Korg 700S #2 - cykong.com

Mods on the Korg 700S #2
\| IEC Mains Socket \| Proper CV/Gate \|

Divided into 4 parts...

	Part 1: F.A.Q.
	Part 2: Schematics & Stripboard & Parts
	Part 3: Wiring -1. Power -2. CV Wiring / Switch -3. Gate Wiring / Switch -4. Repeat Wiring
	Part 4: Mounting

Part 1: F.A.Q.

Q1:
Why "PROPER" CV/Gate? I've got a socket kit....
Q2:
Why "PROPER" CV/Gate? Can't I just solder some sockets to the Mini-Korg and give it CV/Gate?

A1 & A2:
Socket Kits or just basic soldering (tapping to the required pads) to give CV/Gate to the 700/700S is possible, but there are some limitations/disadvantages:

The 700/700S Gate requires some 'kick start'' voltages. The 700/700S works at +20V, and somehow it needs some extra voltages to start triggering. (This is because the original gate also incorporates extra voltages from the REPEAT/BENDER/PORTAMENTO circuitry - see below).
You can get past this by holding down a key on the actual Mini-Korg keyboard, or the good old ''masking tape'' trick, taping down a key while you feed and play external CV/Gate.

The Gate Input does not incorporate the REPEAT / BENDER / PORTAMENTO functions, so these three functions (Switch & Slider & the actual effect) are NOT available if it's a simple solder on socket.

A straight CV in can result in pitch shifted - i.e. a ''C'' key you play on External CV, the Mini-Korg might sound an ''A''.

This simple ''Proper Full CV/Gate'' Mod, will take care of all these, so feeding it External CV/Gate, the Mini-Korg will respond to it, no need to touch the Mini-Korg keyboard, plus all the REPEAT / BENDER / PORTAMENTO functions will work with external CV/Gate.
The trimmer lets you ''tune'' the CV, thus putting matching the external CV range to exactly that of the Mini-Korg, so it's also in pitch, in tune with the rest of the outside world.
In fact, the external CV/Gate will act exactly like you're playing on the Mini-Korg keyboard.

Q3:
Does the Mini-Korg requires Hz/V or 1V/Oct?
Q4:
Is it normal +ve Gate, or is it switched (S-Trig) trigger?

A3:
The Mini-Korg requires Hz/V.

A4:
It requires a +ve Gate.
For this ''Proper Full CV/Gate'' Mod, any normal > +1V (or > +1.2V) positive gate would work.

Q5:
I have a Korg 700, not a Korg 700S. Are you sure the CV/Gate is the same? Isn't the Korg 700S and enhanced version? Would this CV/Gate mod works with a Korg 700?

A5:
The 700 and 700S differs only that the 700S has an extra PCB and the top left panel. The basic 3 PCBs are identical. So the electronics / PCBs that the CV/Gate deal with are the same.
This ''Proper Full CV/Gate'' Mod will work for both the Mini-Korg 700 and Mini-Korg 700S.

Part 2: Schematics & Stripboard

[Click Image to Enlarge]

The schematics for this ''Proper Full CV/Gate'' Mod comes from a Japanese website, E-Lab Japan, but since Summer 2004, his webpage has vanished.

When I first wrote my pages for the 1st 700S, his page was still alive, so I actually linked to his site, since I didn't want "steal" any of his credits and things. But now his site cannot be found....

I have saved his page, in Japanese, mirrored here, with some notes and rough translations.

But since I was gonna add CV/Gate to my 2nd Mini-Korg 700S, I decided to take more detailed photos, so I can try showing it step-by-step, it's very similar to his page, but the followings are all in English, with bigger sized photos and additional notes...

[Click Image to Enlarge]

I translated the schematics into a stripboard (or veroboard).

OP-07 are used because the Mini-Korg uses +20V which is above the usual Op-Amp working voltages.

I dun do custom made PCBs, so I convert everything into stripboard. (It's nice, and everyone without PCB making equipment could do it.)

PARTS LIST

Resistors:	Quantity
10K	x8
100K	x8
1M	x1

Trimmers:	Quantity
20K Trimmer	x1

I.C. / Transistors / Diodes	Quantity
OP07 Op-Amp	x2 (see notes below)*
2SC1740 (or equivalent)	x4
2SA1015 (or equivalent)	x2
2SK30 (or equivalent N-Channel FET)	x1
1S1588 or 1N914 Diodes	x5

Misc.	Quantity
Stripboard	x1 (at least 40x15)
+ the usual wires, solder, connectors etc

* Note:
OP-07 is used because the Korg 700/700S run at 20V. OP-07 is an Op-amp with the correct rating. It is possible to use other Op-amps, or even modify the board to use a dual Op-amp, but the power has to be dropped down from +20V to +15V with a 7815 or a 78L15. (see the side box in E-Lab Japan's schematic drawings)

[Click Image to Enlarge]

This is roughly how the stripboard looks like when built.

NOTE:
The last 100K resistor, that goes to the ''TO REPEAT Pad11'' is missing on this stripboard because I planned to solder the 100K resistor on the wire connection instead of the board.
Otherwise everything else is identical to the stripboard drawing diagram).

Part 3: Wiring

Open up the Mini-Korg:
1. Removed the Wooden Cheek Sides (unscrew the 3 screws one each side)
2. Remove the Top Covering Panel (2 screws on top left/right, four on the rear)
3. Remove the 2 screws (on the left) that hold the top left panel, so panel can become loose
4. Remove 4 screws (at the bottom of the synth) that hold the keyboard
5. Remove keyboard (altho Keyboard lead (3 wires) still connected...

Then we see the 3 main PCBs.

And the CV/Gate/Repeat Mod Locations are:

1. Power (+20V/0V/-10V) for the stripboard & Stripboard Mounting

Power can be drawn from the Power PSU PCB, which is the PCB on the left.

