Discussion about the Korg PolySix synthesizer

Hi Hans,

The voltages from the power supply are good, and I have tried the initial part of the calibration procedure several times as I don't have an oscilloscope.

The DAC part never works correctly: I have lights 1 & 5 On only, and varying VR6 and VR7 does nothing...

Yash

Hi Bob,

I have no oscilloscope as yet, but a nice digital voltmeter and a cool Logic probe by Elenco, both very affordable.

Not too sure how I could use the Logic Probe to detect any faulty IC, but looking to try that, so any help with this is much appreciated.

When I check the voltage sweep from the center pin of the resonance pot (all other pots to 0, i.e. Min value), I get 5V -> -5V (Res 0 to Res 10).

The data pin on the board left to it shows a smaller value sweep: -4.37V to -5V, so I guess that means the multiplexing is working?

When I check the voltage on KLM-367 - IC9-P7 I get the voltage stuck on 0.35V in normal mode, and stuck on 10V in Manual mode.

Yash

As for the DAC, in the KLM-267 board I have its a MC1408.

I also received a new DAC chip from Newark Canada (that's where I am), and it's the DAC08-CPZ from Jed's KLM-367 clone board parts list.

One thing I noticed though is that in the clone board IC-33's pins 7 & 8 are connected, whereas in the original KLM-367, pin 8 is floating.

Yash

For Cutoff on the Potentiometer boards, I get: -5V -> 5V (Cutoff 0 to 10), and -4.36V -> -3.74V on the data pin.

IC33-P8

Starting with all pots at Center position, I do get 0V on the data pin.

All pots to Min: I get -4.35V

All pots to Max, I get 4.35V.

Since Res is the only pot which works the other way round, I get this is ok?

Avg sweep for the Cutoff pot, I get a variation from -4.35V to -3.73V...

I was using the Res line on KLM-367 as it seems it is the easiest one to verify, but I agree, I should focus where the issue seems to be and if it's the DAC, investigate there. Also agree with the need for a scope...

Yash

Funny, I used the logic probe on CPU pin 1, and suddenly I get control over Cutoff, Res and EG Intensity (but it's flaky, as sometimes it happens).

Nothing changed, except the keyboard got disconnected and reconnected.

All right, I'm going to check these., the list is fine as the mails are opt-in, and if it helps somebody else, it will be good.

Not sure I am ever going to get this Polysix working though...

IC27-P1: stuck on 3.88V in Manual mode, stuck on 0.88V when in normal mode.

You bet! Going to find if I can get one.

There does seem to be some funny things happening around CPU-P1, R72, D10 and even D11 (both ends of the latter are high...).

Will head out to the P.O. for a while and will be back to test these.

Bob, how do you reckon I should test all these? I'll first check the traces again I think, then I'll perhaps need to replace some of these.

Yash

Made a mistake earlier referencing the two pins for IC 33 (the DAC), because of course, if you're looking at the board from below....

So, on the clone boards, Pins 1 & 2 are connected.

On my KLM-367, Pin 2 is connected to a smaller rail underneath the board, but Pin 1 isn't connected to anything...

Weird no?

No worries about the CPU, I'm taking as much care as I can, and I do have a spare one with code.

Something else I found while using the Logic Probe after searching the archives for 'Pulse Train', is that while going through CN10 - P1-14, I get pulses on all except Pin 5. This leads from a green wire to the Bank/Program board.

The KLM-367 traces for CN10 - P5 are ok.

Not sure if this is normal or not since the Banks are selected by Switches, nor what I should do here.

Yash

Did a few things (see below the soundcloud link) and here is what it sounds like (it does sound like a Polysix should sound to me judging from Youtube videos but you'll hear me looking to make the Cutoff latch on by varying other pots like Resonance):

https://soundcloud.com/yashn/korg-polysix-resurrection?utm_source=soundcloud&utm_campaign=share&utm_medium=email

Sometimes, from Cutoff Min to Cutoff Max, you also get some Resonance, even though Res is at 0. Also, the Cutoff sweeps up a while, then maxes, then near Cutoff 10, it warbles rapidly. Other pots do seem to be affecting Cutoff sometimes too, like the Effects Intensity pot.

Checked the diodes and resistors around CPU-P1, they look good.

Found that the diode installed for the battery replacement was not connecting well via the via. Reflowed and pushed the diode lead.

Installed new battery holder with leads that I salvaged from my PC motherboard. Goodbye PC. Added new coin battery. Now I can store my Polysix sounds and recall them.

