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Korg MS-20 Mini modifications... My head hurts

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Korg MS-20 Mini modifications... My head hurts

Postby vinyl_junkie » Mon Mar 13, 2017 6:39 pm

I want to bang my head on a desk now but here we go...

Long story short the MS-20 mini has the capability to have both the K35 and the later style filter fitted like the kit and desktop versions.

If you've ever taken one of these apart you would of noticed a large area of PCB space not populated by components but with PCB tracks, numbering etc..

Don't ask but crazy enough I have populated my MS-20 with said components but having issues switching between the two filters.

The new MS series uses NJU-7301 CMOS switches (U1 to U6), I wasn't able to originally source these so used what appeared to be 99% identical variations (NJU-201A)

First tests proved underwhelming, after a little nose around I noticed I needed to remove the 0 ohm jumpers around the CMOS switches, wit that done I got some noise but had "issues" switching between the filters using the jumpers.

So I then ordered the 7301's, fitted them and pretty much the same ol' same ol'.

What's the issue? I can't get it to reliably switch even just using hard wired jumper wires... She does strange things. The K35 filter sounds ok but the "new" filter is hard to select, some times silence depending how it switched, distorted or just a waveshaper lol
More on this when I get some time.. In the mean time some pictures

Click for high res images;

https://c1.staticflickr.com/1/644/33381897866_4a2406f072_o.png

Before I removed the 0 Ohm jumpers and other oh shit it doesn't work bodges.
https://c2.staticflickr.com/4/3835/33039315010_07f0763860_o.jpg
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Re: Korg MS-20 Mini modifications... My head hurts

Postby artgiles » Wed Apr 05, 2017 6:48 am

I have been going back and forth on buying a MS-20 Kit just to get the part values for the Rev 2 area and see if I could make it work on the Mini. I got linked to this from one of your posts on another thread about MS-20 mods. Thank you for the schematic!

I have two guesses on why Korg might have omitted the parts on the Mini. They could have already known a full size was on the way and decided to give the full size a bit more of an edge over the Mini in the market place. Or the circuit board was never fully functional in regards to Rev. 2 filter section.

The Kit boards are not just a spaced out version of the Mini. There is completely different routing when comparing just the back traces. So it may not work without full layout tracing to find faults. But let's assume it will work.

As I have no idea of your knowledge of circuits please don't be offended if I give some advice that is obvious to you. I truly just want to see you succeed.

So the first thing I'm curious of is how you are setting the two jumpers 10 & 20? The schematic shows that you have to use two jumpers at a time and rotate them to select one or the other filter. As in two jumpers vertical to select "old" filter and two jumpers horizontal to select "new" filter. So you feed +9v to close the switches for the filter you want to hear and connect GND to open the switches of the filter you want to silence.

A couple other things I would check are the trimmers for cut off calibration on the HPF and LPF PT16 & 17. One of them or both could be cutting off the signal if they haven't been calibrated yet. Is power getting to all the added IC's? Have you traced the path with an audio probe or oscilloscope to see where it stops working? Curious about what happens when setting the res to high and sweeping the frequency, any oscillation getting through?

I'm very interested to see if this works out as I'm more of a DIY'er so I like the challenge.

By the way your solder work looks great! Did you use an iron or hot air?

You might also try posting at http://electro-music.com/forum/ Lots of folks are into DIY and music electronics there and willing to help others.

Well I hope you get this going if you haven't already and please post back if you do or if you have more questions.
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Re: Korg MS-20 Mini modifications... My head hurts

Postby Martin Walker » Thu Apr 06, 2017 9:32 am

Hi artgiles, and welcome to the SOS Forums! 8-)

With such a helpful first post like that, you're quickly going to win a lot of friends here.

Oh, and welcome too to a fellow MS20 Mini modder (I haven't dared attempt the Rev 2 filter addition, as my SMD soldering skills aren't up to the challenge ;) )


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Re: Korg MS-20 Mini modifications... My head hurts

Postby artgiles » Sun Apr 09, 2017 4:09 pm

Thank you for the warm welcome Martin!
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Re: Korg MS-20 Mini modifications... My head hurts

Postby synthphreak » Fri Jun 23, 2017 12:16 am

Any word on this project?
Would be great to see if it will work!

Im also interested in other modifications. Would like to make my ms20 mini full modular.
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Re: Korg MS-20 Mini modifications... My head hurts

Postby Martin Walker » Fri Jun 23, 2017 6:09 pm

synthphreak wrote:Im also interested in other modifications. Would like to make my ms20 mini full modular.

Hi synthphreak, and welcome to the SOS Forums! 8-)

I've done quite a few mods to my MS20 Mini, as described on my web site here:

http://www.yewtreemagic.co.uk/soundworlds3.php

Image

I include details of the circuit mods on that page too.

Hope it's of interest!


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Re: Korg MS-20 Mini modifications... My head hurts

Postby baward » Sun Jun 25, 2017 12:20 pm

Martin Walker wrote:
Oh, and welcome too to a fellow MS20 Mini modder (I haven't dared attempt the Rev 2 filter addition, as my SMD soldering skills aren't up to the challenge ;) )


Martin

Martin's modifications are excellent.
Ben
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Re: Korg MS-20 Mini modifications... My head hurts

Postby Martin Walker » Wed Jun 28, 2017 3:46 pm

baward wrote:Martin's modifications are excellent.
Ben

Why thank you Ben (bows deeply and doffs his hat) ;)


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Re: Korg MS-20 Mini modifications... My head hurts

Postby synthphreak » Thu Jun 29, 2017 3:18 am

Hello Martin, and thank you!

