Project Korg Polysix (KLM369 & KLM370)

Breaking rules again and fixing new things before the boards I started are done. Tonight KLM369 & KLM370 were fixed. These are the boards that has the knobs for the envelope, filters, amp, waveforms et.c. These were very straight forward to fix. A few electrolytic capacitors and one 4051 on each board.

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The KLM369 located to the far left on the synth can be seen above. This one had a few connectors that was kind of hard to remove. But some bending with a screwdriver made the glue crack. All the knobs on the front has to be removed and the small screw-nut beneath the knob needs to be removed. Then these two boards are connected with a flat cable that can be seen in the picture below.

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This one is kind of hard to put back if the left card is put back first. It can be done but it´s a good idea to fix both these cards at once and then mount the KLM370 first, connect the flat cable between them and then mount the KLM369.

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Very few capacitors and chips on this one. All the pots and switches on these boards do work so no need to fix them now. The volume knob has some slight noise when turned but that kind of thing will be fixed at the very end.

After this step much of the harsh noise is gone. GREAT. The “hiss” is still there in large amounts but this was a great step towards a not so noisy P6. The acid arpeggio above was recorded when the unit was tested after these two boards were fixed.

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Project Korg Polysix (More capacitors)

A small change of plans this time. There was not enough time tonight to start replacing chips tonight. So instead all electrolytic capacitors on the power supply board were exchanged. After that the power supply was calibrated according to the excelent Korg Polysix service manual. So lets look at it before and after.

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Originally there´s two larger caps lying down and a bunch of smaller ones standing up. All of them are cheap standard components.

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After the replacement this board has only standing capacitors. There´s a VM connected to the checkpoints of the board (15v). It´s not that hard to calibrate everything to 15v, 5v, -5v and -15v. As soon as it was calibrated everything was connected and mounted back into the synth again. Too bad that the noise level were not a single bit reduced. There´s probably a problem in the amp section. We will get there in a while. At least the oscillators has some more bite and now they sound as a Polysix is expected to sound. Listen to this simple bass unison arpeggio for example:

At least now one of the boards are totally refurbished, no need to change anything more on the KLM376.

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Project Korg Polysix (CPU board capacitors)

Every electrolytic capacitor on the cpu-board is now exchanged to a new one from Panasonic. When the Polysix was switched back on everything seemed to work exactly as before but some of the program/bank buttons are dead or works sometimes. I guess this has nothing to do with my exchanged caps, the buttons obviously needs to be cleaned or exchanged. This has to wait for later. Actually the unit already started to sound better, I find that kind of strange but perhaps some caps actually were broken and that problem affected the audio in some way. There is still instability and noise so there is a long way home. Not much to see or hear in this stage. Now there is the final stage left for the cpu-board where all 4000-series chips (cmos) will be exchanged. This has to be done in several steps. When that is finished some real improvements of the audio can begin.

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Project Korg Polysix (New battery)

The first part of this operation succeeded.  I actually only made one little mistake wich was easily fixed. As you can see in the image below that hateful leaking battery is replaced with a CR2032 Lithium Battery that is placed in a socket. I actually had to put the battery upside down in the socket since the socket itself could not fit with +/- in the right direction. Thats really a shame but I promised myself to mark the socket in some way before I close this project.

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To the left of the battery there is now an empty socket where the damaged 74LS08 used to sit. To the left of that socket there is another new 74LS08 seated in its socket. I changed both of them when I was at it. Actually you can see the only mistake I made in this picture. Yes I forgot to put one of the 74LS08 chips back. When I switched the Polysix back on 4 of the 8 Program switches was lit. When that chip was put back in its socket everything started working. Finally to the right of the battery there used to be one capacitor (C40) that is now removed and just above that there used to be a resistor (R91) that resistor is replaced with a diode. The purpose of this diode is to protect the battery from voltage when the power is switched on. The new battery is not rechargeable so feeding it with power is a bad idea.

Once I had the Polysix up and running I downloaded the factory presets as wav-files and send the via my soundcard to the synth. I had to turn up the volume once but the Polysix updated the banks with the factory presets almost at once. Nice engineering with tolerance Korg!!! I never owned a working Korg Polysix so I had no idea what the presets sounded like. But now I know. They sound like shit! Well they are obviously from the eighties and my Korg Polysix still sounds very bad and noisey. But that´s what this project is supposed to fix in the end. Listen to this as I switch through the presets and in the end turn to a sawtooth and try out the filter and finally the chorus effects.

This Polysix sounds kind of weak to me. Some patches suddenly has some obscene clipping, perhaps they were in unison I didn´t check. And the noise level is terrible. Well there is plenty of work to do. Nothing that I do to the cpu-board will help with the bad sound but I am going to replace all the electrolytic capacitors and all of the 4000-series chips to expand the lifetime of this unit. I will report back as soon as the caps are exchanged. Soon enough I will try to get rid of the noise.

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Project Korg Polysix (Analysis)

I got my hands on some 30 year old vintage analogs. I will try to refurbish them and incorporate them in Interdictors future productions. One of them can be seen below. A Korg Polysix that seems to be produced in 1984. I am going to do a total makeover this one. I will document every step I take with this classic synth and it´s going to start today with some inspection and analysis. So here it is in all it´s glory.

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First of all I connected it and tried to browse through a few presets. I tried all buttons to find out what state it was in. The presets were sounding odd. I had a hard time to use some of the buttons and the lfo seemed to modify stuff that it wasn´t supposed to. I tried creating some patches but some leds where blinking in strange rytms and the noise level of the instrument was terrible. Some typical signs of a common Korg polysix problem, battery leakage. As you can see below two black keys were tilting and broken and the mod wheels were in pretty bad shape. The wooden sides are damaged from some touring and the arpeggio latch is totally broken. Overall it´s in pretty bad shape.

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So it was time to look inside. You  can find a very good guide of how to open the synth over at Oldcrows. In the four corners of the front panel you will find the screws that hold the front panel in place. These are the large screws. Remove these. Then it´s time to look under the synth. There are four more screws in the front end under the keyboard that should be removed. Now you can gently tip the control panel up in a “hood-up” position and start looking inside the unit.

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You can see above what it looks like inside. Located as the second board from the left is the KLM-367 board. That´s the cpu-board that more often than not has some evil battery leakage. So we start by removing this board for some further inspection. First of all we need to desolder a black ground cable that is soldered to the top left of the board. Then all connectors must be decoupled. Since the connectors are fastened with glue we need a a scalpel or some other sharp knife to crack the glue and then gently pull out the connectors from it´s sockets. Finally there are four Philips screws holding the board in place. Remove these screws. The board is now loose and can be slided upwards away from the keyboard and finally lifted up.

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So this is what it looks like.Beautiful engineering from the beginning of the eighties. Notice the large blue battery in the bottom of the picture. And to the left of the battery some of that infamous battery corrosion. This wasn´t that bad I guess. The leakage needs to be cleaned and as you can se the chip to the left of the battery a 74LS08 is affected by the corrosion so it´s best to change that. This is what I am going to do until the next time:

  • Desolder and remove the battery
  • Desolder and remove both the 74LS08
  • Put in a new battery using a socket
  • Put in 2 new 74LS08 using sockets
  • Try to read back the presets to see if this part of the refurbish procedure was successful

I will be back with some news in a day or two!

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