The Pads with BLACK wires are ones for 0V (GND).
The Green ones are -10V. Orange ones on the right are +20V.
Solder 3 wires to to these pads.
In the photo, you can see my Black/Blue/Red (0V/-10V/+20V) soldered.

These 3 wires will give power to our interface stripboard. I tied them to the bunch of cable nearby, to route them a a space where I can mount eh stripboard:

Cable Tidy: I used connector blocks and screwed the blocks to the top left of the middle PCB. My stripboard is to be mounted on the top of the PCB on the right. Some wire clips are added and screwed to the existing screw mounts on the original PCB, to keep the wires organised.

Here you can see the interface stripboard mounted on the top of the PCB on the right.

2. CV Wiring

The CV in is located at the 680R resistor, in the middle PCB, near the round 741C Op-Amp.

The PCB in the middle.

There is a round big transistor look-alike - which is actually an old 741C Op-Amp.

The 680R resistor is just left to that.

[Click Image to Enlarge]

Here you can see clearly, the 680R resistor, just next to the 2.2k and the 10M.

[Click Image to Enlarge]

Another shot, with the 680R resistor circled in red.

C'mon, anyone can see that now!!

[Click Image to Enlarge]

There is some orange protective paint on the leg of the 680R resistor, since we need to cut this resistor (the loner leg part) and solder on to each sire, it is easier to scrap off the orange paint before we cut it.

I used a cutter to scrap off the orange cover, so revealing the metal conductive legs of the resistor.

[Click Image to Enlarge]

Then we need to cut this long leg of the 680R resistor.

Cut somewhere higher up, so we have a fair amount on both ends, so we can solder to each end.

[Click Image to Enlarge]

Here you see the 680R resistor cut.

[Click Image to Enlarge]

Two wires are soldered to each cut legs of the 680R resistor

NOTE:
The blue wire, soldered to the leg w/o the 680R, is the original keyboard CV, ie the CV that comes from the Mini-Korg keyboard.

The red wire, soldered to 680R resistor, is the CV TO SYNTH.

See the next photo.

[Click Image to Enlarge]

SWITCH

I used a switch, to switch between Internal Keyboard or External CV/Gate. (Or you can use sockets with switching connectors) .

Wire the RED wire above (the one that is soldered to the 680R) to middle pole of one side of a DPDT switch.

The KCV From Int. Kbd (Blue Wire above) goes to one pole on this side of the switch.

The remaining pole is connected to the CV Out from the interface stripboard.

3. Gate Wiring

The Gate is located at Pad 38 in the middle PCB, just a little bit to the right of the 680R resistor we were working on.

De-solder the GREY wire at Pad 38.

[Click Image to Enlarge]

The Grey wire is the GATE FROM the Internal Keyboard.
We need to extend this to the switch.

[Click Image to Enlarge]

Solder a wire to Pad 38. This wire is the GATE TO SYNTH wire (see next photo).

SWITCH

I used a switch, to switch between Internal Keyboard or External CV/Gate. (Or you can use sockets with switching connectors) .

Wire the RED wire above (the one that is soldered to Pad 38) to middle pole of the other side of a DPDT switch.

The Gate From Int. Kbd (GREY Wire above) goes to one pole on this side of the switch.

The remaining pole is connected to the Gate Out from the interface stripboard.

Well, the CV/gate wiring and switch is done.
The switch now switch between Internal Keyboard, or External CV/Gate.

4. "REPEAT" Wiring

We need to get the REPEAT/BENDER/PORTAMENTO output from Pad 11, which is located on the PCB on the right. Just solder a wire from Pad 11 to the Interface Stripboard.

Pad 11, located on the PCB board on the right.

[Click Image to Enlarge]

Pad 11 is the one with a lag on top.

We can solder to this lag.
But BEWARE of DRY JOINT.

I scraped the lag a little with a cutter, so that it can stick to the solder properly.

[Click Image to Enlarge]

Here you see a wire (green) soldered to Pad 11

This Wire from Pad 11 goes to the remaining FROM REPEAT point on the interface stripboard.

How this Pad 11 thing works:
The REPEAT Switch/Slider, the PORTAMENTO Switch/Slider, and the BENDER Switch/Slider each generates a certain amount of control voltages, and are then passed from the slider to circuitry on the PCB (near Pad 10 / Pad 11).
Then the BROWN wire at Pad 11 actually goes to the Internal Keyboard - where this extra CV is added to the keyboard CV/Gate, before they go down to the 680R resistor and Pad 38.
This is why normal wiring to the 680R and Pad 38 won't get you the REPEAT/BENDER/PORTA functions.

Part 4: MOUNTING

On this Mini-Korg 700S, I mounted two mini-jack sockets and the switch at the far rear right of the keyboard.

[Click Image to Enlarge]

And from outside:

This completes the installation guide to the Proper CV/Gate mod.

TUNING & TESTING

When first connected to External CV/Gate, the Mini-Korg might not sound, or you get a very low oscillation. This is probably due to the TRIMMER on the interface stripboard, not set up properly.
It needs to be turn, one way or the other, then you should start getting a wider range.

Also, you need longer (> 1 to 2 sec) external key presses, just send external CV by holding down 1 key for a few seconds, then turn the trimmer. Once the CV/Gate gets to a suitable range, the mini-korg will trigger.

Then all you need is to tune the external CV so that the mini-korg is in tune with the outside world.

	Synths
	*Last Updated:*
*KORG 700S (Mini-Korg K-2)* #2**
Includes Step-by-step Proper CV/Gate Mode Installation Guide
\| INTRO \| MODS \| Mod: IEC Mains Socket \| Mod: Proper CV/Gate \|