I plan to do all those mods from your website. They sound fantastic.
Ive seen other mods from other websites i also may try.
Could you please explain the ring mod that you have on the top of your ms20. I didnt see a schematic for it posted on your website. Im just interested if its something useful i may use on my unit.

I am a big fan of the version 2 filter of the ms20. I had a vintage unit about 10 years ago with the version 2 filter but had to sell it.
For now, I use an old Frostwave Resonator connected the audio out of the ms20mini. Its much more smoother and easier to create bass tones than the dirty filter. I was hoping someone would figure out a working version 2 filter on the ms20mini for the diy crowd.

I wish there was something that can be modified for the sample and hold on these units. It seems to not sound musical in any sort of way.

Chris.
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Re: Korg MS-20 Mini modifications... My head hurts

Postby Martin Walker » Thu Jun 29, 2017 7:52 pm

synthphreak wrote:Hello Martin, and thank you!

I plan to do all those mods from your website. They sound fantastic.
Ive seen other mods from other websites i also may try.
Could you please explain the ring mod that you have on the top of your ms20. I didnt see a schematic for it posted on your website. Im just interested if its something useful i may use on my unit.

Hi Chris!

The ring mod is classic diode-based (I used Schottky diodes for their very load forward voltage drop), but with a simple input buffer/booster. Here's my circuit:

MW-Retro-Expansion-Ring-Mod.jpg


I do think it sounds more authentic/retro than the Cross Modulation already featured in the Korg MS-20, especially since you can input triangle/sawtooth waves as well as pulse ones for more mellow results, but you can hear it in action in my audio demos and judge for yourself ;)


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Re: Korg MS-20 Mini modifications... My head hurts

Postby synthphreak » Thu Jun 29, 2017 11:34 pm

Martin, thank you again!
I appreciate you sharing this information. I look forward to adding this to my ms20 mini.
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Re: Korg MS-20 Mini modifications... My head hurts

Postby vinyl_junkie » Mon Jun 18, 2018 12:07 pm

artgiles wrote:I have been going back and forth on buying a MS-20 Kit just to get the part values for the Rev 2 area and see if I could make it work on the Mini. I got linked to this from one of your posts on another thread about MS-20 mods. Thank you for the schematic!

I have two guesses on why Korg might have omitted the parts on the Mini. They could have already known a full size was on the way and decided to give the full size a bit more of an edge over the Mini in the market place. Or the circuit board was never fully functional in regards to Rev. 2 filter section.

The Kit boards are not just a spaced out version of the Mini. There is completely different routing when comparing just the back traces. So it may not work without full layout tracing to find faults. But let's assume it will work.

As I have no idea of your knowledge of circuits please don't be offended if I give some advice that is obvious to you. I truly just want to see you succeed.

So the first thing I'm curious of is how you are setting the two jumpers 10 & 20? The schematic shows that you have to use two jumpers at a time and rotate them to select one or the other filter. As in two jumpers vertical to select "old" filter and two jumpers horizontal to select "new" filter. So you feed +9v to close the switches for the filter you want to hear and connect GND to open the switches of the filter you want to silence.

A couple other things I would check are the trimmers for cut off calibration on the HPF and LPF PT16 & 17. One of them or both could be cutting off the signal if they haven't been calibrated yet. Is power getting to all the added IC's? Have you traced the path with an audio probe or oscilloscope to see where it stops working? Curious about what happens when setting the res to high and sweeping the frequency, any oscillation getting through?

I'm very interested to see if this works out as I'm more of a DIY'er so I like the challenge.

By the way your solder work looks great! Did you use an iron or hot air?

You might also try posting at http://electro-music.com/forum/ Lots of folks are into DIY and music electronics there and willing to help others.

Well I hope you get this going if you haven't already and please post back if you do or if you have more questions.

Reviving an old thread wooo, been a while eh...

Basically artgiles you hit the nail on the head. I was miss reading the schematic for the jumper and doing it wrong.
I used a regular Pace temp controlled soldering iron for all the work with a mini wave tip.
I think the part number is 1121-0490-P5, I love using these tips for all types of work.

After my switch blunder I was successfully able to switch between both filters which worked although required some tweaking other than tuning, more on this later.

The ironic thing is I didn't like the sound of the Rev2 and just put it back to Rev1, even with all that terrible hiss and DC bump that distortion is magical :lol:

Here is some one who's done the same mod and is far more knowledgeable.
If you watch the whole video and read some of the comments it'll piece together the last bits of the puzzle i.e. getting the OSC volumes correct and lowering the CV bleed.

https://www.youtube.com/watch?v=hauqT5cPUi8
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Re: Korg MS-20 Mini modifications... My head hurts

Postby Martin Walker » Mon Jun 18, 2018 4:32 pm

vinyl_junkie wrote:The ironic thing is I didn't like the sound of the Rev2 and just put it back to Rev1, even with all that terrible hiss and DC bump that distortion is magical :lol:


:headbang: :beamup:

Never mind - at least you now know that the Rev 1 is definitely the most characterful ;)


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