Vintage Synth Repair

Hi gang,

I bought a beatiful Korg MS-10 last week, but couldn't travel to 
check it out first. There are some problems that I'm hoping someone 
here can give me a hint about.

Firstly, the keyboard doesn't trigger the synth itself. I've neatly 
circumvented the problem by triggering it from another vintage 
synth, so no biggie. 

Main issue is that all the oscillator waveforms except for Noise are 
completely silent. Well, if the volume is very loud I hear a slight 
noise signal when keys are pressed and released. Everything after 
the VCO seems fine, the filter self oscillates like crazy (beautiful 
timbre!), and the filter works fine with the Noise waveform. All the 
LFO modulations work too, aswell as most of the patch panel outputs 
when I patched them directly to my mixer. The thing I like most 
about this synth is that the LFO shapes aren't perfect, so if I mod 
the filter cutoff in self oscillation, with a sawtooth, I get a 
wonderful "smack" when it resets the cycle.

But kind of a drag when only filter oscillation and Noise are the 
only things coming out of the machine.
Envelopes work fine too btw.

So I've narrowed it down to either dead VCO (is it a chip or a 
complex circuit?), or a problem with the VCA section so the VCO 
signals don't get through to the filter. But then I shouldn't hear 
the Noise signal either and be able to filter that?

There are very few analogue repairmen here in Norway, and the few 
there are charges LOTS. So if anyone could give me a pointer, maybe 
it's simply a loose wire or something..I'm no electrician so I 
didn't open it up and check..but you recognize the symptoms, shout 
out :-)

Thanks for reading all this.

-Tom from Norway-

Hi,  If it's any help http://markglinsky.com/  says it has a service manual for your keyboard.

bitexion <tom-rf@...> wrote:
Hi gang,

I bought a beatiful Korg MS-10 last week, but couldn't travel to 
check it out first. There are some problems that I'm hoping someone 
here can give me a hint about.

Firstly, the keyboard doesn't trigger the synth itself. I've neatly 
circumvented the problem by triggering it from another vintage 
synth, so no biggie. 

Main issue is that all the oscillator waveforms except for Noise are 
completely silent. Well, if the volume is very loud I hear a slight 
noise signal when keys are pressed and released. Everything after 
the VCO seems fine, the filter self oscillates like crazy (beautiful 
timbre!), and the filter works fine with the Noise waveform. All the 
LFO modulations work too, aswell as most of the patch panel outputs 
when I patched them directly to my mixer. The thing I like most 
about this synth is that the LFO shapes aren't perfect, so if I mod 
the filter cutoff in self oscillation, with a sawtooth, I get a 
wonderful "smack" when it resets the cycle.

But kind of a drag when only filter oscillation and Noise are the 
only things coming out of the machine.
Envelopes work fine too btw.

So I've narrowed it down to either dead VCO (is it a chip or a 
complex circuit?), or a problem with the VCA section so the VCO 
signals don't get through to the filter. But then I shouldn't hear 
the Noise signal either and be able to filter that?

There are very few analogue repairmen here in Norway, and the few 
there are charges LOTS. So if anyone could give me a pointer, maybe 
it's simply a loose wire or something..I'm no electrician so I 
didn't open it up and check..but you recognize the symptoms, shout 
out :-)

Thanks for reading all this.

-Tom from Norway-





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The service manual is also available a few places on-line including:

 

http://www.lazyblueoctopus.com/html/vintage_synth_docs.html

-----Original Message-----
From: Sandra Garcia [mailto:createsjg@...] 
Sent: Wednesday, February 23, 2005 7:26 PM
To: vintagesynthrepair@yahoogroups.com
Subject: Re: [vintagesynthrepair] Dead VCO on Korg MS-10?

 

Hi,  If it's any help http://markglinsky.com/  says it has a service manual
for your keyboard.

--- In vintagesynthrepair@yahoogroups.com, "masada wilmot" 
<masada.wilmot@c...> wrote:
> The service manual is also available a few places on-line including:
> 
>  
> 
> http://www.lazyblueoctopus.com/html/vintage_synth_docs.html

I already found the service manual, but being no electronics expert 
the various print charts mean little to me..There is no 
troubleshooting section in the back that presents my exact syndromes, 
which isn't exactly expected either. I was only hoping someone had an 
MS-10 with similar problems. I realise it could be lots of things, 
and probably hopeless to answer without looking at the synth itself.
But I've narrowed it down just by tweaking the controls to:

1. Dead VCO section
2. Something with VCA that prevents the oscillator signal to pass to 
the filter. 

I've had enough synths to check that the EG isn't set to 0 sustain 
level :) Another thing..wouldn't someone that is well-versed in 
video/TV repairs be able to fix an old synthesizer too?
That broadens the field quite a bit when it comes to finding 
repairmen. They share lots of the same things, don't they?

