Circuit Bending

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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.

Mute for the instruments in the rythm section

Mute for Bass Drum and Claves

Bass Drum Mute

Claves Mute

Mute for Snare Drum

Mute for HiHatOpen

Mute for HiHatClosed

Mute for Tom1

Mute for Tom2

Mute for the Casio-Chord and Bass Accompaniment

Chord Mute

Bass Mute

Accompaniment Hold

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

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):

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.

Diode

Required by Octave and Key-Hold switches

Speaker Switch

Phones Switch

Line Out Switch

Distortion Potentiometer

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

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.

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:

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 november 17th 2016

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:

Does it bend? ;)