Casio MT-68

This is what i bought:

What you see here is a Casio Keyboard from 1983 - it has an internal speaker in the top left corner, which had to go to make room for the knobs. Speaking of knobs - If you look closly you'll notice that 3 sliderknobs and one rotary knob were missing...I replaced them with selfmade "copies" made of gorillaplastic (thats something like sugru). The new knobs don't look perfect, but work well. Otherwise it was working well when i bought it.

Please note: MT-68 is identical to MT-65 (in all but the color which is white for the MT-65) MT-100 uses nearly the same wireing and the same chips, but ommits a few functions of the MT-65/MT-68 and has a graphic equalizer, which the MT-68 / MT-65 do not have.

Other Casio Keyboards use the same chips, but may differ stronger in the way they are build: MT-45 , CT-410V, MT-400V - check shematics (See links in top left corner) for more info on that.

This is what it looks like "finished" (for the time beeing):

The Speaker, now on the bottom side:

Functions of the switches and Potentiometers:

Internals

Here you see the internals of the MT-68 unaltered. Central is the M440-MAIM PCB. On the right side u can see its main chip, a NEC D930G, the actual sound synthesis takes place in the D931 which is directly beneth the letters saying "M440-MAIM". On the left side you see the amplifer/external conector board named M440-MA2.

Here you see the component side. Because of the ribbon cable it can not be accessed with the same ease as the other side. For conveniance, casio even named the analog rythm section instruments, BD for bassdrum, CL for claves and so on.

step by step and spoiler alert

To help others, especially people like me which are more or less new to circuit bending, I show what I did in a step-by-step tutorial - just pick the steps u like and copy. Use all info at your own risk! That is: stay away from higher then battery level voltages and curent, and ask someone who knows a little about soldering and electrictiy to assist you if you are a complete newbee. Be aware that also the bended "toys" can suffer, up to breaking completly. Also im not liable for any errors that this page may contain.

Using shielded cables when possible may help to sustain the sound quality.

I use microphone cable from the electronics store: its a twisted pair with shielding, and its relativly thin at around 3.5 - 4 mm or so; also, it was fairly cheap.

Also, a "loop" should not be formed: If a wire goes from the PCB to a switch or potentiometer and another one back to the PCB, as this will act as an antenna. Better use twisted pairs of wire.

On all photos, if there is a line with a letter or number at one end, i ask you to solder onto the OTHER end of the line, the lines are just there for better visibility.

Speaker Move

In this step the internal speaker is moved to the bottom side of the MT-68, to make space for lots of knobs and switches on the top of the MT-68. Of course, if the MT-68 stands on a table, it will be far quieter then before. I don't mind, as I use it with headphones or via external speakers most of the time.

remove the screws and unsolder the cabel on the speaker terminals
use the speaker as a gauge to mark holes (B)
drill holes using a plasic or wood drill into the 4 (B) and one into (A)
use a jigsaw to cutout (A)
file down the edges of (A)
find some old speaker grill (some metal mesh, could even be a kitchen sieve) and cut it to match (C)
decide if the mesh/grill goes in the inside of the case, or on the outside
screw in 3 screws at (D). Use plastic or self-cutting woodscrews, and screw from out- to inside, so one can not cut oneself on the tips of the screws.
screw the speaker into the new position
find a kabel that is long enough to connect the speaker (give it some extra length, as the whole MT-68 is better handable, if this is not so short) and connect to the old cable and the speaker
use heat shrink tubing to make shure that no unwanted contact is made between cable and PCB.
close case and test function

Mute for the instruments in the rythm section

Mute for Bass Drum and Claves

Bass Drum Mute

desolder one Leg at (I) (at the little red dot)
solder one wire to the PCB long enough to reach where u want to put the switch
solder one wire to the open leg of the capacitor (may use heat shrink tubing to prevent short circuit)
connect switch
and test

Claves Mute

desolder one Leg at (III)
solder one wire to the PCB long enough to reach where u want to put the switch
solder one wire to the open leg of the capacitor (may use heat shrink tubing to prevent short circuit)
connect switch
and test

Mute for Snare Drum

desolder legs of resistor and capacitor at AA and BB
connect the open legs of resistor and capacitor to each other, and connect this point to a switch
connect BB to other side of the switch
and test

Mute for HiHatOpen

desolder one leg each of the TWO capacitors at Y
connect the two open ends with each other
connect the solder pad at Y with the switch
connect the two capacitors with the switch

Mute for HiHatClosed

desolder on leg of capacitor at HHC
connect wire pair to HHC and open leg of capacitor
connect to a switch

Mute for Tom1

desolder on leg of capacitor at T1
connect wire pair to T1 and open leg of capacitor
connect to a switch

Mute for Tom2

desolder on leg of capacitor at T2
connect wire pair to T2 and open leg of capacitor
connect to a switch

Mute for the Casio-Chord and Bass Accompaniment

Chord Mute

important: use a shielded twin cable, idealy twisted
connect the shielding to ground
desolder resistor Leg at (C)
solder one wire to the PCB long enough to reach where u want to put the switch
solder one wire to the open leg of the resistor
connect switch
and test

