Korg 700S Mods1 - cykong.com

Proper External CV/Gate In

Again, like the 800DV, the Japanese E-Lab site has all the info and the proper CV/Gate Mods Schematics on the Korg 700 (site in Japanese). Since the 700S is essentially a 700 plus the Ring-mod board, the basic CV/gate section is the same and is compatible.
[I don't fully understand Japanese, but can read certain Chinese characters that Japanese uses and also I do know a fair amount of the "English" characters in Japanese symbols, so I could roughly understand the info on the site. Also, info on the schematics are given in English.]

The circuit/schematic that he provides, also incorporates a CV Tuner, which is very useful!

E-Lab site actually provides 2 different CV/Gate mods, one BASIC and one FULL which would keep all the Repeat, Bender, portamento, Vibrato working under External CV. This was the one I headed for.

UPDATE (2005/07):

Before, I didn't put up the full details and schematics on this site because I don't like to look as if I am stealing E-Lab Japan's great work(s), but...
Since E-Lab (Japan) site has vanished (as from mid 2004)...
I did manage to save some of his pages for offline viewing.
So, here are the schematics diagrams for the CV/Gate Mods: BASIC and FULL.

The Stripboard

700S Full CV-Gate Stripboard:

Click to Enlarge

As suggested by E-lab Japan, I used the OP-07 op-amp instead of having to use a stepdown voltage regulator (as the 700S operates at 20V, but most op-amps' max supply voltages are < 15V)
The circuit is again built on stripboards...

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 (for the FULL Mod)

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)

This is roughly how it looks like when built:

Click to Enlarge

Mounting and Wiring

POWER Tapping

The +20V/GND/-10V can be tapped from any of the 3 Korg boards (or on the 4th board on the 700S).
Basically they're on the pins on each board,
ones marked +Vcc is the +20V (usually with orange/red wire soldered to),
ones marked GND is the 0V (usually with black wire soldered to),
and the ones marked -Vee is the -10V (usually with blue wire soldered to).

Easiest is to fins them on the left-most board (the PSU board):

Picture courtesy / from E-Lab Japan

Other Mod Locations....

UPDATE (2005/07):

Originally I didn't put up full details on where to solder the CV/Gate mod, because I respected E-Lab Japan's work, so I put a link above to link to his page, where all the instructions are given. (Although in Japanese, the images are in English, and should be very helpful).

However, after his page has vanished in mid 2004, I have a few enquiries about the CV/Gate mod.
And in July 2005, this really cool guy called Dirk has decided to follow my stripboard to build his CV/Gate mod.
so it's kind of my responsibility that one fully understands what's going where, but that reference page has vanished!

BUT WAIT! I have the offline contents saved on my harddisk.
Well, the page is basically in Japanese. I don't understand Japanese, although I can read the Kanji (same as Chinese characters) and also I understand the Japanese English Character Symbols (i.e. some symbols that when put together and pronounced, it becomes an actual English word).
So I decided I'll write some notes underneath his original Japanese paragraphs, so anyone reading them would get some idea of what it's about, instead of just looking at the images!!!

---> Full (Proper) External CV/Gate Modifications <---
by E-lab Japan, with notes by C.Y.!!

UPDATE2 (2005/07):

Well, I have actually modded my 2nd Mini-Korg 700S with Proper CV/Gate.
This time I have taken a lot of photos and written a step by step guide, similar to E-Lab Japan's one, but in English. Please see the Korg 700S #2 Page.

---> Step-by-Step Proper Ext. CV/Gate Mod <---
by C. Y., on Mini-Korg 700S #2

Mounting the Stripboard

Loads of space around the VCF PCB area....so i mounted the CV-Gate stripboard above the VCF PCB.

I moved the name plate on the rear to the centre - because the extra Ring mod board is located vertically in the rear centre, and there's no space to mount any sockets. By moving the plate to the centre, the area behind the VCF board provides lots of spaces for many many sockets!

The REPEAT function is tapped from Pad 11 on the VCF board:

I use a small ultra miniature DPDT switch to switch between the Internal keyboard and External CV.
On the left panel, there's a space but was covered up by a name plate. I removed the plate and put it somewhere above, then mounted the Int/Ext switch there:

External Aux CV to VCF Traveler
At first, I thought the CV->VCF would be like the 800DV VCF CV In, but i was wrong, since the 800DV is NOT a dual 700/700S but a dual 770... In fact the 700/700S VCF CV is slightly simpler...
External CV to VCF Schematics: Click to Enlarge!	The HPF CV in is actually PAD 6 and LPF CV in is PAD 3. The Traveler sliders are actually 10K sliders, so in theory, wiring a supplying Ext CV to a 10K resistor tapping to pad 3 and pad 6 should work the Traveler. Pad 3 and Pad 6 are located at the bottom right of the VCF PCB board.


I used a DPDT Switch to switch between Internal and External VCF control. Also, I added another SPDT switch that shorts the external CVs together, so i have a switch tat 1 Ext CV controls both the HPF/LPF linked together.
I mounted the 2 switches next to the VOLUME slider at the front. Had to drill holes on the front panel and the inside panel, but it is not too hard....

NOTE
The internal Traveler actually feeds the +20V to pads 3 and 6. I was using a Kenton ProSolo feeding Aux CV to these, and it only goes from 0 - 12V, and only affects the VCF to roughly 3/5th - logical - around 12V out of 20V. So there is a need for an additional circuit to "boost" the Aux CV to full 0-20V range for this to work exactly the same as the internal Traveler.

	Synths
	*Last Updated:*
KORG 700S (Mini-Korg K-2)
MODS - Rear Panel & Sockets, Proper CV/Gate Inputs
\| Synth Index \| Korg 700S \| 700S Mods1 \| 700S Mods2 \| E-Lab Japan CV/Gate Mod \|
\| IEC Mains Socket \| Proper CV/Gate Input \| Aux CV to VCF \| Key Hold \|

IEC Mains Socket
The 700S came with a little compartment at the back to hold its power cable & power plug, similar to the 800DV. It's quite neat, but it's still a flying lead.... so I added an IEC Mains Socket and removed the original flying lead. The lead compartment is now empty....

I placed the IEC mains socket next to the compartment, in fact it's quite close to the fuse holder. I could have put it in a less cramp area, but i needed the other spaces for other sockets.
Note: To make the IEC Mains Inlet hole, I use a screwing hand tool, which is an M10 Radius Hand Hole Cutter, 28x21mm (RS Cat.#543-614 or or 541-501), with matching 10A Snap In PCB Mount IEC 320 Plug Socket (311-8031 or 488-191).

Keyboard Key Hold Switch
Simple switch, wired to the internal keyboard PCB, shorting out a key just like you pressed one key - this is useful, for example if say using external audio in processing thru VCF, so i dun have to use masking tape to tape one key down to trigger the envelope!
	Whichever one to short doesn't matter, just short it to the tip to the brown wire. I shorted the C2 key. Mounted the switch to the right of the Traveler on the front panel.