gThe Mini-3 Keyboard
The Mini-3 Keyboard
Find A Keyboard
The first step in building a analog synth is the keyboard itself.
The best option is to buy a broken keyboard from a music or thrift store
and convert it. I have done this three times so far the most expensive
costing about $7. The Casios seem to be the easiest to convert because
the PC-board for the "diode matrix" is mounted directly to the back of
the keyboard.
Remove The Keyboard
Remove the screws from the case and the cut, carve, pry, unscrew, and
pull the existing electronics from the case. Be careful not to break or
damage the PC-boards on the keyboard itself. Cut the keyboard away from the
plastic case. Be careful to leave enough plastic on the sides of the
keyboard to mount it to another case.
Cut on the dotted line
Remove the PC-boards
Remove the screws holding the PC-board(s) to the back of the keyboard. Then
remove the PC-board. Save the screws and mounting hardware we will put
the boards back on.
Remove The Diodes
The keyboard is really a bunch of switches connected by a diode matrix.
We need a resistor chain so the diodes must be removed from the board
and the solder pads cleaned.
Install The Resistor Chain
The chain is made with 100 ohm 1% resistors, one for each key minus one.
So if you have 32 keys you need 31 resistors. It does not matter how
many keys you have (up to 61 anyway) the keyboard circuit can be scaled.
I was lucky enough to find 100 ohm 1% resistors in a five pack it was
simple to hot glue the resistor pack directly to the PC-board and then
route the resistors to the pads.If you are not as lucky as I was you can
do the same thing with discrete resistors.
What we want to do is turn something that looks like this.
Into something that looks like this.
Reassemble the keyboard
After all of the resistors are in place bridge the solder pads
on the edge of the board to make a common point for all the switch pads
to connect. Solder the connection wires to the ends of the chain and the
common point before you screw it back to the keyboard.
Testing the keyboard
Attach a digital ohm meter to the common point and one end of the chain.
Press a key and measure the resistance. It should be 100 ohms X the number
of resistors between the pressed key and the end of the chain. IE.. The
lowest key should be 0 and the highest key 3100 ohms (on a 32 key keyboard).
