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Korg MS20 Kit Mods

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Re: Korg MS20 Kit Mods

Postby Wonks » Wed May 11, 2016 11:54 am

Pics on the web showing its innards all look pretty discrete to me, though spread over multiple smaller boards.
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Re: Korg MS20 Kit Mods

Postby Wonks » Wed May 11, 2016 11:56 am

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Re: Korg MS20 Kit Mods

Postby BigRedX » Wed May 11, 2016 1:49 pm

Hugh Robjohns wrote:
BigRedX wrote:Have Korg added the ability to be able to modulate each oscillator separately on the latest versions? Because that certainly wasn't possible on the original MS20 which IMO doesn't make adding oscillator sync very useful.

On the ms20m desktop model, yes... But not the mini or the ms20 kit.

That's far more useful then.

It's a pity these features weren't on the original MS20. Once I'd got over the novelty of the patch bay and leads, it quickly because apparent that without other equipment to connect to it, there wasn't really anything anything it could do that any other 2 oscillator, 2 envelope generator synth of that time could do also (and probably more easily since the routing would have been done with switches rather than patch leads).
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Re: Korg MS20 Kit Mods

Postby Zukan » Wed May 11, 2016 1:57 pm

Hugh Robjohns wrote:
Zukan wrote:Why do you want to do all this H? After all, it's just an MS 20.

;)

I had a hankering to play again with a simple monosynth, away from the computer screen. Nostalgia!

I've owned various over the years, but mostly Rolands of various sorts; I've never had a Korg, and the opportunity arose... but there are few things that I'd like to do but which the Korg doesn't support directly.

I must say I'm very impressed with the quality of this re-issue. The mainly surface-mount components don't make it the easiest to modify, but it's still do-able... I think...

H

Go for it! Great fun modding simple gear into tasty muthas. Ask Martin.

I found the older synths easier to work with. I modded my Arp Solus but only with basic stuff as I'm not handy with soldering anymore and I am not a Jedi like Martin or Will.
I also modded my SH3a with a pitch wheel and got James Walker to get it to do a couple of other insane things but can't remember what they were.
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Re: Korg MS20 Kit Mods

Postby Martin Walker » Wed May 11, 2016 4:14 pm

Hugh Robjohns wrote:I must say I'm very impressed with the quality of this re-issue. The mainly surface-mount components don't make it the easiest to modify, but it's still do-able... I think

The MS20 mini was my first experience of modding a surface-mount design, and I can't say it was that enjoyable - I needed a magnifying glass, strong lighting, and an extremely steady hand :beamup:

One sneeze at the wrong moment and you can lose hundreds of SMDs in the carpet :blush:


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Re: Korg MS20 Kit Mods

Postby Hugh Robjohns » Wed May 11, 2016 6:20 pm

I'm making good progress... I've worked out all the connections now. I think I need more powerful reading glasses -- SMD really does your eyes in! ;)

I've decided to provide switches to modulate the PWM directly from the LFO or EG2. I won't bother with an external socket input for now, but can easily add it later if needed. Another switch will provide VCO2 Sync. I'm abandoning the FM mode for now as it gets very complicated very quickly when trying to separate out the mod inputs to the two VCOs. The Filter switch is trivially simple, and the last switch is to hold the VCA permanently open, which is handy if I'm processing external sounds or just don't want to have to keep a finger on the keyboard!

It appears that the MS20 Kit version has been designed with the option of bringing various useful signals and inputs to access pads on the PCB, but in most cases the final linking components (usually buffer resistors to or from op-amps) have been omitted, which is why most of the pads appear 'dead' when tested! Grrr...

The designated pads for the Sync option are not what I expected at all. They appear to extract the signal from VCO 1 after the oscillator's output FET buffer, and inject it into VCO2 in a different way to the simple routing diode method previously discussed -- I think its using a low-impedance part of the circuit, but by my reckoning the signal also needs to be inverted... so, not too sure about that yet. I need to build the thing and run a few measurements...

Hopefully, the only real modification to the PCB is the removal of a 0-Ohm resistor above the PWM pot, so that I can inject an external control voltage without it being pulled up to 15V all the time. Everything else just involves dabbing wires carefully onto test-point pads, op-amp pins, and a decoupling capacitor to pick up a +15V supply...

I can't squeeze the switches onto the front panel, sadly, there just isn't enough room, so I'll mount them on the top, where there's plenty of space for other controls or sockets if needed later.

H
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Re: Korg MS20 Kit Mods

Postby awjoe » Wed May 11, 2016 8:55 pm



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Re: Korg MS20 Kit Mods

Postby Folderol » Wed May 11, 2016 9:35 pm

A while ago, visiting a customers own engineering workshop, I saw the ultimate in D.I.Y. bench lamps.

There was a very large magnifier with a tightly packed ring of LEDs. This was suspended from an extreme angle-poise type assembly. Only this thing was jointed and balanced so well that it stayed exactly where, and at what angle you put it. There were no springs - just friction bearings and counter-weights.

Oh, and those LEDs? They were RGB+W. Not only were they very bright full on, but you could vary both the brightness and the colour temperature!!! It's astonishing how much difference the latter can make to the detail you can pick up.

