Any fellow Elektronauts have a Korg MS10? & time to help?

The Lounge Other Gear
#1

Here’s the thing,

I just bought a very clean vintage Korg MS10 that I really like a lot. But…

The square wave sounds ‘funny’ under certain conditions, and I’m trying to find someone who I can compare notes with me on this to see if this is normal or a fault. I can’t tell.

By sounding ‘funny’ I mean this:
When I turn the PWM knob thus narrowing the pulse width, and play at a low octave there are audible ‘artifacts’ in the sound of the wave. It sounds like an irregular ‘clicking’ sound. I have an audio example of what it sounds like when I turn the PWM knob here below. All settings are a basic patch with nothing plugged into the patchbay and set as follows:

Scale-16
Wave-Pulse
PW-sweep from off to full and back again.
Sustain - full
Filter - 100% open
Res - Off
all others off

Korg MS10 pulse wave - artifacts

No other wave has this sound, and the knob itself is not making this sound. I noticed on an oscilloscope image that every now and then 1 wave will have almost no ‘width’ and I assume this is the sound that I am hearing. Like this image below.

Anyway, the place I bought the MS10 from has a good return policy so no loss there. But I’m really hoping to hang on to it because I really like the sound overall. So if someone can spare a few minutes and compare sounds for me it would be much appreciated!

Thank you in advance for any input on this.

#2

Do you mean this high-frequency “sizzling”, which can be identified during your PWM sweep?

IMO the extremely thin spike in your image does definetly not belong to an ordinary PW. Since the MS-10 has one VCO only, this could be some electronic component (possibly a capacitor?), which is passing out. It would be interesting whether the spike is generated perdiodically or at random.

I have an MS-20. One of the VCOs provides PW too. If I find the time, I can compare, but I think, only judging from the image, there might be an issue, because I had the impression that the “sizzling” started quite early in your audio-recording.

There might be a good chance for a repair, though :wink:

#3

Precisely that’s the word I was looking for a ‘sizzling’ sound! That is exactly what I would call it.

And yes it starts right after I start to turn the pwm knob and the wave narrows. The spike is very random, and in the various images I’ve looked at, it is not even always the same thinness. Sometimes like this wave it is very thin, and other times it is a little thicker.

It seems to be less apparent in the higher notes. Maybe due to the tightness of the wave in higher notes with less gap in between?

Thanks for replying on this, I’m thankful for any insight into this synth’s problem.

#4

If it’s random, as you describe it, I would let an electronic guy have a look.

My guess that a capacitor might be the culprit is derived from some humble experience in electronics. AFAIK in the old days most timing has been done with classical RC-circuits (resistor/capacitor). The spike in your image seems to lack the typical exponential short release-time, until the signal drops vertical. Since resistors are long living things, the issue shouldn’t be caused by such a component, but in the old days capacitors have been used, which could age such that they lost capacitance or developed internal electrical leaks, which could cause electrical short-circuits. This said, theoretically it’s possible that a capacitor in your VCO-circuit short-circuits every now and then, if it receives a voltage high enough, which might be caused by the typical “instability” of old-days analogue circuits. If the voltage level is low enough, the VCO oscillates as it should, but if the critical level is exceeded, a short-circuit cuts down the release time to nearly zero and we observe the spike.

If it’s like I assume, then a technician should be able to identify the component and replace it.

But … TBO … it’s only an educated guess :wink:

#5

Thanks for sharing what you do know about this! Especially the information on why you think it may be a capacitor. That’s an interesting theory on how a short-circuit would cause a cut to the release time like in the image.

I downloaded a copy of the service manual and skimmed over the schematics and parts list. I’ve done some small electronic work and almost want to poke around myself but… the shop I bought it from may send it out for repair while on warranty. So, your advice to have it looked at by an expert is what I’m considering now.

I’m glad that at least someone else thinks that the ‘sizzling’ sound is not normal. I didn’t think this was typical for an MS10, but you never know.

#6

this would be my first guess…

Bildschirmfoto 2017-02-24 um 14.55.05.png1562×1194 482 KB

cheers, nik

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#7

Yep … if the OP was sparkling and driving the time-constant to zero … this would explain it too :thumbsup:

#8

You are welcome. It’s always be wise to make use of granted warranty.

#9

Thanks for narrowing things down for me on the schematic. I’m just checking the meaning of each symbol online so I can grasp what I’m looking at.

Sorry but the ‘IC6’s’ amplify the signal? I undersatnd where the resistors, and grounds are, and can follow the signal flow but… I really never studied the guts of my synths much.

