Modify your Monotron, ADD CV + GATE

Get the schematics first!

It had got to be big topic that there is the mark that what kind of signals comes around when monotron had been released.
Let’s check next 6 lines. it’s VCC, GND, GATE, Bias, Pitch and CutOff. What kind of signal we can get from there? let’s get check it out there.
Korg had published the schematic of Monotron and anyone can download it. Follow next link and get it.

you can find “Download” button at the bottom of the page. Let’s get check the schematics.
off couse you may loose your warnnary if you remove the back panel of monotron. Do it with your own risk.

What kind of signals we can get?


Monotron is driven by 2 batteries. it has DC/DC converter inside and it provides 5V for all the schematics. You can get 5V that monotron made.


this is the same voltage of spring of batteries. the value of VCC is difference from here.


For driving Opamp, we need bias voltage. And this voltage also output from monotron to outside. this line is bfferd by IC2B. R10 and R9(each of them are 27kand10k) divide VCC, and it’s 1.35V or so.
Monotron use LM324 as Opamps. It’s single rail type. Normal Opamp needs the margine from power line. this opamp also needs margine to VCC. but lower side is around 0V. so you can use around 0V up to 4V or so in 5V operation.
Bias voltage is just little lower than the center of the vltage that we can use. suppose why it should be. that the oen of fun of analog schematics!


It seems to be input for external for gate signal. Inside of monotron, the gate signal is made by compalator. that’s IC2C.
In the schematic that I trace from original, the Ribbon is seems to be a retio-stat. Actually when you touch it, it’s as is. And when you don’t touch it, the sweeper is not touch anything.
But there is bias voltage thorough R5 from – input of IC2A. + input of IC2A gose to bias, so there should be bias level because of vertual short.
In the other hand, the – input terminal of IC2C as compalator gets the voltage that divided from bias to VCC by R13(100k)and R14(2.2k). it should be just a little bit but surely upper than bias level.
Each end of ribbon is coverd by the enclosure, so when you touch the ribbon you get the voltage upper than bias.
you can use this page instead of hit the DENTAKU key when you want to get divided voltage from 2 registors.


VCO is driven by summing amp that builded by IC2A. the input of summing amp are adding voltage from ribbon (R5) and signal from LFO (R4). there is the rand for “Pitch” so you can add external voltage to there with some registor.

by the way, What do you think about the R7 on bias block?

Single Supply Op Amps that’s used for monotron can get around 0V by single power source. it’s is great. But there is also small weak point.
there is distortion point around 0V (or bias). because of the switching output stage around it. it’s called as zero closs distortion.
The R7 at the output of bias block bleed current to shift the point of distortion.
there is same technic is used for R73 at last stage of VCF. in this case, it’s got down to the ground. That’s one of basic technic for using affordable chips.


VCF is driven by double inverted amp (IC4B and IC4A).Q12 will change the cuff off frequency. Source of the voltage of cuff off is coming from VR6A. it around 0V to 2.98V ( that’s made by 10k of VR6A and R75(6.8k) it’s also summing you can add registor to add the voltage outside of the monotron there. 

Add external gate input

Let’s go straight ahead. Put the gate signal into Gate rand. What happens there? When we feed the gate signal, Pitch CV also be fed into summing amp. so VCO should be driven by next two source.

  • Pitch CV though registor that you add to there
  • Gate voltage through R11


if you don’t mind its pitch, as FX sound or something, then there is no treat around this. put gate signal no mods. in this case, the voltage of gate singal affect to the pitch. If you want drive it from MIDI or something, you may need more accuracy around pitch or so. but the gate voltage may get bad affect to the pitch.
most simple way to remove the affect of gate voltage to the pitch is removing R11.this methods cut the path to make pitch and gate singnal inside of monotron. Blue circle is the R11 on the picture.
Removing Chip registor from PCB is not easy for everyone.
Adding the solder to a chip registor that we attempte to remove. make registor floating in molten solder then tip of soldering iron will grips it by the surface tension of molten solder. there is no need to scrape or pull. You should better to try this way first. there needs some practice for beginners. there is no need to re-use the parts. you should remove it without damage for the board.

this mod allow you to add gate signal without any affect of gate signal level or something. but it kill the ribbon. Monotron should be getting not to be able to get sound itself. Your need gate signal from outside.

if you no need to touch to the ribbon, then this mod is simple and easy.


by using this methods, marks on the back side of the PCB is as is.

but the line “pitch” and “cutoff” is conected on invert input pin of opeamp directly.

this point has so high impedance. So you should better to not to draw the line too much. you may get noise around there.

