Add a MIDI input to your Casio SK-1 Sampler

Here's how. You'll need:

The rest of this page has all the instructions (and pictures) for installing the mod in the SK-1 (or SK-5). Also, the original August 1987 Keyboard Magazine article is now available for you to read.


Now available: a PCB layout for the SK-1 MIDI Mod! To download this layout as an Adobe Acrobat (PDF) file, click here. This layout is 1:1, and is provided AS IS. It is untested (i.e., I've never made a board from this layout) and is provided as a way for the enterprising DIYer to make a board for this mod. I HAVE NO BOARDS so DO NOT ASK.

A List of Necessary Parts

To build the SK-1 MIDI mod, you will need the following parts. Most of these parts are available from electronics distributors such as Digi-Key, Marshall Electronics, Jameco, B.G. Micro, and others.

Qty Part Number Description Reference Des.
1 80C31 Microprocessor, 128 byte RAM, 4K EPROM U3
1 2732A EPROM U5
1 74LS373 Address Latch U4
1 PC-900 Opto-isolator, Sharp U2
1 LM7805 5 Volt Regulator U1
1 5 Pin Din Connector, 180 Deg., PCB mount P1
1 Crystal, 12 MHz X1
1 1N4148 Diode, Silicon CR1
4 Resistor, 10K, 1/8W R1, R2, R3, R4
1 Resistor, 270, 1/8W R6
1 Resistor, 220, 1/8W R5
1 Resistor, 8.2K, 1/8W R7
2 Capacitor, Elect., 10uF, 16V C1, C4
2 Capacitor, Cer., 27pF C2, C3
1 14 Conductor Ribbon Cable, 13 inches
1 Stranded Red Wire, 4 inches
1 Stranded Black Wire, 4 inches
1 4-40 Screw, Nut, Lockwasher
1 24-pin Low-Profile IC Socket

Substitutions: If you can't find the Sharp PC-900 opto-isolator, substitute the more common 6N138. It uses a different pinout, so you'll need to connect it like this:


Likewise, other size EPROMs can be made to work by grounding the extra address pins. You can use an 80C51 in place of the 80C31 without modification.

Schematic Diagram

[Schematic goes here]
Click on the image above to view a larger drawing, suitable for printing, mounting, and framing using a tasteful custom frame.


Some tips to make the construction phase easier:


Open the case of the keyboard by removing the screws from the bottom of the instrument and carefully opening it up. The top of the case, with the keys, buttons, and circuit board will flip up and the two halves of the case will then lay flat on your workspace. Orient the top of the case nearest you and the bottom furthest away. See the picture below.

Fig. 8. After removing the recessed screws, open the SK-1 and set the
front panel on its face.

The circuit board is attached to the keyboard and front panel by several ribbon cables. These will act as a hinge and allow it to lay down in the other half of the case. Carefully pry the volume knob off of its shaft, remove the 7 Phillips-head screws that attach the main synthesizer circuit board to the front panel and lift the circuit board up. Then lay it down on the bottom half of the case, like this:

Fig. 10. Remove the main circuit board and set it aside. The new circuit
board and the end of the ribbon cable are visible at right.

You can just see the MIDI mod circuit board in the upper right corner of the picture. The mod will go into the cavity next to the speaker, nearest the back of the case. You will need to drill a 5/8 inch hole in the case to provide access to the MIDI connector, using this template to mark the location of the hole.

Fig. 9. Template (shown actual size) for drilling the new hole in the
rear panel to accommodate the MIDI jack.

Use a forrestner bit or start with a 1/8 inch bit and drill larger and larger holes until you reach the final size. Very important: before you drill, make sure you are drilling in the proper location. Measure it several time, and then measure it again. Drill the small hole and check the location of the connector by placing the mod in the cavity. By drilling the hole carefully you will keep from making a mess of your nice SK-1.

Once you have the hole in the correct place, fit the mod into the cavity. Route the ribbon cable over to the back of the SK-1's circuit board. The mod should look like this now:
Fig. 11. The new circuit board is now in place. The ribbon cable
extends over the end of the main board.