#10

I’m just curious what does ‘OP was sparkling and driving the time-constant to zero’ mean? What is ‘OP’?.

#11

OP means op-amp or operational amplifier and as its name reveals, it is amplifying :wink:
there are so many ways how. sneek peek here https://en.wikipedia.org/wiki/Operational_amplifier

this little bugger is a 4558 Dual-Opamp, i guess from JRC

…it seems that the “lower part” is still working where it is shaping the sinewave,
the “upper” part may be faulty …this is what “narrows” the pulse cycle.
if you cant get a 4558 you could try a TL-62, TL-72 or even a TL-82.
use an ic-socket so you can swap them later easily.

cheers, nik

2 Likes
#12

Hi, just stumbled across this thread while having a similar problem. Picked up an ancient ms10 that has been collecting a lot of dust and not used for god knows how many years, a bit crackly here and there. The square wave worked for a minute and then suddenly stopped producing any sound and the pw was jumpy before it went, other waves are fine. Might it be as simple as replacing this ic? Should I worry about the brand of replacement ic will it make a big difference?

Hope this finds somebody, it’s already been a very helpful read

#13

I Know! I have no helpful knowledge as to your problem, but chiming in to just say that this was a great read, and very interesting.

#14

It deffo looks like the top portion of the IC on the schematic is generating the pulse wave. It looks like it’s configured as a comparator that has its inverting input fed with the ramp wave via the divider formed by R78/79 and a voltage from the PW pot or external input into the non inverting input. Adjusting this voltage via the pot/external input will alter when the comparator switches hence shortening the pulse width. Any 4558 should work here as would other op amps with the same pin out. As someone else pointed out it a good idea to socket the IC if you’re replacing it.

It’s worth checking around the switch too. It’s possible that’s gone faulty or you’ve got a bad joint. If you’ve got access to a scope it’ll help you diagnose the issue but if you’ve got the other waves there’s not much else beyond the op amp and a sprinkling of resistors that are doing the pulse wave

Welcome to the forum BTW!

#15

Thanks!

I think I’ll take it apart and have a look at the board to see if there is anything obvious around the switch. Other than that i can’t see what else it would be it looks so simple, will let you know how I get on

":slight_smile:

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#16

It could be that the dry joint is on one of the IC legs or the resistors around it. It could also be on one of the pot legs. It might be worth reflowing those joints first as you might not need to swap out the op amp.

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#17

If you decide to go for repairing it yourself, I got myself one of these a few years back to work on my Roland GR-300 guitar synthesizer:

It’s not cheap, but boy did it make it easy to remove and replace components. If you’re not comfortable working on things yourself they by all means take it to a pro, but for my purposes, it worked great. In my case it was also faulty op-amps – if I had more skill I might have been able to test them in the circuit to see which one was not working, but instead I replaced them one by one until the issue was resolved. It’s the component that’s most frequently failed in my gear. If you have any other through-hole equipment you want to be able to work on yourself, something like this tool is a must. Good luck!

1 Like
#18

That costs as much as the synth itself! Are the lower budget ones any good? Like rs components or so

#19

I’ve had good results with the ~$100 Hakko soldering station. Precise and consistent temperature control is worth paying for.

Desoldering braid gave me better results than the $10-$20 solder suckers (which tended to lift traces), BUT I found that it is critical to match the braid with the solder. Braid meant for lead doesn’t work well for lead-free, and vice-versa. The MS-10 will have lead solder. Fortunately, braid is cheap so buy several types and try them out.

With patience and care you can desolder with anything down to the $15 bargain irons. If you plan to do other DIY projects, it is worth investing the $100 in a good Hakko or Weller.

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#20

Yea, it’s gotten really expensive (I bought mine a few years ago and it wasn’t so costly, but that was pre-pandemic, so who knows). I look at it this way: if I’m going to have any vintage gear, it’s going to break down eventually, so being able to do minor repairs myself is worth the price of a tool that can do it properly and safely, even in the hands of an amateur. I tried braids and solder suckers, but unlike @obscurerobot I didn’t have much luck with them – you certainly may so those things are a great place to start. In the case of my GR-300, I figured that having a professional even look at it, much less repair it, would have cost me at least as much as the cost of the desoldering tool. I had a pretty good idea of what the problem was, so I went for it, and lucky for me it worked out. Now I have the tool and I’ve used it on a few other projects as well (and went through a period of building a lot of Euro kits, inevitably doing some things wrong as I was learning that required some desoldering), so at this point it’s mostly paid for itself.

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