Casio SK-1: Overhaul, Part Two

Saturday, January 11, 2014 at 9:20 pm

Casio SK-1 Gate Input and Individual Outs

Casio SK-1 Gate Input and Individual Outs

With all the groundwork laid out the previous time, ’twas time to actually start making things more useful! I decided to add a gate input, so I could sync the SK-1 to my other gear… and to build a breakout box for the port I added last time… click on for more.

First off, I was tired of fretting about yanking off the battery wires… so not only did I extend them, I made the whole thing detachable!

SK-1 Detachable Battery Fing

Tinned the wires on one side, and soldiered a little connector onto the other end. Shove the wires in; fits neatly in behind the speaker. Easy to extend it (so you can keep working on it with the batteries connected) or disconnect entirely (when drilling more holes and so on). I’ll probably wind up re-using this idea on other projects.

Next up… I wanted to be able to sync the SK-1 to my other music gear. The SK-1 is certainly a blast to play the old-fashioned way, but those of us with less-than-perfect timing would often prefer to let the computer play. 🙂 As I have MIDI to CV/Gate, I can use a a tap-tap-tap of MIDI notes to generate clock pulses at any sort of tempo or rhythm: From that MIDI, through CV/Gate conversion, and we get a 5v pulse out. Simple to work with, but still — how to get the SK-1 to understand it?

Casio SK-1 Gate Trigger for One-Key PlayWith an NPN transistor and a little fiddling around, you can usually get a 5V gate pulse to trigger any button on a device. I used a 2N3904 transistor, but any of the common NPNs should work just as well. Find the part of the circuit the key press closes, and put the transistor there, collector and emitter straddling the two points. Due to the nature of the circuit (keyboard scanner) direction CAN matter, and you may need to add a resistor on the input, even a cap, just stop it from being flaky — sudden flutters of keypresses, unreliable response to gate pulse, that sort of thing. My advice is to test it thoroughly before sealing it in, because it probably won’t work on the first go!

SK-1 Gate / One-Key TriggerAfter you get it working, seal it in with electrical tape, and wire the jack’s ground to ground.

Casio SK-1 Gate Input and Individual Outs

Casio SK-1 Gate Input and Individual Outs


Next, I have that lovely bend port that I’ve been using with a fragment of wire clipping. This is, to say the least, unsatisfying. Time for a breakout box!


How The Squirrel Box originally looked: Pre mod, pre-castration. The horror

I’ve dubbed this box ” The Squirrel Box” for obvious reasons. I think it was originally meant to be hung on the wall and house keys, as it had a series of hooks inside. It was about the right size, and it was lying around… so, why not?

As the “nuts” took up a lot of area, I made the executive decision to castrate the squirrel box. I’ve saved them, though (I put them in a jar, just to be poetic), and as they’re metal, they may find future use as contact points.

Mounting terminal blocks in squirrel box

Laying out terminal blocks.

Lots of Wires

Lots of Wires

Lots of Wires, Tinned

Lots of Wires, Tinned

Wiring terminals in squirrel box

Wiring up terminal blocks

Squirrel Terminals, all wired up

Terminal blocks all wired up. Some of the wires in the cable were duplicate
ground wires, which was really annoying to map out!

Squirrel Box patchable controls

Adding some patchable controls with neatly twirled and tinned wires.

The idea is that I can now connect The Squirrel Box (the breakout box) up to my SK-1’s bend port, and have fun all day with alligator clips… then close the lid and jam a bit! Modular bending all day; no removing the speaker for a patchbay. Disconnect everything, and it’s a normal SK-1 again — portable.

Bent SK-1 with squirrel box (open)

Bent SK-1 with squirrel box (closed)

SK-1 with Squirrel Box, patched and ready to jam. You can see I have a foot pedal connected
there too; I found myself wanting one after trying to play switches and keys at once.

…But, back in the studio, it’s much more useful.

Casio SK-1 Gate Sync

It has individual outputs for each voice, and for the rhythm section. The individual outs bypass the amp, allowing for a vastly cleaner recording. They also cut out some filtering, which I am on the fence about. Even though it’s crappy ol’ RC filtering, it’s still part of the SK-1’s sonic character. Can’t do much about it, though, as they’re shared across voices and such. So, no way to have that filtering on the individual outs unless I clone the filter circuit a few times. In any case, it still beats the pants out of the noisy amp… not to mention that the drums have always been too loud, and being able to set their level independently is vital to me not going deaf! I think it might make sense to hardwire in a level control for rhythm and then wire in another output that is post-mix, post-filter, but still before the noisy amp, just to provide an additional way to record it. As it is, though, I really prefer the individual outs. Things get nice and wild when you start processing each voice differently!

The gate input triggers one of the one-key-play buttons. I’ll program the SK-1’s 3-track sequencer, then use the gate input so the SK-1 plays along in time. I had moaned that the SK-1 could be as good as the Roland SH-101, if only I could sync it, and control it in some way other than manually playing the keys… so, while this isn’t perfect, it’s still awesome. The sequencer is a bit tough (like the SH-101’s) but it’s usable if you’re patient. The one real fly in the ointment is getting it to loop; you have to start it over when your sequence ends. I hope to solve this soon, somehow… any ideas?

Another thing that bugs me (along with many others) is the auto-off feature, through which the SK-1 will automatically power itself off after seven or so minutes of no keys being pressed. Given that the sample gets erased when the power goes off, this can be heartbreaking. The only solution I’ve seen on the internet involves literally building a slow-ramp timer with a 555 dedicated to pressing a key every few minutes. I feel like there’s some potential to use the shutdown signal against itself, however: It sends a line on the CPU low; this could be used with a PNP transistor to press a key. I took a cursory stab at it, but I was unable to make it work. Given you have to wait a few minutes each try, it’s slow going… so, in the end, I just sealed it up.

After all, I wanted to play with it…

SK-1 fun


Categories: circuit bending / SDIY, gear, sampling, Uncategorized

3 Comments on “Casio SK-1: Overhaul, Part Two”

  1. Hi! I’m working on a SK1 too! I’ve only very recently gotten into the electronics world so my stuff is more rudimentary. I’ve been working on using a cmos 555 timer to trigger the auto-play button, which will automatically loop the sequence programmed into the SK1. In your case you could just add a gate input to the auto play button like you have a gate input to the one key play buttons.

    Question: How did you attach the gate input to the one key play button? Did you use a NPN transistor? My attempts with a JFET and more recently a MOSFET have not been working and i’m not sure where i went wrong.

  2. Oh whoops I guess you did use a NPN. I’ll have to try that again, I thought my first attempt with it didn’t work because of leakage current.

  3. Glad ya got her finished and buttoned up 🙂 I did a similar project with a Ct370 but used a hard drive ribbon cable as my patchbay and little breadboard floppy jumper wires for patching 🙂 Nice and tidy and you can even get some contact bends by pluggin a wire in and touching it. Anyhoo, looks good and glad you have it in your collection 🙂

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