After I'd finished drooling the guy said "Before you ask, No" :lol:

I'm very tempted to try building something like it myself. The electronics would be easy enough, but I don't have the mechanical skills to build the support system.

... now, where can I get a really big f*** off magnifier? :tongue:
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Re: Korg MS20 Kit Mods

Postby Wonks » Wed May 11, 2016 9:58 pm

Hugh Robjohns wrote:Hopefully, the only real modification to the PCB is the removal of a 0-Ohm resistor above the PWM pot

It's got superconductors inside? Wow! :D
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Re: Korg MS20 Kit Mods

Postby Martin Walker » Wed May 11, 2016 11:00 pm

Folderol wrote:A while ago, visiting a customers own engineering workshop, I saw the ultimate in D.I.Y. bench lamps.

That does sound great.

I bought myself one of these Redtone daylight lamps offering 10,000 lux illumination, which is very effective for its original design purpose and for soldering SMD components ;)

https://www.amazon.co.uk/Redstone-SAD-Light-Box-Medically/dp/B000YSDHZ8/ref=lp_248565031_1_5/276-6399338-5341435?s=lighting&ie=UTF8&qid=1463003821&sr=1-5

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Re: Korg MS20 Kit Mods

Postby Zukan » Thu May 12, 2016 8:52 am

No wonder you're permanently tanned.
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Re: Korg MS20 Kit Mods

Postby Hugh Robjohns » Sun May 22, 2016 9:35 pm

Just a quick update on the MS20 Kit modifications I discussed earlier. Managed to finish the mods and put the whole thing together today. Everything working as expected/wanted.

Here's a schematic of the mods I've introduced:

Image

Most of it is very obvious and straightforward.

PWM -- I have two switches, the first selects normal or modulated, and the second the modulation source, which is either the LFO (MG) or the second Envelope Generator. The LFO source comes from IC5 pins 1/2 (bottom left of main board), while the EG2 source comes from IC25 pins 6/7 (bottom right). The EG signal is attenuated slightly to make it the right voltage range.

The connection points are the marked pads on the rear of the main board (see below) The only modification required to the board is to remove a '0 Ohm resistor' (R275) just under the PWM pot (see image below). I simply cut it in half with cutters, and unsoldered the remaining bits! This resistor is there to provide a link between TP59 (marked jack switch, which is connected through a resistor to the 15V supply) and TP60 (marked jack in connected through a resistor to the PWM pot), and it needs to be removed to allows effective external PWM control. The switch effectively restores the 0 Ohm link in its normal position.

Of course, if you wanted an PWM control input socket instead of the selectable MG/EG2 that I wanted, you could simply wire a TS socket (with normalling contact) to the three pads. TP59 goes to the normalling contact, TP60 to the tip contact, and TP61 to the sleeve contact -- but you'll still need to remove the 0 Ohm resistor R275.

SYNC ON -- the Sync mode requires a little extra circuitry if you want to make use of the oscillator circuit buffering that's already on the board -- as opposed to the simple diode link referred to above. It's just a simple inverter built around one transistor.

TP56 (VCO1 sync out) connects through the switch to the base of an NPN transistor (I used a BC550) via a 1000pf ceramic cap and a 100K resistor. The base is coupled to the grounded emitter via a 47K resistor and the collector to the +15V supply via a 10K resistor. The inverted output from the collector is returned to TP62 (VCO2 Sync In). I derived the +15V supply from the lower side of capacitor C419 which is near the top right, just below the connecting loom on the top edge of the main board.

VCA HOLD -- This +15V supply was also fed via a switch and a potential divider (comprising two 10Ks in the top leg and one more 10K in the lower leg to ground) to the anode of D31 which is a control input to the VCA. D31 is just above and to the left of C419. The VCA requires 5V to be fully open, hence the potential divider! When activated the VCA stays fully open regardless of the EG2 settings. I find it useful when setting up a sound or processing an external sound, instead of holding a key down!

The only other mod was to borrow the three-pin header plug from a redundant computer fan and connect that to a switch to select the two filter modes.

The circuitry and wiring was all built on veroboard and mounted on the back of the switches which I fitted to the top surface of the front panel, aligned with the control sections they affect.

Image

Image

Hope that helps anyone else interested in tweaking their MS20 KIt.

I can't say how these mods relate to the MS20 MINI I'm afraid, but I expect the circuitry is very similar. The sync circuitry is essentially the same as that in the MS20M kit (keyboard-less desktop version).

Needless to say, any mods are undertaken entirely at your own risk. The mods as described work for me, but I can't guarantee they'll work for you! ;)

H
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Re: Korg MS20 Kit Mods

Postby Stephen Bennett » Mon May 23, 2016 8:40 am

Thanks Hugh—can we make it a sticky? I'll have a go in the summer.

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Re: Korg MS20 Kit Mods

Postby Hugh Robjohns » Mon May 23, 2016 9:59 am

Done

H
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Re: Korg MS20 Kit Mods

Postby Stephen Bennett » Mon May 23, 2016 10:55 am

Hugh Robjohns wrote:Done

H

Excellent—it's going to remind me now every time I come to the forum.

Regards & thanks

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