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 Forum index » DIY Hardware and Software
Korg MS20 Oscillator Sync Mod
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ericcoleridge



Joined: Jan 16, 2007
Posts: 889
Location: NYC

PostPosted: Thu Jan 25, 2007 2:46 pm    Post subject: Korg MS20 Oscillator Sync Mod
Subject description: Anyone here know of a mod for osc sync on MS20?
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I've been thinking of doing a little work on my MS20, such as PWM, RM input, and individual waveform outs, but one other thing I'd like and can't figure out myself is how to achieve Oscillator sync.

Has anyone here attempted this mod on an MS20?

I can post a schematic if anyone wants to check it out.

Thanks
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Rob



Joined: Mar 29, 2004
Posts: 580
Location: The Hague/Netherlands/EC
G2 patch files: 109

PostPosted: Thu Feb 01, 2007 10:53 am    Post subject: Re: Korg MS20 Oscillator Sync Mod
Subject description: Anyone here know of a mod for osc sync on MS20?
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This one is really simple, takes only a single diode like a 1N914 or 1N4148. And a simple toggle switch to set sync to on or off. Very Happy

Regrettably it is not so easy to let the second osc sweep to get a nice sync sweep. For this you need to add an extra pot, and there is ample room for that. There is no standard way to sweep the second osc (without sweeping the first osc) without a modification. Sad

I will try to find some time this weekend to open my MS20 and take a picture on where to connect the diode. Please be a little patient...

/Rob
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ericcoleridge



Joined: Jan 16, 2007
Posts: 889
Location: NYC

PostPosted: Thu Feb 01, 2007 12:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks so much Rob. I had a feeling sync would be simple enough --it has been on other synths I've modified-- but I couldn't for the life of me figure it out.

I'm really looking forward to checking out your mod; But I'll be patient Sad .


As far as sweeping Oscillator 2, isn't there a dedicated CV jack for Osc 2 immediately to the left of the main Kybrd CV input--under the output volume pot? I don't have my MS20 in front of me right now, but I could have sworn there was...

anyways, thanks again.
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Rob



Joined: Mar 29, 2004
Posts: 580
Location: The Hague/Netherlands/EC
G2 patch files: 109

PostPosted: Thu Feb 01, 2007 4:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

ericcoleridge wrote:
As far as sweeping Oscillator 2, isn't there a dedicated CV jack for Osc 2 immediately to the left of the main Kybrd CV input--under the output volume pot? I don't have my MS20 in front of me right now, but I could have sworn there was...

anyways, thanks again.


True, but that one will disconnect VCO2 from the keyboard voltage, it is a V/Hz override input over the keyboard voltage.
What one would like is to have both VCOs track the keyboard and additionally sweep VCO2 higher in respect to the keyboard with an extra CV. One would need an external CV mixer to mix the Keyboard Output CV and an Env CV and then route it back into the VCO2 CV input you mention. But you can not do that directly on the MS20 without that extra CV mixer.

You should look at the schematics of the KLM-127 board from the service manual, no difference if it is an (old production) or (new production) schematic.

If you have the schematic in front of you, look at the oscillator sections. You see that both sections have a diode between Q4 and Q3 on VCO1 and Q9 and Q10 on VCO2. To hardsync VCO2 to VCO1 all you have to do is connect a diode from the Q4 collector to the Q10 base. The discharge pulse from VCO1 will now also discharge VCO2 with the anode to Q4 and the cathode to Q10. Works like a charm.

And now that you are looking at the schematic you can also see that the VCO-2 CV IN will disconnect the keyboard through the switch that is in the connector (the little arrow that connects to resistor R41).

To add an env signal to sweep VCO2 you need to connect a resistor of e.g. 47k to 100k to the emittor of the right transistor of the Q7 double transistor (pin 1). The other side of the resistor goes to the taper of a pot. The pot is connected to ground and to point 27 or point 28 at the outputs of EG-1. These are marked on the circuit board. A linear pot of 50k should do the job. The resistor should be mounted close to Q7 and not close to the pot.
But you will have a hard time finding a spot on the front to add an extra pot. You could actually put the pot on the top side of the MS-20, but personally I find that a bit ugly.

I did find a logical place to put a switch for the sync option:
Posted Image, might have been reduced in size. Click Image to view fullscreen.
A small, mini-size switch just fits.

Under this switch I drilled a small hole through the PCB where two wires coming from the switch go through. There is some space for this hole about where the switch is located.
The wires coming from the switch are connected to one side of the diode and the point on the other OSC. Always, use shrink tube to isolate the soldered connections (eg. between the diode and the wires) to prevent possible short circuiting after moving the synth around.

You need to use thin shielded wire and earth the shield of the wire to a ground point on the circuit board to prevent the oscs to sync when they are tuned in close unisono while not in sync mode. If you instead would twist unshielded wire to the switch the capacitance would be big enough to have the OSCs sync while the switch is open and the OSCs are detuned a couple of cents.

