusted screws. It needs a thorough clean-up.

The keyboard itself was only partially secured by two of the five screws it needs and was moving around quite a lot. The bend and modulation section was missing the two screws securing the front, but the wheels themselves were rotating quite smoothly. Beside some dirt and grime, all the keys are actuating properly and the mechanism and contacts don't seem too dirty. There is a weird, sticky, tar-like stain on the E2 key which is going to be fun to remove.

All the other screws are present, and most of them had their black oxide coating eaten off by rust over time. They will probably need to be replaced if I cannot get them clean enough. The serial number and model details plate is still perfectly legible. A quick search on the internet lead me to the Zen & the Art of Synthesizer Maintenance website that lists serial numbers and production years of many Korg devices, and this specific synthesizer was produced in mid to late 1981 based on the serial numbers it has for the PolySix.

Someone cut the power cord for some reason...

That's it for the outside. Not too bad considering I found it in a trash can at a recycling center on a rainy day. I still need to take a look at the inside, and that's where the fun begins...

The inside

There is obvious traces of some sort of water damage. Not sure if it's recent. The thick insulating cardboard under the KLM-425 power input board is badly warped and has some strange white powdery residue on it (no, not that kind of white powdery residue). There is rust in some spots on the inside of the metal enclosure as well, indicating it was stored somewhere humid or was exposed to water at some point in its life. All the cable harnesses (except the power cable) are perfectly fine; their insulation is intact and none of the tiny wires are cut. The three fuses are not blown, but the power input board has a RIFA capacitor that needs to be replaced yesterday. The boards are generally dirty and dusty, and need to be bathed in alcohol with a vigorous toothbrushing.

Power input, transformer, and part of the voice board and power supply board.

And here's the Varta battery, in all its leaky goodness. At first glance, the damages look localized and the leak doesn't seem to extend too far. There's obvious signs of battery juices on the enclosure, so it did leak with some force. The power supply board looks OK, but the large electrolytic capacitors will need to be checked before applying any power.

Other half of the voice board and CPU board. Power supply board looks OK. The corrosion on back of enclosure is scarily close to some cable harnesses.

The effects board looks clean. Although it shares a side with the CPU board, the battery liquids did not make their way to that board. The jacks are in good condition with no corrosion on them. Whether they actually work is another topic entirely.

Effect board and jacks. Corrosion is visible on the battery itself and the back of the enclosure.

Here's a close-up of the battery with some of the damage visible on the CPU board. The battery was removed and the board was quickly cleaned with alcohol to remove as much of the corrosive gunk as possible.

Varta. Enough said.

That doesn't look good...

That's... surprisingly clean, now...

A quick check with a multimeter, and all the traces have continuity to where they need to go. The pins on IC30 and IC31 look pretty rough and IC32 isn't looking too good either, but there's probably a chance the synthesizer will works as-is without any major repairs. The microcontrollers are socketed, so they can be easily changed if there's anything wrong with them. The rest is all soldered in place, except some select ICs and the envelope generators and filters for each of the voices.

Quick list of the different boards and what needs to be done

KLM-425 (power input board)

RIFA needs to be replaced
Needs a new power cord

KLM-376A-PS (power supply board)

Electrolytic capacitors need to be checked
Voltage rails need to be checked

KLM-366 (voice board)

General cleaning
Tuning of the six voices possibly required

KLM-367 (CPU board)

Remove the battery
Clean up of the corrosion and general cleaning
Replace suspicious ICs (IC30, IC31, IC32)
Check ground wire

KLM-368 (effects board)

General cleaning

KLM-369, KLM-370, KLM-371 (boards where all the switches and knob are)

General cleaning
Spray DeoxIt in each potentiometer and switches

I/O jacks

Cleaning of each jack with DeoxIt

General

Find replacements for the missing screws
Try to remove the rust on the screws that were present
Clean up the entire exterior, including knobs and keys

