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Advice on calibrating

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  • Alain
    I ve just finished re-calibrating my MP4 from top to bottom. Took me 2 hours. And in the course of doing so, I realized the importance of certain points.
    Message 1 of 5 , Feb 8, 2010
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      I've just finished re-calibrating my MP4 from top to bottom. Took me 2 hours. And in the course of doing so, I realized the importance of certain points. First, there are some mistakes in the service manual. Not really mistakes, but more misunderstandings. The whole manual, and the calibration section, is based on the assumption that an old version of the MP4 is to be serviced.

      However my MP4, as well as a LOT of others, is a newer version. So there are a few extra steps to take while reading the service manual.
      But how can you tell if you have an older or newer version ?
      Simple, when opening your MP4, if you see a small circuit board on the left marked KLM327 then you have an old version. On the newer version, that board is missing entirely.

      So for the new version on the KLM355 board, first there is no more VR11 filter keyboard tracking adjustment. That VR11 is now part of the final VCA adjustments instead.
      Second, when calibrating the offset of each VCO, the wire color reference for connecting the DVM is completely different. But a correspondence chart is provided at the end of the manual.
      But here they are for quicker access:

      OLD NEW
      VCO1 Red Pink
      VCO2 Yellow White
      VCO3 Blue Purple
      VCO4 Gray Green

      Also, the trim pots correspondence for the offset adjust is not indicated. So here it is:

      VCO1 -> VR2
      VCO2 -> VR1
      VCO3 -> VR4
      VCO4 -> VR3

      Another extremely critical point, is that you have to FIRST calibrate the voltages on the PSU. Then, calibrate the keyboard CV generation by adjusting the Key Assigner KLM356 to very precise values (a high precision DVM is essential here).
      Don't start the VCO calibration unless you have confirmed that the assigner is generating EXACTLY 1 volt per octave, as described in the calibrating procedure. Otherwise you'll have a very hard time getting things right.

      Personally, I prefer to use a digital frequency meter instead of a tuner for the VCO calibration, and a keyboard frequency reference chart. Numbers don't lie. Also, it makes it easier to compare octaves.

      The end result is not absolute perfection, after all this is not a digital machine. But I've managed to get the tuning on each VCO to within +/- .1 hz over the 6 octave range, which is more than acceptable. But it takes methodical work and patience, going back on previous trim pots several times to confirm their value didn't change.

      So a well spent 2 hours, with an end result beyond my expectations.
      Before I calibrated it, my MP4 showed a +/- 5.5 hz difference on all VCOs across the whole 6 octave range, and the up-normal-down transpose switch showed a +/- 6.2 hz difference ! It didn't sound good.

      Hope this can be of some help for those courageous enough (and well equipped enough, a 4 digit DVM is essential) to undergo this precise procedure. Otherwise you'd better leave this kind of job to a professional, or end-up with a badly out-of-tune MP4. You've been warned.

      Alain.
    • Florian Anwander
      Hi Alain do you know my instructions on this: http://fa.utfs.org/diy/korg_monopoly/vco_adjustment.html ? ... Very good point! I ll add it to my instructions
      Message 2 of 5 , Feb 9, 2010
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        Hi Alain

        do you know my instructions on this:
        http://fa.utfs.org/diy/korg_monopoly/vco_adjustment.html ?


        > Another extremely critical point, is that you have to FIRST calibrate the voltages on the PSU. Then, calibrate the keyboard CV generation by adjusting the Key Assigner KLM356 to very precise values (a high precision DVM is essential here).
        > Don't start the VCO calibration unless you have confirmed that the assigner is generating EXACTLY 1 volt per octave, as described in the calibrating procedure. Otherwise you'll have a very hard time getting things right.
        Very good point! I'll add it to my instructions

        Florian
      • Alain
        Hello Florian, Yes, I was aware of your page on this. And thank you for adding my humble contribution to it. I was about to write you a note about this
        Message 3 of 5 , Feb 9, 2010
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          Hello Florian,

          Yes, I was aware of your page on this. And thank you for adding my humble contribution to it. I was about to write you a note about this possibly missing info.

          Although since I use a frequency meter, the service manual from Korg was clear enough for me.
          I just use A4 and A1, instead of A#4 and A#1. (easier numbers to work with: 440hz, 55hz, 3520hz)

          My own procedure is quite simple, and follows a path similar to the Korg manual. And for those of you who would rather use a frequency meter than a tuner, here it is in 8 easy steps (well 32 to be exact):

          Transpose switch to NORMAL. (A voltage of exactly +5.250 V must be confirmed at the right side of R8 in the KLM356 CV1 output before continuing).

          1-Octave switch to 16'
          2-Play A4, adjust vr102 to read 440hz
          3-Play A1, adjust vr103 to read 55hz
          4-Repeat steps 2 and 3 until values displayed are EXACT.

          5-Octave switch to 2'
          6-Play A1, adjust vr101 to read 440hz
          7-Play A4, adjust vr104 to read 3520hz
          8-Repeat steps 6 and 7 until values displayed are EXACT.

          Repeat steps 1 to 8 for VCO 2-4 with their corresponding VRs.

          You're done.

          Note: you can tolerate a +/- 0.2 hz value fluctuation to account for analog drift.

          Alain.



          --- In korg_mono-poly@yahoogroups.com, Florian Anwander <fanwander@...> wrote:
          >
          > Hi Alain
          >
          > do you know my instructions on this:
          > http://fa.utfs.org/diy/korg_monopoly/vco_adjustment.html ?
          >
          >
          > > Another extremely critical point, is that you have to FIRST calibrate the voltages on the PSU. Then, calibrate the keyboard CV generation by adjusting the Key Assigner KLM356 to very precise values (a high precision DVM is essential here).
          > > Don't start the VCO calibration unless you have confirmed that the assigner is generating EXACTLY 1 volt per octave, as described in the calibrating procedure. Otherwise you'll have a very hard time getting things right.
          > Very good point! I'll add it to my instructions
          >
          > Florian
          >
        • chuddy023
          Hi, Thanks so much for your post, your method is by far the easiest to follow. Last night I successfully calibrated and tuned my new (to me) MP and the pitch
          Message 4 of 5 , Dec 17, 2013
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            Hi,Thanks so much for your post, your method is by far the easiest to follow. Last night I successfully calibrated and tuned my new (to me) MP and the pitch is bang on within a few cents right across the octaves...I used an ossicilscope to measure the frequency on A 440 and the relative notes...Next up is the VCF and VCA.... =)thanks again..
          • niklas.ehrlin
            Hi I have followed the instructions on the tuning procedure - and was hoping to get a fairly good result from doing the power supply and key assigner
            Message 5 of 5 , Nov 10, 2014
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              Hi

              I have followed the instructions on the tuning procedure - and was hoping to get a fairly good result from doing the power supply and key assigner calibration. And the perhaps followed by an offset change on VR1-VR4.
              This was to pehaps not have to deal with removing the whole VCO PCB (KLM-354), as I guess there is no other way if you need to adjust the VR101-VR104, VR201-204.. ... ..and so on.
              But the offset did not work for all OSC. So I have to remove the whole PCB. Any suggestion on how to do this in a safe (not to damage it) and easy way?

              Regards
              Niklas
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