Modified Casio SK-2

Modified Casio SK-2 - You more commonly see the SK-1 modded. The SK-2, which is rarer, seems to be avoided by a lot of circuit-benders. That's not to say it isn't bendable. Not as bendable, perhaps, but bendable nonetheless. Here are the schematics for these mods.

When I found this keyboard at the re-use centre (where people take their less disposable trash rather than throw it in the landfill), it was covered in grime, spilled juice, etc...; had four broken black keys; and the battery compartment was missing. It cost less than fifty cents, and, despite outward appearances, was in perfect working order.



The first thing I did was take it apart; removing all electronic components, switches etc... from the top half prior to giving the plastic parts a jolly good wash. The broken keys were then epoxied back in place, and some that were in danger of breaking were also epoxied. With that done, it was all put back together ready for "probing."

It was immediately evident that a patch-bay was in order, for the number of bend combinations on the large ICs was innumerable. I'd never attempted a patch-bay before, but was confident enough to give it a go. I also looked about for pitch modifications, and while there was no way to pitch downwards (which is always more interesting), I found some pitch & tempo points which surpassed the range of the keyboard and already-present tempo select buttons.

The pitch mods made the whole instrument out of tune, even when taken out of the circuit; so I added those first so I could still access the hidden side of the main circuit-board where resides a tuning trimmer. I retuned it using my unmodified SK-1 as a guide.


Next came the tricky part: the patch bay. Cutting holes in the case was straightforward enough, except for the unforseen phenomena of the diamond dremel blade melting the plastic only for it to solidify again into slag that obstructed the blade. There were no major accidents though, and in short order I was looking at three gaping holes where the left speaker used to be.


Strips of RCA jacks come in set numbers. In this case, there are fourteen pins on the IC, so I needed 28 jacks. Thus I bought two strips of 8 and one of 12. The latter wouldn't fit comfortably on the case, so I cut it down by about 3/8". Be warned, the dust from cutting the mounting board is incredibly fine and toxic-smelling. Just how toxic I don't know, but I got the breather mask out just in case. I failed to buy any mounting screws so I had a rummage and used some that I had in my parts-box.

Having prepared 28 lengths of thin solid-core wire (I usually use stranded), I next prepared myself for the most nerve-wracking part of the build: hooking the jacks to the IC pins. It went surprisingly well once I got into the rhythm of things. I tested now and then just to make sure I hadn't killed the unit with excess heat from the soldering iron.

Success! All that remained was to fit the reset switches and an output jack. I didn't keep track but the build took approximately one evening and the better part of the next day. I'd intended to do it in dribs and drabs, but, as usual when I'm circuit-bending, once I started I became obsessed with getting it finished.

Oh yes, I also built a switch box with four leads for connecting patch-bay points. Each lead has an on/off switch, an interrupt switch, and a 100 ohm resistor to minimize stress on the circuitry. The box came with wood-screws in it: $1.50 at the local dollar store.

Modifications:
- 28-point patch bay. Connecting these various points causes loops and glitches and other such shennanigans when running the demo tune.
- Pitch/Tempo/Mangle knob. A 1k ohm potentiometer combined with an 8 ohm resistor. In theory it increases pitch and tempo of both rhythm and tone to warp speed. I also discovered a quirk in this bend after I'd installed it. As well as crashing the demo tune when turned all the way up, it will also corrupt whatever sample happens to be in memory. It adds distortion spikes on top of the audio information, resulting in sounds that Trent Reznor would be proud of. Mysteriously, playing around with the play/record/repeat selector will sometimes return the corrupt sample to normal again.
- Pitch/Tempo Tone knob. This one effects the tempo of the rhythm and the pitch of the tone separately, as opposed to the pitch of both. Another 1k pot./8ohm res. combination here. Both these knobs share an on/off and selector switch circuit.
- Envelope/Volume knob. This does a number of things. First of all it effects the sound of the synth tones, makes them sound a bit more synthetic and beefy. It also decreases tone volume and length. It's paired with an on/off switch.
- Soft & hard reset switches.
- 1/8" output.

Sound clips:

Alien modem A snippet of glitched-out weirdness.
Demo song + pitch/tone The demo song, which I've now heard more times than a person should, being cranked through the pitch/tone knob.
Hyper demo The demo song dropping out of warp speed via the pitch/tempo knob.
Crazy band The demo song again, now almost unrecognisable as the A.I. band begins to improvise.
Flute + envelope, part 1 The flute tone: the first few notes are raw, the rest have envelope mode engaged.
Flute + envelope, part 2 More of the same, with a touch of pitch shifting at the end.
Flute + envelope + pitch The flute tone sounding very high indeed.
Piano + envelope The piano through the envelope mod this time.
Sample mangle This is what happens to samples when you turn the dial to "mangle."
Mangled sample + env This is the same mangled sample heard above, through the envelope mod.
Glitches Mmm, crunchy.
Pitch loop A stalled bit of audio data, with some pitch shifting.
"Pop vweep" Similar to the above.
Rhythm glitch Trust me, this isn't a standard rhythm on this machine.
Traffic noises Glitches somewhat akin to synthetic traffic sounds.

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