Let's make some basic repairs and go from there

First thing first: that RIFA capacitor has to go. Now. With extreme prejudice. Unfortunately, I could not cut the pins from the board, I had to desolder it the old fashioned way. That's nothing a solder pump and some copper wick can't take care of. I did manage to lift and break part of the trace on the board for one pin. Thankfully, the pins on the replacement capacitor were long enough that I could remove some of the conformal coating on the board and use the pin itself to bridge the gap. I used a TDK B32922P3333K000 0.033µF polypropylene film capacitor with an AC voltage rating of 305V and a DC voltage rating of 605V. They are about 0.58$ each from DigiKey, and are the closest from the original RIFA specs I could find. More importantly they are rated X2, which means they are made to be used in that application and will not randomly blow up like the RIFA would.

Marbling is great on a steak, not on a capacitor. This has to go.

Much better.

Next on the list is the power cable. A cheap two prongs cable later and we're in business. I'll figure a way to put the grommet back later. The large electrolytics on the power board are within specs and we're ready to plug it in.

First try

This is an historic moment. This synthesizer didn't have power applied to it in who knows how many years and anything can happen. It can explode, catch fire, or just, you know... work.

Before applying power to the boards, we need to make sure the transformer and the power supply board are giving the right voltages. We are expecting to see 39.5V and 29V from the two leads out of the transformer (AC current), and the power supply board should regulate that down to four DC rails: +5V, -5V, +15V, and -15V.

All the power cables going to the boards were disconnected, and the PolySix was powered up. Nothing exploded or caught fire. That's a good start. The different voltage rails were within 10% of their expected voltage, which is good enough. I'm not sure if this power supply gives the same output with and without load, so I'll assume it drifts high when there's no load applied. We can connect the rest of the boards.

Once everything was connected as it should, power was applied and the PolySix lit up. LEDs were flashing and pressing buttons made LEDs change. So far so good. No Magic Smoke™ was released yet. I think it's safe to connect a speaker to it. I used my sacrificial Marshall MS-2 stack (with one full watt of output power), just in case something was terribly wrong with the output signal. Turns out there is no signal from the output jack, but the headphones jack works perfectly fine. It gets better: it makes noise.

It's alive! Sort of.

It makes noise, but the noise it produces are... not great. It's the kind of noises dying electronics make. It definitely needs more work, and that's also where I made a mistake. I decided to order some replacement ICs and the appropriate sockets based on my brief observation of the CPU board's state. When I received the parts and got to work on removing the ICs, that's when I noticed the corrosion was much, much worse than I thought.

Where's ~~Waldo~~ the vias? The pins, traces, and vias of IC31 were completely obliterated...

As you can see from the above picture, the vias are completely gone, and the corrosion started attacking the fiberglass of the PCB itself around pin 12 of IC31. This IC was still partially working because it was held in place by rust, but some pins were not even connected to the chip's internals anymore. Simply removing the pins of this single IC took well over 30 minutes when it's usually a two-minutes job. As I need to replace multiple chips in that area and they all look similar, I suspect the vias and part of the traces they are connected to will be gone as soon as I apply heat to desolder them. Some traces on the underside of the PCB were also marginal or just plain missing. All in all, in this current state, I'm not equipped to restore this board and it would not make sense to do it either. We need to find an alternative.

There's still hope: the KiwiSix

Now that we figured out the KLM-367 board is completely shot, what can we do? A quick Google search didn't provide many answers. Beside a few MIDI retrofit kits, only one is a full replacement of the KLM-367 CPU board: the KiwiSix.

The KiwiSix is manufactured by KiwiTechnics, the synthesizer repair division of PC Services Ltd. from Auckland, New Zealand. They offer replacements and retrofit kits for select Roland synthesizer models and for the Korg PolySix. The latter is offered in two flavours: with or without the power supply board upgrade. 395$US plus 52$US shipping (or roughly 610$CAD at the time of writing this article) for the KiwiSix and power supply board upgrade is a major expense, but it replaces the two boards that are the main sources of issues.