Reflowed all joints on CN10. Now I have a Pulse Train on CN10-5. Momentarily lost Pulse Trains on CN10-6 and CN10-7 but now it seems it's working.

I guess a calibration procedure is in order.

Another thing I'm thinking of is bridging Pins 1 & 2 of the DAC. It looks from the MC1408 that both should be grounded, but.. on my KLM-367, only Pin 2 is, Pin 1 isn't connected to anything...

Yash

Ok, will leave P1 of my DAC as is.

Good progress for today, so thanks a lot, Bob.

Yash

That rings a bell, Bob: I can see Malte also once wrote to me saying the signals are multiplexed and passed through CN11, and that there could be a malfunctioning 4501. In fact, in the thread, I even write myself that the 4501s on the control boards need to be checked (I most probably saw this from an even earlier thread with Andrew troubleshooting somebody's issues), but I think that in the meantime, I forgot about this and thought they were 4051s on the KLM-367.

Now, I do have a couple of brand new 4501s that I had ordered from Newark!

So the plan is to remove the control boards and swap the new ones in as from tomorrow.

Will give a heads up of the results.

Fingers crossed! Those sounds I got today were well worth it.

Analog, analog, analog...

Spent some more time with the control boards: socketed the 4501 so that I can swap more easily. Tested with the new 4501 and it does look like I get control over the pots more easily than before, but it is still flaky.

I checked all the D and P lines with the Logic Probe, ensuring that I get a pulse everywhere, so that's good.

CPU-P1 does not provide a pulse but a varying level now.

Did calibration again, this time making sure that EG intensity was at 0 and not at Min (an easy mistake to make!)... Of course, not having an oscilloscope blocks the total calibration. I tried using my Arduino for that but the oscilloscope which did work does not give measurements precise enough. On the other hand, I could see the waveform when adjusting MG.

Based on the schematics, I gather that all the pot values are multiplexed by the 4501s on the control boards, then sent through CN11 as INH | A| B | C | DATA to KLM-367 so that the CPU uses them as well as the 4501s at IC18 and IC19 to Demux them, and there's a comparator involved too.

I don't have enough new 4501s to swap them all, which might have been cool, so I may have to test them individually in a circuit.

It would be interesting to know which end points I can test by either the Logic Probe for a Pulse Train or else with the Digital Voltmeter.

Yash

When re-doing the calibration, at the DAC level, I still get only LEDs 1 & 5 On, and no amount of variation of VR6 and VR7 changes a thing, so I still also want to make sure the old MC1408 DAC is still good.

Actually, while searching the archives, a post by Andrew confirms what I found in the datasheets and also the discrepancy between my KLM-367 board and the clone boards which use the newer DAC08-CPZ:

Old board (which probably uses either MC1408 or another old DAC mentioned by Andrew): DAC Pin 1 floats, whereas the applications in the MC1408 datasheet show P1 grounded (yes, I have seen Pin 1 as NC in that datasheet for one version of the DAC, but the applications show ground).

DAC08-CPZ definitely needs a grounded P1 according to its datasheet. Hence, simply plugging DAC08-CPZ in my board would not be good enough: I will have to jumper P2 and P1 for grounding.

Yash

Not what I meant: when the service manual says to put all the sound settings at normal, almost all of the pots have to be put at Min (0 on the panel). On the other hand, the EG Intensity, while at 0 on the panel, is actually not at Min position, but in mid-position.

It is easy to mistake that 0 by turning it fully anti-clockwise, which I was doing, thereby making an improper calibration.

So, when doing the calibration procedure, one must ensure that EG intensity is actually in the mid-position (and at 0) and not at Min position on the panel.

Yash

Looks like I've got a faulty Zener diode D2... In circuit, it passes through 3.5V instead of 3.9V. 

Tested off circuit, it's giving me Infinity with the Digital Voltmeter in Diode/2K mode.

Now rummaging for a part in my salvaged stack and in the couple of Newark boxes if they have indeed shipped me this.

Yash

Replacement Zener found and installed at D2, continuity tested around it, part of calibration procedure re-done, when calibrating the DAC, LEDs 1 & 5 are still on no matter what.

Spent a good time reflowing and checking KLM-371 too.

So now it feels like I am nearing a complete repair since the Polysix sounds really good and clean, and when I do get control over the pots, it's much fun.

Somehow the signals get garbled up, so if swapping the two new 4051s does not help, I will also try a swap of the CPU as well as testing each 4051 individually on a breadboard.

