Korg Ms 50 calibration

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abstraktor

Korg Ms 50 calibration

Post by abstraktor » Fri Jul 13, 2012 4:35 am

I'm not sure if this is the right place to post this but...
I cant find any proper and legible service manual for the MS 50 as I need to re-callibrate both the VCO span and VCA1. There's only one trim recess on the VCA so I'm guessing that's the one for sensitivity, but the VCO has a few and I'd like to know which is which.
Any help appreciated. Cheers.

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sempervirent
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Post by sempervirent » Sat Jul 14, 2012 7:27 pm

Send an email to Modeless Factory, I'm sure he'd be able to help, or provide a service manual. All I can find online is the schematic, which lacks any calibration info as far as I can tell.

Do post back if he provides any info, I'd be curious to know this as well.

abstraktor

Post by abstraktor » Sun Jul 22, 2012 3:34 am

Bumping this - still no joy yet in finding out the trim pots functions - the service manual is impossible to track down. :cry:

abstraktor

Post by abstraktor » Sun Jul 22, 2012 3:58 am

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I'm reasonably sure that no. 4 is the response for the VCA - I'm getting some low level sound leak from this when not triggered.
No's 1, 2 and 3 on the VCO are a bit tricky - they could be HZ/V response or span or tuning or even V/Oct. (They wont be the same as the MS20 btw).
The span is way out when patched in to my MS20 - I can hook up to a Kenton and compensate, but I want to play the MS's together -hence the desire to re-calibrate.
Any ideas folks? :despair:

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Post by sempervirent » Mon Jul 23, 2012 10:26 am

I wrote to Modeless Factory myself, since I was also curious. (That's where my MS-50 came from, some time ago.) He said that he actually doesn't have a service manual, but that the schematics reveal the following (the numbers refer to your diagram) and that's how he does the calibration:
1) is for SCALE adjustment, justify the scale knob. You need to do 1st of all.

2) is for width adjustment

3) is for pitch adjustment
Hope that helps.

abstraktor

Post by abstraktor » Mon Jul 23, 2012 10:46 am

thanks mate that is helpful - not sure what the difference is between scale and width though - unless one is for HZ/V and the other for V/OCT which would make sense.
I'm going to try out a cal tonight :tu:

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Post by AntManBee » Mon Jul 23, 2012 10:53 am

I'm embarassed to say that I hadn't noticed that the scaling was out, which perhaps says something about the way I was using the MS-50, but hope you get it sorted. Here's the procedure from Dave Manley via Analogue Haven:

Part I - linear scale adjust (Hz/V CV)

1. center the trim pot. set the scale to 32'. adjust the coarse and fine tuning to zero beat the VCO output against a reference.

2. set the scale to 16', adjust the trim pot for zero beat against the reference (VCO will now be one octave higher than the reference.

Repeat until you don't need to adjust VR1. This should get you close. To fine tune repeat the process again, but this time switching between 32' and 4'.

Part II - expo scale adjust (Oct/V CV)

There is a trimpot (should be somewhere near IC2, Tr1) labeled VR3. This should affect the expo scaling. If the intonation is ok using the Oct/V CV IN with a Oct/V source, then you shouldn't mess with this. If it isn't than the standard procedure for trimming an expo based VCO should work fine.

1. Set the CV In (OCT/V) to 1V. Adjust the coarse and fine tuning to zero beat the VCO output against a reference.

2. Set the CV In to 4V. Adjust VR3 to zero beat with VCO three octaves higher than reference

Repeat until the scale is correct. I don't know what the tracking range is for an MS-50, if you feel lucky repeat again switching the CV In from 1V to 5V (or 6V).

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Post by sempervirent » Mon Jul 23, 2012 11:06 am

abstraktor wrote:thanks mate that is helpful - not sure what the difference is between scale and width though - unless one is for HZ/V and the other for V/OCT which would make sense.
I think the "scale" trimmer is for the Scale pot (the octave switch). It probably has an effective range that needs to be "centered" around certain usable pitches.

And "width" is probably the pulse width (it's located right above the PW/PWM knob, and needs to have a "center" which would be a square wave).

As for the pitch trimmer, I'd guess that there is just one pitch calibration (try middle C?) and that between the separate Hz/V + V/Oct inputs and the oscillator, there are separate circuits that compensate compensate for the difference between those two standards. In fact, I just checked the schematics, and if you look at drawing A40044 that seems to be the case.

EDIT: I started my post before the previous one. Looks like there might actually be separate calibrations for the two pitch CV standards.
Last edited by sempervirent on Mon Jul 23, 2012 11:14 am, edited 2 times in total.

abstraktor

Post by abstraktor » Mon Jul 23, 2012 11:12 am

:doh: should have asked you first ! that's exactly the procedure I was looking for.
Actually I never noticed how out the span was until I patched in a melodic thing with the Ms20 and played each in unison.
(Hope you're getting as much fun from yr serge as i am from this).

Thanks for the help guys.

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Post by NewEnglandAnalog » Mon Feb 22, 2016 8:41 pm

I took the boards out and located each trimmer in the schematics. Here's a chart indicating what everything does. I added a little note to help calibrate the VCF as well, since it's the only thing that isn't really straightforward.
Note that there is no PWM adjustment trimmer. If you want to set the PWM limits on the PW pot, I recommend using diode clamps with trimmers on pin 1 and 3 of that pot.
-Rob
www.NewEnglandAnalog.com
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