VCC, GND and Bias lines are power source. you can feed power to something you connect outside of monotron.

but controlle cv inputs are just Gate, Pitch, Cutoff and GND. you may be able to use 3.5mm mini jack in monotron enclosure. anyway to find the room to put the connector is another difficulty for this DIY.

in this methods, it’s so easy to do that but you can’t get sound fron Monotron itself.
We need to do some other methods, when Adding MIDI-IF, then you can drive monotron from MIDI. but removing it from monotron, then you can’t get sound monotron itself.
this methods kill the ribbon.

Don’t kill the ribbon!

Monotron Delay has so wide frequency range than Normal monotron by its ribbon controller. just one flick of ribbon can get over 5 octave of frequency sweep. That’s the charm of this gear. In normal Monotron, I had recommended that more easy method to adding Extra gete input. But for Delay, I want you to try add transistor methods. Don’t kill the ribbon.

Adding transistor methods it based on Mr. masa921 developed when the normal Monotron had been released. the methods that indrduce to you here is almost same as his did.

His Original page is here

the provider site that he use not allow to access from some of over sea. I can’t believe that!.


As the same as Monotron, Korg had published the schematic of MonotronDelay and you can also get it. Follow next link and get it. you can find “Download” button at the bottom of the page.

There is small change around parts# or something, but Monotron and Monotron delay is almost same schematic especially around VCO.

This is the schematic of extra gate input for Monotron Delay.

The key of this method is .. by shorting R18 by added TR, get down the compare voltage low to make compalator on instead of getting ribbon voltage high.
masa921 use DTC114 but it’s already discontinued item and it’s not easy to get now. so i use FJN3302 as another choice of digital TR.


the reason to use Digital TR is only the space. if you have enough room to put parts then you can use simple tr and base registors.

Put down to flat face where notes its name of transistor to the LM324(Quad ope-amp) then most left pin (3) is base. it’s just in the air. it don’t connect anywhere on the PCB.

The 2nd pin is corrector. it gose 9 pin of LM324.

The last pin of FJN3302 gose to the grand. I use 11 pin of LM324.

base pin of transister is just in the air. When you finished to check if it works or not, glue the IC and the transistor together, as soon as possible. It’s so easy to tear off around there by pulling wire of the base.
you can this lead as additional get input. just draw out the wire there.

this drawings may be help for you, also.


this methods based on for monotron, so you can use this for monotron also.

Adding connector, you can make it removable monotron and another equipments. When you connect MIDI-IF something, you can drive monotron from outside. and when you remove it then you can play it by build-in ribbon.
Big probrem is there is no connector that suits for monotron, I think. Anyway, it’s too small.
MR. masa921 did like this.

I did almost same things just like this.

1V/Oct adapter


This mod give monotron a chance to connect something to drive it by another equipments. How about 1V/Oct Adaptor ?
I add a portament function on it. I realy love the portament. I don’t need any volume knob. but I need portament knob. I hope that every synthe lover think as same as me.

Build it using universal pcb board or something.this is one of DIY project. if you want to adapt it to monotron Delay, you may need pitch change function and more something. think about it and share it, please.



You can get MIDI-IF kit for monoton and monotron Delay at
visit next page and put an order.

this kit have some options. if you build it for normal monotron, then I recomend that getting #205 Connector and pot set of MIDI-IF for monotron with #200.

if you want to use this kit for monotron Delay, then order #207 Extra parts set for Monotron Delay with #200.

Picure shows how to put the kit into some encrosure. (the kit don’t include enclosure )

Extra parts set for Monotron Delay

This parts set includes extra parts that you need when you mod this kit for Monotron Delay.
it’s not for beginner’s project. check insruction manual closely. 