Yash

On 08.01.2014 14:52, josh.nursing@gmail.com wrote:
>
>
> Replacement Zener found and installed at D2, continuity tested around
> it, part of calibration procedure re-done, when calibrating the DAC,
> LEDs 1 & 5 are still on no matter what.
>
>
> Spent a good time reflowing and checking KLM-371 too.
>
>
> So now it feels like I am nearing a complete repair since the Polysix
> sounds really good and clean, and when I do get control over the pots,
> it's much fun.
>
>
> Somehow the signals get garbled up, so if swapping the two new 4051s
> does not help, I will also try a swap of the CPU as well as testing each
> 4051 individually on a breadboard.
>
>
> Yash
>
>
>
>

Sounds like a good idea, Bob, thanks. Will try that after re-capping the power supply, and doing some continuity testing on KLM-366, as the board seems to have some of the symptoms of KLM-367, i.e. some darkened tracks, some green on resistor legs, one chip which has very blackened legs in the Key assigner section, etc... Touched everything up with some vinegar for now to neutralise the corrosive base from the battery.

Speaking of KLM-366, I was following the VCF Mod line up to CN-05 and I get something like 7.35V to -6.35V on a complete sweep of Cutoff. It looks like I miss a part of the range here although the change is smooth.

However, assuming the range is fine, I tried the Logic Probe on IC15 on KLM-366 because a member mentioned this on one of my previous threads. It appears that on KLM-366 and IC15 ( I forget which pin ) but it warbles, just like the Cutoff/Res does.

This suggested to me that there might be an issue on KLM-366 itself, hence the need to check it out.

Yash

Johannes, many thanks, this is just the kind of information I am looking for. After the power supply recap and the KLM-366 check, I will go through this procedure.

I have found a lot of great information in the archives thanks to your post too.

The test are time-consuming but I am learning a lot in the process. I used not to like the electronics class as I was more into software, but now I do!

Yash

Not sure I did this correctly as I swept one knob from Min to Max, then put it back to Min, then swept the other knobs similarly for each pin, because then I do not get cumulative voltages for controls working together, but for preliminary diagnosis, it's ok. I feel like a Korg Factory QA employee.

Here are the results (I also have it as a large PDF if needed. the blank squares indicate no change from initial first read of first row when probing): 

https://drive.google.com/file/d/0B9VKnWFwKzxKVHJkd1I1M1hMRGc/edit?usp=sharing

IC18-2 (Res CV) picks up all of the signals apparently, so I guess a track checking around this is in order.

Also, when monitoring and changing EG Sustain, it will rise fast initially, but settle on the Min voltage quite rapidly. Same thing for EG Attack and EG Decay. MG Delay does this too.

As I probe the Cutoff pin, the filter opens.

Yash

Here is what a Cutoff sweep from Min to Max sounds like in my Polysix for now. You will hear quite a bit of noise in the form of fluctuations in the signal. In the Min position at the beginning, what you hear is probably some Cutoff or VCA or MG bleed as calibration wasn't up to par.

https://soundcloud.com/yashn/korg-polysix-cutoff-sweep

Got to check the traces around IC18-2 (Res) and also try to follow the rest of Johannes's instructions.

Yash

Interesting results, Bob: at the DAC calibration step, only Effects Speed makes Leds 1 light up to 4 (in the very first values near Min), all the while LED 5 staying put, and this only when Cutoff is at 0. If I put Cutoff at Max, then the LEDs stay at 1 & 5 On.

I reckoned from the Service Manual that VR 7 makes the two LEDs 'open sideways' until they reach 1 & 8, so I used FX Speed to set the LEDs 4 & 5 ON, and the turned VR7 (which I had reflowed and continuity-tested recently), and it does work.

So, both VR7 and VR6 do work, it's just there's an issue at or near the line which deals with LED 5 onwards.

Any thoughts?

Yash

Meant IC18-12 instead of IC18-2 for Res.

Very good advice as usual, Bob, thanks. Will do that right after the current KLM-367 INH/DATA/P2x check and I am also trying to fit a jumper/socket for the large GND cable, because it is a lot of trouble to desolder and re-solder that cable when troubleshooting. If that works, I'll put some pics in the files section.

I checked *all* the IC18 connections after building the matrix of Pots vs Opamp Pin for the CVs as shown in the document I linked to earlier based on Johannes's instructions - no issue, no apparent short (pin 7 & 8 or 6 & 8 do show some resistance values), removed the chip, cleaned the area around and the socket and reflowed the latter.