The KiwiSix is a completely new KLM-367 replacement board. Beside replicating the original board's logic, it has a bunch of updates and new features. Here are some of them. The full list is available at the link above.

512 patches saved in non-volatile memory, with 64 factory patches provided (no battery required!)
MIDI in and out, and MIDI control for pretty much everything the PolySix can do
New metal plate for the power cord and MIDI ports, with a standard IEC socket (power cord not included)
Key transpose (-1 to +2 octaves)
Two modes of Polyphony (7th note override or 7th note ignored)
Arpeggiator and Sequencer clocks can be divided from the master clock

The power supply board replacement is exactly what it says on the tin: a full replacement of the KLM-376A-PS board using modern components with built-in safety systems to prevent overvoltage. Two ferrite clamps are also provided with the kit to attach around the keyboard and the oscillators settings cable harnesses to minimize noise.

Unboxing the KiwiSix

Once shipped, it took about 10 days for it to arrive to The Great White North. Import duties and fees were not that bad at roughly 55$CAD, bringing the total cost to about 660$CAD. While waiting for it to arrive, I did the rest of the maintenance on the other boards. Beside very rusty screws, no other issues were identified. I also cleaned the inside of the metal enclosure to remove the rust and dust.

The boards were neatly packed in ESD-safe bags with bubble wrap and some sort of cotton pad packing material, and the kit includes everything you need to install it.

Installing the KiwiSix

Installing the KiwiSix is relatively simple:

Remove the old board
Install the new board
Make sure the cables are connected the right way
Remove the microcontroller on the voice board
Connect the supplied interface cable between the KiwiSix and the voice board

Some mods are required in order to make the wheels MIDI-compatible and require some soldering. The instructions to install the boards and make the mods are clear and easy to follow.

Replacing the backplate also requires a bit of soldering (for joining the IEC connector to the KLM-425 board) and potentially drilling a hole in the enclosure to relocate the ground, but only if you have an early revision of the PolySix that was never serviced.

The power supply board upgrade is a tad more complicated and require salvaging the connectors from the original board. Removing the original board is risky business due to the thermal compound used when it was installed (as mentioned previously), so make sure to wear gloves and a mask. The board itself is screwed into two U-shaped brackets, and the brackets are screwed to the enclosure with a metal plate in between. This metal plate is where the thermal paste is, assuming it didn't dry out completely or was not removed during a previous maintenance. The plate, the enclosure area where it is screwed, and the brackets needs to be cleaned thoroughly with 99% IPA.

The connectors we need to move are made out of unobtainium and melt at a relatively low temperature. Moving them from one board to the other requires a hot soldering iron, fast hands, and frequent breaks. It may sound unintuitive, but a hotter iron allows you to dump more heat on the solder a lot faster without the pin transferring too much heat to the surrounding plastic. Work quickly with a good copper wick and take frequent breaks to let the pin and plastic cool down, and it should be uneventful. Make sure to solder them the right way on the new board, though...

KiwiSix board fully installed.

KLM-376 replacement board installed.

The replacement plate for the power cord and MIDI ports has screw holes for the original serial number plate.

Once the replacement power board is installed, you need to make sure the different power rails are correctly adjusted before plugging everything else to it. They each have their own trim potentiometer and test point. Power up the synthesizer, wait 15 minutes for the electronics to stabilize, and check each voltage rail, adjusting when needed. Once that is done, you need to follow the same steps but with all the cables connected. At this point, there should be little to no adjustment required.

Final adjustments

This is where things get interesting. Now that we have everything installed and connected together, we need to tune and calibrate the voice board. The KiwiSix instructions contain a section with everything that needs to be checked, tuned, and adjusted. Keep in mind that each PolySix is different, and they all have their quirks and particularities. Some numbers provided in the instructions might not be what your PolySix needs or expects. You should not be performing these adjustments unless you know what you are doing.