>Another thing..wouldn't someone that is well-versed in 
video/TV repairs be able to fix an old synthesizer too?
That broadens the field quite a bit when it comes to finding 
repairmen. They share lots of the same things, don't they?<

 

Categorically NO.  They are different beasts altogether.  Yes your highly
qualified engineer, note I said highly qualified engineer and not
technician, should be able to work it out but there would be a big learning
curve first and that could be expensive.

 

Regards

 

Brian








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you all may be interested in reading the doctoral thesis at http://njohnson.co.uk/ 

Brian Davies <briang3oyu@...> wrote:v\:* {behavior:url(#default#VML);}o\:* {behavior:url(#default#VML);}w\:* {behavior:url(#default#VML);}.shape {behavior:url(#default#VML);}
 

>Another thing..wouldn't someone that is well-versed in 
video/TV repairs be able to fix an old synthesizer too?
That broadens the field quite a bit when it comes to finding 
repairmen. They share lots of the same things, don't they?<

 

Categorically NO.  They are different beasts altogether.  Yes your highly qualified engineer, note I said highly qualified engineer and not technician, should be able to work it out but there would be a big learning curve first and that could be expensive.

 

Regards

 

Brian







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http://groups.yahoo.com/group/vintagesynthrepair/
  
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neil johnson of neils webbly world is in the roland w30 group with me and has his doctorate and teaches audio electronics--you might try to get his input.  his website is  http://njohnson.co.uk/ 

bitexion <tom-rf@...> wrote:
--- In vintagesynthrepair@yahoogroups.com, "masada wilmot" 
<masada.wilmot@c...> wrote:
> The service manual is also available a few places on-line including:
> 
>  
> 
> http://www.lazyblueoctopus.com/html/vintage_synth_docs.html

I already found the service manual, but being no electronics expert 
the various print charts mean little to me..There is no 
troubleshooting section in the back that presents my exact syndromes, 
which isn't exactly expected either. I was only hoping someone had an 
MS-10 with similar problems. I realise it could be lots of things, 
and probably hopeless to answer without looking at the synth itself.
But I've narrowed it down just by tweaking the controls to:

1. Dead VCO section
2. Something with VCA that prevents the oscillator signal to pass to 
the filter. 

I've had enough synths to check that the EG isn't set to 0 sustain 
level :) Another thing..wouldn't someone that is well-versed in 
video/TV repairs be able to fix an old synthesizer too?
That broadens the field quite a bit when it comes to finding 
repairmen. They share lots of the same things, don't they?




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   To unsubscribe from this group, send an email to:
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Educated guess as this has happened to Korg VCOs in other 
instruments: replace the FET F3.

Crow
/**/

--- In vintagesynthrepair@yahoogroups.com, The Old Crow 
<oldcrow@o...> wrote:
> 
>   Educated guess as this has happened to Korg VCOs in other 
> instruments: replace the FET F3.
> 
> Crow
> /**/

Exactly the kind of thing I was hoping to see :-) 
I'll check out the other links aswell.
We're talking a MOS-FET transistor here? Standard electronics gear?
Or is it Korg specific? Specifications?

I took electronics classes many many years ago but never practised 
it afterwards due to pursuing a different career path, as I never 
got any good at the stuff.

All the FETs are typical 2SK30(A) JFETs.  You can either order some 
2SK30As from a place like americanmicrosemi.com or just use a 
general-purpose modern part like a 2N3819.  Just make sure you observe the 
transistor lead order as they are different.

Crow
/**/

On Fri, 25 Feb 2005, bitexion wrote:

> >   Educated guess as this has happened to Korg VCOs in other 
> > instruments: replace the FET F3.
> 
> Exactly the kind of thing I was hoping to see :-) 
> I'll check out the other links aswell.
> We're talking a MOS-FET transistor here? Standard electronics gear?
> Or is it Korg specific? Specifications?