  • pot:100k,1k
  • trimmer:10k
  • DIN5 pin jack
  • FJN3302(1 piece)
  • registers (2X47k, 4.7k, 15k and 36k)

Connector and pot set of MIDI-IF for monotron

Din 5P Connector for MIDI and a 100k pot (Linear)

MIDI-IF KIT for monotron V2

MIDI-IF for monotron kit (includes #201, #202 and #203)
w/o enclosure, MIDI-Connector and 100k(B) pot. We recommend that getting with #205 Connector set for monotron.
it’s not beginner’s projects.




ここでは現在比較的、安価に手に入れやすくなった鍵盤として、MIDIキーボードを取り上げ、これの出力するMI DI信号をアナログシンセで使えるCV+GATEの信号に変換するモジュールが公開されました。



この部品のサポートに関しては、著者の運営するサイトで進めるべく準備中 です。



MIDI-CV変換CPU+専用基板 : DIY Parts
上記書籍に掲載された回路の中に使われている、CPUと専用基板のセットの頒布です。(2016-02-17:CPUのみの頒布は終了しました。#508 MIDI-CV変換CPU&専用基板をご検討ください)
MIDI-IF for SX-150 mk2 : Ready Made
キットとして頒布されている学研SX-150 MK2のMIDI-IFを組み立てて、ケースに組み込んだものです。SX-150 (オリジナル)版もオーダーメイドで制作します。
CMU-800 MIDI interface : Ready Made
RolandのCMU-800 をMIDI化するレディメイドのMIDI-IFです
MIDI-IF KIT for monotron V2 : kit
MIDI-IF KIT for SX-150 V2 : Kit
MIDI-IF KIT for SX-150 mk2 V2 : Kit
MIDI-IF for Monotribe : Easy Kit
Mini MIDI-IF : Easy Kit

Monotribe MIDI化



ELECTRIBE Life / Posts Tagged 改造



このほかにも多くの方が挑戦されて、当時はレポートがいろいろなところから飛び込んできてワクワクな毎日でした。僕は、Aire Variableさんのページに深い印象を受けて、ずっと拝見していました。

Aire Variable / Korg monotribe分解

その後、MIDI Bareという、MIDIクロックをmonotoribeのシンク信号に変換するハードウエアのキットで登場する機会を得ました。回路図はこちら

氏の解析を受け、Gameboy Geniusというサイトに、nitro2k01氏が”Monotribe, MIDI and me”という記事を投稿され、この記事の内容を追試してみました。MonotribeのMIDI化です。

Monotribe MIDI化




MIDI送信に関しては、”Monotribe, MIDI and me”
“MIDI, the electrical side of things”というセクションの最初に出ている回路図のR1,とR2に当たる抵抗については、出力が3.3V出ているのであれば、フォトカプラのLEDの電圧降下分の2Vを引いて1.3V。これに5mAだから、抵抗は260オーム。受信側に200オームは入っているに違いないいので残りは60オーム。nitro2k01氏も同様の計算をされているようです。



U1 6N138 Phtocpller
D1 1N4148 Switching Diode
R1 220R 1/4W carbone
R2 270R 1/4W carbone
C1 0.1u Ceramic
MIDI-IF for monotribe BOM


air Variablesさんの記事

  • 1.IC11-20(/BOOT)
  • 2.IC11-12(RXD0)
  • 3.IC11-11(TXD0)
  • 4.VDD(3.3V)
  • 5.GND
  • 6.IC11-29(SCK1)























Breakout Box for Monotribe



This box will be able to drive your Monotribe by CV+GATE signal. If you have something equipments that can produce CV+GATE then connect it to your monotribe. let’s get more groove from your monotribe! 

I recomend to use EMS but this item can use registerd small packet. I want to make shipping fee more affordable. Send me a mail about that.

#You need something equipments that can output CV+GATE
#You need to up your monotribe OS ver.2.1 or over
#monotribe is trademark of KORG INC.


Instruction manual for kits

English version

Japanese version


Old version for maintenance

MIDI-IF KIT for SX-150 mk2 V2

MIDI-IF for SX-150mk2 kit (includes #201, #221 and #223)
w/o enclosure, Connectors and pots。We recommend that getting with #222 Connector set for SX-150.
it’s not beginner’s projects.

MIDI-IF KIT for SX-150 V2

MIDI-IF for SX-150 kit (includes #201, #202 and #211)
w/o enclosure, Connectors and pots. We recommend that getting with #212 Connector set for SX-150.
it’s not beginner’s projects.