This implies a faulty 4501 @ IC18? Will have to try.

I did try a swap of the 4501s int he front panel, and reverted back until I can pinpoint rationally and understand myself where the issue is since I've got only two new ones. I do have 2 old stock ones too, but if I make sure the lines are fine first, then I'll be more confident to make proper swaps, knowing if I'm swapping a good one for a faulty one.

Examining KLM-367 again yesterday, guess what? Greenish residue on all the 6 electrolyticcaps on the left side of the board.... Either the caps themselves leaked or the battery alkali attacked them. The caps on the right seem fine as I examined everything with Macro photos and zooming in.

Couldn't find enough proper caps in my Grade A salvage box, so had to dive into the salvage circuits. Found some great Elna and Nichicon caps in DVD players that people threw away next to my door. The Elna didn't fit cap-wise, so the Nichicon were usable.

Re-capped using 4 Nichicons and 2 caps from a previous salvaging operation, of a more dubious brand. Was happy to have the Nichicons for the crystal and near the Opamps, and used the other ones for the power rails.

Tested continuity for *all* of these connections and ensured they were all good. It looks to me that this is a must-do because on following the lines, these caps deal with a lots of the important chips and traces, including CPU-P1, etc... If these caps are mis-behaving, then a lot of issues can occur on the board. I have hope here.

Thanks a lot for all the valuable advice, it helps a lot.

Breakfast, then more troubleshooting/hacking.

Yash

What could be the reason that IC18-P12 (Res) picks up the signals from all other Pots?

There do not seem to be any continuity or shorts problems around IC18, and the traces for P2.0 - 2.3 & INH2 are fine. The CPU should be okay too and I swapped new 4501 at IC18 & 19 at one point, and a second time reverted these and used the new 4501 on the two left-most panels.

Doesn't this narrow it down to either the DAC or the comparator at IC6?

Yash

I can understand your confusion: there are several simultaneous issues and different posts in this single thread detailing all the issues, including new ones I am finding and fixing along the way.

Yes, the DAC calibration does not seem to work since that LED 5 is still stuck.

However, in Normal mode, changing the front panels 4501s to new ones resolved nothing: I believe the front panel is sending the proper signals.

On the other hand, the Demux troubleshooting shows that IC18-P12 (Res) always has a varying voltage, no matter which pot is moved, so I believe the issue is narrowed to IC6 and the DAC area IMO, possibly IC6 providing erroneous Data for the CPU's T0. This, in combination with two potential issues with the DAC area (lack of proper calibration, Pins 1 & 2) could cause this.

I will be stuck on the IC6 replacement: I checked and do not have the component. I guess I can still socket it for now and test it separately until I get a replacement part. The other thing I will test is the DAC area for sure. It will be an easy mod to test the board with the DAC08.

I will go through the resistance check you mentioned concerning the Px paths between the front panel and the KLM-367. If we can get values to compare, that would be great.

If I can get IC-18-P12 not to pick up voltage when I am using Cutoff that will be a good step as it is currently picking up voltage from all pots!

Yash

I never said IC6 sends CV, I only said it's connected to CPU's T0 through the DATA signal.

Also, I didn't find any other pot than Cutoff or FX Speed which changed the LEDs on DAC calibration, but did find interplay between the two (which appears to be expected), but my LED 5 is still stuck ON.

Haven't ruled out some issues with the front panels completely yet since I will be doing the resistance measurements.

Specifically, I want to understand what could cause IC18-P12 (Res) to always have voltage variations no matter what potentiometer is turned. First thought, it's a short somewhere, but it could be something else.

Modded KLM-367 to use the new DAC chip and testing calibration again, plus more exploration on the leftmost front panel, and perhaps the resistance values check is the plan for today.

Yash

What are the measurements at D5 and D6 in-circuit?

I get values both ways in-circuit. Lifted one end of each but the diodes seem good when tested that way. Re-soldered them and get values both directions again. Had to replace R43, and fortunately I had plenty of them in my Newark order.

Tested continuity on both the Volume front panel and the Cutoff front panel. There does not seem to be any issue there.

Finally gone round to removing that CPU socket after verifying that I did get a new 40-pin socket (of which I don't like the build and the pins too much, but it will have to do). Also had to get a new Weller soldering iron as my previous one's tip was damaged. The little bits of solder stuck in the vias are very annoying. I still have about 10 vias I need to clean more before I can envision putting in the new socket and re-testing continuity for the CPU, before mounting everything in the Polysix again.