In order to perform these adjustment, you will need a digital voltmeter/multimeter, a decent oscilloscope, a small screwdriver to adjust the potentiometers, and a tuner of some sort. I used a Klein Tools MM400 digital multimeter, a Keysight EDUX1052 oscilloscope, a standard Phillips #2 screwdriver from my iFitIt set, and my trusty Peterson StroboStomp Mini strobe tuner.

Voice adjustments

The adjustments require the PolySix to be set with the "Normal" settings (most of the time, some need certain settings that are mentioned in the instructions). The graphics from Korg's Service Manual and the KiwiSix manual isn't exactly clear (at least when it comes to the toggle switches), so here they are in text form:

Tune knob: centered
Bend intensity: 0
VCO

Octave: 8'
Waveform: sawtooth (left-most position)
PW/PWM: 0
PWM Speed: 0
Sub Osc: Off (top-most position)

Frequency: 0
Delay: 0
Level: 0
Mod: VCO (top-most position)

Cutoff: 10
Resonance: 0
EG Intensity: 0
Kbd Track: 0

Attack: 0
Decay: 0
Sustain: 10
Release: 0

Mode: EG (top-most position)
Attenuator: +10db

Effects

Mode: off
Speed/intensity: 0

Korg PolySix Normal settings

Every single aspect of the VCF and VCA needs to be checked: offset, level, resonance, envelope generator intensity, and keyboard track. Most of these settings need to be checked on each voice, which is time-consuming. The KLM-366 has tiny LEDs in each "voice unit" that will light up when a voice is active, but they are not very bright and they are very easy to miss.

Voice tuning

The most fun and (potentially infuriating) step is the next one: voice tuning. Each voice need to be tuned so they at pitch. By hand. One. By. One. Remember that the PolySix cycles through the voices on each key press. If you are not careful, you will adjust the potentiometers on the wrong voice.

The tuning is done in three different places on the KLM-366 board. The tune knob on the front panel is the first place and it needs to be centered before changing anything on the voice board. Above CN12 on the board itself, there is the master tuning area which allows you to set the center adjustment, tune low, and tune high. Tune mid and a specific potentiometer for the D#4-E4 adjustment are located beside the unit 0 section, towards the bottom of the KLM-366 board. These will affect ALL voices, and changing any of these will require you to start the tuning process from scratch again on each voice. Changing the master tuning should get you roughly in the ballpark on all voices.

Once the master tuning is "good enough", you can proceed to adjust each voice individually with their tune low and tune high controls. Make sure you turn the potentiometers on the right voice when tuning...

Tuning the six oscillators to perfectly track is impossible. You only need to get it to sound good enough. As the KiwiSix manual states on page 33:

Get things as close as you can and it will not be possible to get every voice tracking exactly over the full range as each voice has it's own oscillator, and after all this is part of the Polysix analog charm and unique sound.

Tuning is hard...

I encountered an interesting tuning issue.

F is the new C.

The key I'm pressing? It's supposed to be a C5 (523Hz), and it's outputting a few hertz shy of an F4 (340Hz, according to the oscilloscope). No combination of master tuning and voice tuning can get me remotely close to a C5. I can get roughly 1½ tone up or down (D# to G#, more or less). Something is terribly wrong, but the oscillators are perfectly fine. I reached out to KiwiTechnics for some guidance, and it turns out this was a PEBKAC/ID10-T error:

Sad ~~trumpet~~ synthesizer noises

Yup, I somehow managed to toggle key transposition when I first tested the KiwiSix and power supply board upgrade... After reading the manual and menu structure cheat-sheet, I disabled key transposition and everything was much better. Setting the master tuning potentiometers pretty much centered brings me close enough, and individual voice tuning took very little adjustment.

520Hz is close enough.

Success, at last!