Still, even without an envelope sweep the sync does add quite nice extra timbres to the MS-20. And you can manually sweep VCO2 by its Pitch control.

If you understand what I wrote and think you can do the sync option now, without me having to open the case to make a picture I would be very pleased if you would notify me. Its a hassle to dismantle the synth and I'm quite busy, so...
Regrettably I also do not have a scanner, so I cannot publish an annotated schematic showing the option.

Let me know about your exploits.

Success,
/Rob
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ericcoleridge



Joined: Jan 16, 2007
Posts: 889
Location: NYC

PostPosted: Fri Feb 02, 2007 12:04 am    Post subject: Reply with quote  Mark this post and the followings unread

Rob wrote:

If you understand what I wrote and think you can do the sync option now, without me having to open the case to make a picture I would be very pleased if you would notify me. Its a hassle to dismantle the synth and I'm quite busy, so...
Regrettably I also do not have a scanner, so I cannot publish an annotated schematic showing the option.

Let me know about your exploits.

Success,
/Rob


Rob, this is very gracious of you to tell me about this mod in such great detail. I don't yet have the schematic in front of me--- but I believe that there is nothing more that a picture would add to the description you have provided. I certainly don't wan't you to have to open up your MS20 for this purpose.

However, I may need to ask you a question, after I look at the schematic as it relates to your above notes... I'm still just finding my way around electronics---which is why I was incapable of figuring out where to link the oscillators... but your description is great, and I think I'll have it now.

Thanks again--I'll post again and tell if it comes out...[/i]
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dvdbr75



Joined: Jan 22, 2009
Posts: 1
Location: boston

PostPosted: Thu Jan 22, 2009 6:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

I posted a message on the AH mailing list to help with some questions on this mod. I'm posting it here as well.
The mod works pretty well but there's a few things I am hoping to improve:

1. The waveform glitches at some points when changing the tune of the slave VCO (it is modified so can sweep it independently). It appears to want to glitch more when the two vco's are close to multiples of each other (when slave VCO is 2x, 3x freq of master etc.. ), although it can glitch at other times. When I say glitch I mean you can actually see the waveform jumping back a forth a bit on an oscilloscope, and it is audible. Any idea what causes this?

2. I lose sync after a few octaves. There seems to be a finite maximum
frequency that the slave vco can get to before the sync breaks down a cuts out. For example, if I have the master VCO at 64Hz I can adjust the slace VCO to about 4x this (say 256 Hz) before the sync breaks down. If I have the master at 128Hz I can adjust the slave to about the same 256 Hz before sync drops out. As a result, at higher frequencies I can't really do sync sweeps because it drops out of sync too fast. Any idea what determines the possible sync range? Does it have something to do with the reset pulses not being clean and sharp and quick enough to work at higher freq's?
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Poster



Joined: Jul 19, 2005
Posts: 27
Location: Netherlands

PostPosted: Tue May 11, 2010 3:09 am    Post subject: Reply with quote  Mark this post and the followings unread

hello all,

I want to do this mod as well and just want to make sure I understand the instructions correctly before I pick up that solder iron Smile ..

Did I make the right connections?
Especially those connections to the two transistors Q4 and Q10..
I just guessed that is the right EBC order?

thanks..


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Rob



Joined: Mar 29, 2004
Posts: 580
Location: The Hague/Netherlands/EC
G2 patch files: 109

PostPosted: Tue May 11, 2010 7:31 am    Post subject: Reply with quote  Mark this post and the followings unread

The switch and the diode have to be in series, on your pic it looks like the switch shortcuts the diode, which is not the way to do it...

So, a wire goes from one transistor to the diode, then to one pole of the switch, then the other pole of the switch goes with a wire to the other transistor. Just keep in mind that when there is the diode the second oscillator hardsyncs. So, to not let it sync the switch should interrupt the diode connection by interrupting one of the wires that go to the diode.

I don't have a MS20 anymore, so regrettably I cannot post a pic of the internals with the mod.

Poster wrote:
hello all,

I want to do this mod as well and just want to make sure I understand the instructions correctly before I pick up that solder iron Smile ..

Did I make the right connections?
Especially those connections to the two transistors Q4 and Q10..
I just guessed that is the right EBC order?

thanks..
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Poster



Joined: Jul 19, 2005
Posts: 27
Location: Netherlands

PostPosted: Tue May 11, 2010 9:14 am    Post subject: Reply with quote  Mark this post and the followings unread

thanks for the reply!

so in short: I just cut the signal after the diode.. thanks..

just two more questions..

- is there a difference in using either 1N914 or 1N4148: can I use both?
- is the EBC order (or when rotated 180 degrees: CBE) universal?

As I said I'm not really into electronics but I looked at transistor specs and they all seem to have the order as drawn in the image..
So it's safe to say my assumptions are right?


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