Yash

CPU Socket replacement took a very long time: it's really hard to first, remove solder and then empty the vias. I sometimes used a mechanical pencil lead to help in addition to a solder bulb. Tested continuity of the CPU pins, and apparently only one trace was damaged during the operation, so I had to add a jumper for CPU-30 to C11-1 and to C10-5 if memory serves me well.

Pulse trains on C10 are okay. Pulse on CPU-1 isn't, but sometimes this comes back.

The Polysix reacts mostly the same: no control for a while, until some combination of the pots make them 'latch'. I noticed loss of control of some pots like MG: MG seems fixed at high frequency.

The Cutoff pot seems to have less noise now, i.e. less affecting Resonance, so I guess that's a plus. When it sounds, the Polysix sounds great.

I guess my next step would be to test voltages again between the pots and KLM-367. Oh and also socket IC5 and IC6, but I've done an overdose of desoldering since yesterday, so I may leave that for another time.

Yash

I know, and there does seem to be some shorts somewhere, assuming all the chips are functional (remains to be confirmed), not to mention any potential short I may have additionally caused by replacing the CPU socket.

When I did continuity testing between C5-9 and C5-10, 11, 12, 13 when powered on, I got connections there. This isn't normal, is it?

Yash

Getting closer (see below)! Unsoldered and socketed IC5 & IC6 and checked continuity thoroughly.

Noticed a few things along the way: I thought all Electrolytics caps on the right side were okay, but hiding beneath the negative leg of C19 were some greenish residue and dust... Didn't have any spare 0.33uF/50V in my box so used the next closest one: 0.47 uF/50V Nichicon. Noticed that on clone boards, a tantalum is used. I may have this, need to check values with those cap codes.

Each of C1 and C2 (new caps I had to put in) had a pin unsoldered today, so had to redo those as well.

I tested the resistance of R17, the resistor array and two pins were giving me infinity. Reflowed and re-tested. Fine.

Now, on powering on the Polysix with IC6 and IC5 swapped, I get all LEDs ON, and this won't change no matter how long I wait.

Swapping back the previous IC6 in its IC6 socket reverts the Polysix to its previous functionality. It powers on with one voice activated, but I guess that's just the Reset/VR1 calibration.

Therefore, IC5 is most probably bad. Maybe IC6 also has some issues. Too bad I don't have any spare LM393! I saw a few ones on the voice boards, but I'm loather to use those. I don't want to touch the voice board yet if not absolutely necessary.

Next I will test each of these LM393 comparators on my small breadboard to pinpoint where the issue is.

Onward!

Yash

Meant IC6 is most probably bad, especially since when I put it in the place of IC5, I cannot even do the reset procedure.

Yash

Received a small batch of new LM393N comparators that I swapped for IC 5 & 6, and TADA! All pots working, clean VCF Cutoff sweep, Cutoff is not bothering Resonance, etc... 

Of course, I was nearly sure of some improvement of swapping in new chips based on my previous swap of IC5 and IC6 giving me two different machines.

Here is a link for the new Cutoff sweep: 

https://soundcloud.com/yashn/cutoff-sweep-normal-with-res

During minimal testing tonight, I think there is still some issue: the MG pots might be interfering with the Arpeggiator speed, which was working perfectly before, so I will look into that.

So Korg Polysix Resurrection done, and 'sans oscillator' although I won't be able to calibrate it closely without one yet.

Next step: calibrating the beast, I am confident the DAC calibration will work now and that I will have no more LED 5 stuck, which will help with the rest of the calibration and functionality.

Once calibrated and tuned (I may have to do the tuning by ear, referencing my Kurz), I may do some of the additional mods listed by Terje and others.

Happy.

Yash

Tried the calibration again, and as foreseen, no issue with LED 5 being stuck on the DAC calibration step, 1 & 8 they are.

I don't understand the part of KLM-366 calibration with VCF and Resonance: it seems that Cutoff is at Max at the knob ('Normal') but the manual says to look for 0V at CN5-7.

My voltage sweep seems to be off: I don't get the full range of -5V +5V for Resonance on CN5. I don't know if that is correct.

Calibration without an oscilloscope cannot be done properly, so until then, I would like to thank Bob and Johannes and all those who responded and helped, including those who shared their experience in the past.

Regs,

Yash

All pots working, tried tuning but it's not yet satisfactory. Love the fat SSM sound, even in Poly and not Unison mode. Here it is with some custom patches:

https://soundcloud.com/yashn/polysix-resurrection-fully

Yash