Form the other end of the ribbon cable into a curve, as shown in the next picture. Trim it to shape and strip and tin each of the wires. Attach them to the pins on the back of the SK-1 circuit board where the small, 12-pin ribbon cable (from the keyboard) is already attached (just above the "TUNE" adjustment). Make sure that both ribbon cables are soldered to the pads and that none of the pads are shorted together. Check the orientation of the ribbon cable from the MIDI mod. Pin 1 of the ribbon cable should be on the right. Connect the power wires to the power connector on the SK-1 board--red wire to the terminal nearest the back of the case and the black wire to the innermost terminal. If in doubt, use a voltmeter to check the polarity.

Fig. 12. The other end of the ribbon cable is stripped and soldered to the
main board as shown here.

Once everything is wired correctly, fold the circuit board back onto the top half of the case and close the SK-1. Use a couple of screws to temporarily hold it closed.


The MIDI mod has several modes of operation. By default, the interface will set itself to receive on the first channel that it hears. For example, if the first message that it receives (after being turned on) is on channel 5, from then on it will only respond to MIDI data on channel 5. The interface will forget its receiving channel when power is removed.

To force the instrument to respond only to a particular channel, connect the "F" jumper and set the address using the "1", "2", "4", and "8" jumpers. See the table below for help.

The SK-1 can only play four notes at a time. Normally, the MIDI mod will reassign notes on a first-played first-reassigned basis if more than four simultaneous notes are played via the MIDI input. If the "A" option is jumpered, notes after the fourth one will be ignored. This is similar to the way that the SK-1 handles voice assignment using its own keyboard.

Because it is designed only to work with its internal 31-note keyboard, the SK-1 can only play notes from F2 to C5. The MIDI interface will normally wrap notes that are outside of this range to the nearest playable octave. So, if F1 is played, it will sound the F2 on the instrument. Likewise, C#5 will wrap to C#4. To disable this behavior, connect the "X" option jumper. With "X" selected, out of range notes are simply ignored.

A F 1 2 4 8 X Option Description
0 x x x x x x Enable Voice Reassignment
1 x x x x x x Disable Voice Reassignment
x 0 x x x x x Auto Channel Assignment
x 1 0 0 0 0 x MIDI Channel 1
x 1 1 0 0 0 x MIDI Channel 2
x 1 0 1 0 0 x MIDI Channel 3
x 1 1 1 0 0 x MIDI Channel 4
x 1 0 0 1 0 x MIDI Channel 5
x 1 1 0 1 0 x MIDI Channel 6
x 1 0 1 1 0 x MIDI Channel 7
x 1 1 1 1 0 x MIDI Channel 8
x 1 0 0 0 1 x MIDI Channel 9
x 1 1 0 0 1 x MIDI Channel 10
x 1 0 1 0 1 x MIDI Channel 11
x 1 1 1 0 1 x MIDI Channel 12
x 1 0 0 1 1 x MIDI Channel 13
x 1 1 0 1 1 x MIDI Channel 14
x 1 0 1 1 1 x MIDI Channel 15
x 1 1 1 1 1 x MIDI Channel 16
x x x x x x 0 Wrap Out of Range Notes
x x x x x x 1 Ignore Out of Range Notes

Testing and Troubleshooting

Connect an Casio 7.5 volt power adapter to the keyboard and turn it on, then off and back on again. For some unknown reason, the instrument becomes confused when the power adapter is connected. The SK-1 should respond normally to its own keys. If not, recheck your wiring.

If it plays correctly, connect a MIDI keyboard to the SK-1's MIDI input and play some notes. The SK-1 should make the appropriate sounds in response. If it does, congratulations!

If the MIDI input does not work, check your wiring. Make sure the EPROM is properly programmed and is inserted in the socket correctly. If you have an oscilloscope, check the pins of the 12MHz crystal to see if it is oscillating. You should see the pins on the EPROM bumping and grinding as well. The most common problems involve the connections between the microprocessor, address latch, and EPROM. It is all too easy to swap a data or address line.

Once everything is working, and you've selected the options you want, attach the mod's MIDI connector to the back panel using a small amount of super glue or hot glue. Close the case, reinstall all of the screws, and put the volume knob back on its shaft.

Pat yourself on the back for a job well done. Enjoy!

Copyright (c) Paul Messick, 1998.
SK-1 MIDI Retrofit 8051 firmware is Copyright (c) Paul Messick, 1986, 1988.