Other adjustments

The Service Manual and the KiwiSix installation manual also give instructions for the headphones and line-level internal volume adjustments. I had to do fiddle with the offset and internal volume trim potentiometers in order to get a signal on the line-level output. I still need to re-check the signals going to the BBD chips on the KLM-368 since the effects are very subtle. It's not preventing anything from working, but chorus and phaser are practically identical and they should not be.

Time for cosmetic work

There's really not much I can do cosmetically to make it look better. It was cleaned and disinfected with 99% isopropyl alcohol, the stains on the keys were scraped off, the knobs where cleaned in Dawn, and the dust bunnies were evicted. I did find three ~~old~~ vintage Gibson "Thin" picks underneath the KLM-368 board.

However, most of the screws were severely rusted. The black oxide coating did provide some protection, but it can only do so much. Cleaning them out with soap and neutralizing the rust with vinegar had the side-effect of removing what was left of the black oxide layer. Cleaning didn't do much for the pitting that occurred on the screw heads, and that pitting made some screws brittle and just unsightly. I could not find any reliable online sources that would tell the sizes of the screws used in assembling the PolySix. The only source I found is just plain wrong and I ended up buying a screw size gauge and measure every screw I removed. Hopefully this helps some other poor soul to restore a rusted-out PolySix.

All screws are metric Phillips pan head machine screws. Some requires larger heads, but a fender washer (the larger ones) is good enough. I suggest you keep the original nylon washers as they can be hard to find new.

Bend and intensity wheels, underside

2x M4x50 with fender washer

Keyboard, underside

5x M5x20 with fender washer

Metal enclosure, top, corner screws

4x M4x15 with nylon flat washer

Metal enclosure, underside

4x M4x20 with fender washer

Control board brackets, screwed from top of enclosure

12x M3x6 with nylon flat washer

Power board, back of enclosure

4x M3x10

Power input plate (where the IEC and MIDI connectors are)

4x M3x10

I/O jacks support bracket

3x M3x10

Finding metric screws in Quebec is near impossible. Almost no hardware store carries them, and when they do, it's usually with Robertson heads (or worse: flat heads) in sizes nobody needs. Trying to find black oxide finish is even more complicated. The only reliable source I know is Canada Bolts. They have all the sizes of screws and washer you can think of, but they did not have any with a black oxide finish. Stainless steel is good enough and that's what I went for. Be careful when opening the package containing the screws. You don't want to slice your thumb open like I did...

The only thing left is to find a way to fix the back corners of the wooden sides to at least prevent them to degrade too much, but that can wait.

The end?

The PolySix is now fully functional and infinitely more stable than it ever was. The KLM-367 and KLM-376A-PS boards, source of 99% of the issues found on the PolySix, have been replaced and the voice board has been tuned. There are a few leftover issues, but nothing that is a showstopper.

There is still a slight noise problem. The ferrite clamps helped, but I will have to do "common ground" mod at some point. I do have the 18-gauge stranded wire laying around to do it.

The arpeggiator is behaving a bit wonky. It is extremely slow for some reason. It's probably a setting in the KiwiSix, but I didn't really look into it yet. The voltages all look fine and the LED is flashing at the correct rate, but it doesn't do much until cranked to at least 7. The LED on the button is also dead, and I'll have to replace it when I decide to work on that.

One of the voices (unit 0, I believe) has a failing envelope generator. It's working, but sometimes it's not envelope-generating much. Replacement SSM2056 chips are hard to come by and are selling for over 50$US. There are a few reliable sources that carries them, and I'll need to get a few, just in case. Might as well get some SSM2044 chips at the same time.

I'll keep working on it in the coming months, but for now, here's a 12-seconds recording of the very first chord it made:

Updated 2024-09-22 - edited for clarity.

Updated #2 2024-09-22 - a section I originally typed before first publishing the article mysteriously disappeared at some point. It's back now.

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Digital Clef

J.P.'s Gear Review v2.0 - Ep. 14 - Let's talk about vintage synths