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Korg MonoPoly - VCO adjustment procedure

The description of the VCO adjustment procedure in the servicemanual is very poor. I wrote this description on a request in the analogue heaven maillist. Please keep in mind that tuning an polyphonic synth is not the best idea if it is the first time in your life that you are tuning a synth. A lot of patience and concentration are required. Don't listen too loud to the tuningsounds, because they will make everyone else in the world go nuts - especially your partner or your landlord... (don't say I didn't warn you)

ATTENTION: TUNING THE MONOPOLY INTERNALLY REQUIRES TO OPEN THE CASE OF THE MONOPOLY. THIS SHOULD ONLY BE DONE BY PEOPLE, WHO KNOW HOW TO HANDLE THE DANGEROUS VOLTAGES INSIDE OF THE SYNTH PROPERLY WITHOUT ENDANGERING THE LIFE OF OTHERS OR THEMSELVES!!! IF YOU DON'T HAVE THE REQUIRED EXPERIENCE - KEEP YOUR HANDS OFF!!! DOING THE WRONG THINGS INSIDE AN ELECTRICAL INSTRUMENT MAY KILL YOU OR PEOPLE, WHO RELY ON YOUR WORK. I WARNED YOU. I REJECT ALL LIABILITY FOR ALL CONSEQUENCES OF THE FOLLOWING INSTRUCTIONS!

An 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 in the service manual. Otherwise you'll have a very hard time getting things right. (Thanks to Alain Hubert for this hint!).
This instruction replaces the topic 3.2 "VCO-1 adjustment" (and 3.3 for VCO 2,3,4) in the chapter 8 of the servicemanual. You still have to adjust VR1 to VR4 on KLM-398 as described in the topic 3.1 "OFFSET adjustment" befor you start the VCO adjustment (thanks to Tor Sigvardson for this hint!).
There are four trimmers for each VCO on pcb KLM-354 (see markings on schematics, pcb_layout from component side, pcb picture from soldering side (as seen when openeing the synth)):
  • VR103 is for the 1V/Oct characteristic in general
  • VR101 is for the 1V/Oct characteristic of the octave range switch
  • VR104 is for the 1V/Oct characteristic in the highest frequencies
  • VR102 is the internal tuning for the VCO.
The numbers are for VCO1. The other VCOs have correspoding numbers, as VCO2 -> VR201,VR202...; VCO3 VR301,VR302,...  The trimming should be done in exactly this order per VCO.

The only things you need for the adjustment is a fine screwdriver for handling the trimmers and a wide range tuner with a analogue meter like a Boss TU-12H (in fact the TU12-H is the only today available tuner which I would recommend).

  • Open the Monopoly.
  • Switch on the Monopoly and let it switched on for 30 Minutes.
  • After those 30 Minutes set the master tune knob and the individual tune knobs of VCO2 to VCO4 on the frontpanel to the most left position and do not touch them while the adjustment procedure unless it is mentioned
  • Connect the tuner to the output of the synth and listen to the synth through the headphone out (or vice versa).
  • Set the synth as shown in the adjustment procedure (http://fa.utfs.org/diy/korg_monopoly/mp_20_adjustment_1.gif)
  • For each VCO turn down the levels of all the other VCOs on the frontpanel.
  • Set the synth to Unisono-Mode.
  • Set Detune on the frontpanel to the most counterclockwise setting.

Setting VR103:
  • Set the octave range switch of the VCO to 16".
  • Set the Transpose switch (above poly/unisono switches) to "down".
  • Play alternating the lowest F and the highest F (you also may setup a slow arpeggio arp-Switches to latchmode and 1-oct and those two notes).
  • Now adjust VR103 and whatch the tuner: It does not matter WHICH note exactly is displayed (it may be a completely different note), but you should hear a clean octave and the meters needle should show the same degree at the low note as at the highnote. As soon as you turn the trimmer also the low note changes - don't get worried about that. The only thing that matters is the difference between the two notes. The absolute pitch of those two notes does not matter at that moment.
If you made it to a clean three octave jump. Then the main thing is done for this VCO.

Setting VR101:
Don't make this adjustment before VR103 is adjusted perfectly
  • Stop the arpeggiator.
  • Play the lowest F
  • Turn octave range switch of VCO-1 forth and back between to 2' and 16'.
  • Adjust VR101 for a clean octave jumps. Again the absolute note does not matter.

Setting VR104:
Don't make this adjustment before VR103 and VR101 is adjusted perfectly
  • Turn octave range switch of VCO-1 to 16'
  • Play the lowest F.
  • Turn octave range switch of VCO-1 to 2'
  • Play the highest F.
  • If there is an annoying detuning showed in the meter then adjust VR104 while jumping between lowest F and highest F. If it is only a slight detune, then leave it.

Setting VR102:
  • Turn octave range switch of VCO-1 to 16'
  • Turn the master tune on the frontpanel to center position. (If this is VCO 2, 3 or 4 also turn the individual VCO tune on the frontpanel to center position).
  • Play the lowest F.
  • Set VR102 to read the tuner an exact F.
  • Now this VCO is done.

Continue with the other VCOs
After all VCOs have been adjusted individually, play the lowest note at 16' and transpose to "down", turn up the volume of all VCOs, tune them at the frontpanel to the lowest as possible beat rate and play in unisono mode over the full keyboard. If now still the VCOs sound detuned at the upper octaves turn down the level of always one of the four VCOs to check wich one is still the detuned one, and repeat the procedure with this VCO again.

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