Bass Mute

important: use a shielded twin cable, idealy twisted
connect the shielding to ground
desolder Leg at (B)
solder one wire to the PCB long enough to reach where u want to put the switch
solder one wire to the open leg
connect switch
and test

Accompaniment Hold

This switches a Note-Hold, or if Accompaniment is on, a Accompaniment-Hold in the Left Section (Bass-Section) of the keyboard.

desolder one leg of the diode between A and O
insert switch in series to the diode
use heat-shrink-tubing to prevent shortcuts

Key Hold

Played notes are sustained. Works with up to 4 notes, the 5th note replaces the 1st, the 6th replaces the 2nd and so on, so that never more than 4 notes play at any one time (as the melody-section of MT65/68 is 4 voice polyphonic):

solder one wire to the solderpad at (Y)
solder one wire to the solderpad at (Z)
also add the diode (see the passage "Diode")
connect a switch
and test

Octave switch

The Octave switch moves the whole melody-key-section one Octave up (or down if it was switched to "up"), only works while MT-68 is in "chord" mode.

solder one wire to the solderpad at (A)
solder one wire to the solderpad at (B)
also add the diode (see the passage "Diode")
connect a switch
and test

Diode

Required by Octave and Key-Hold switches

use any simple cheap standard diode

Speaker Switch

Just cut one Wire to the Speaker and insert a switch

Phones Switch

desolder one leg of resistor at P and connect 2 wires to a switch ( second wire in this photo is on other side of PCB) for Phones switch

Line Out Switch

desolder one leg of resistor at L and connect 2 wires to a switch ( second wire in this photo is on other side of PCB) for Line Out switch

Distortion Potentiometer

if possible, use a shielded twin cable, idealy twisted
connect the shielding to ground
connect one wire to (D)
connect one wire to (G)
connect one end of the cable to the center pin of a 100K Ohm Potentiometer
connect the other end of the cable to one of the outward pins of the 100K Potentiometer
as this works only in a small portion of the potentiometer, it might be better to put a resistor, value somewhere between 20K Ohm and 80 Kohm and a 10 or 20 K Potentiometer in series, if you try this and get a good result, please tell me! you can contact me here.
test

Little Distortion Switch

For me, this adds a little extra distortion to the Potentiometer, however, this seems very little and somehow seems to be part of the same circuit as the "Distortion Potentiometer"- Mod - im looking for better connectionpoints for this switch

if possible, use a shielded twin cable, idealy twisted
connect the shielding to ground
connect one wire to (U)
connect one wire to (V)
connect switch
test

Key Volume Potentiometer

With "Key Volume" i mean a potentiometer that allows to regulate the volume of the played note without changing the volume of the Casio Chord / Bass etc. This needs a shielded cable. connecting the shielding to ground close to the connection on the PCB is recomended.

desolder one leg of the resistor at KV
conect the shielding of the cable to ground, I extend the shielding and connected to point G
connect the two inner wires of the shielded cable to KV and the open resistor leg (on other side of PCB)
on the other end of the cable connect the two inner wires to the middle and one outer leg of a 470 K Ohms Potentiometer
also recomended to connect shielding on the potentiometer side to ground, too
test

Pitch

As I wanted to keep the "tone" regulator (originally a 100K Potentiometer) on the back of the MT-68, didn't find a better conction then directly to the tone Potentiometer, and it did not work well with a 100K Potentiometer in parallel, this is somehow a little more complex:

use a desolder pump if possible
desolder the 100 K Potentiometer, remove all solder as far as possible from the pads (A,B,C and S)
get two 220 K Potentiometers
solder 1 Potentiometer into the same place as the old "Tone" Potentiometer was.
shorten the Potentiometer with sharp cutting plieres (I left about 1 cm for better control)
file down the edges of the axle
prepare 3 wires of the same length, mark both ends of one wire (remove a little insulation)
connect marked wire to B
connect the other 2 wires to A and C
braid all 3 wires to one
solder the marked wire to the central pin of your 2nd 220K Potentiometer
connect the other 2 wires to the 2 outer pins of the Potentiometer
test

Last Words about the MT-68

These final two pics are of the connected wires inside, at least most of them, and of the switch and potentiometers rear view. I think these pics are more a reference in "How not to do it", but may give an impression of the resulting chaos, if you, like me, just add one bend after the other without any real plan on sorting and organizing the stuff. I had to break away some plastic-wall around the speaker inside the keyboard, to make room for the wires going in and out of this box when the top of the keyboard is fitted to the bottom.

thats alll for now... ...more to come... page last updated June 27th 2018

Thank you's go out to Michael for the 1st constructive feedback!

Thank you to Dennis also, who pointed me to a bad mistake: The Snare Drum Mute did not work, but it should work now!

Feedback, positive or negative, about the page or its content, via the contact form. also, i'd be very happy to read of any improvments, error corrections or additions on the MT-68 bends. so ask yourself:

Circuit Bending