SX-150 Mark I Synth

SX-150 Synthesizer (3300 yen): Ok, I finally broke down and bought the synth. Actually, I’ve been wanting to get this kit for weeks, I was just waiting to reduce my “snack money for kits” debt, while also squeezing out as much remaining entertainment value as I could get from the other kits I already own. However, that opened me up to a rather rude shock. One Sunday, I’m in two different bookstores in Akihabara, looking at the synth kit mook to see what’s required to add a 6V power adapter jack. The following Tuesday afternoon, the synth is sold out from both stores. Since I work Tuesday evenings, I didn’t have time to try looking anywhere else, so I had to sit and wait. The next day, I go to another shop – also sold out. This place had an entire stack of the kits a few weeks earlier, and now, nothing. Which is a little strange, since the synth had only come out last August and it’s going out of stock so quickly. But understandable since the smaller stores want to free up shelf space for selling the newest products. Anyway, I went into Kinokuniya in east Shinjuku, and they had two kits there – I bought both since my current plans are for using up to 3 kits on my homemade keyboard. Moral – buy what you need when you can (I’ll get the third later if my plans work out).

(The kit, once it’s assembled.)

The SX-150 is very similar to an analog synthesizer that PAiA Co. used to sell back in the 1980’s (PAiA’s still in business selling synth kits, but this one’s not in their catalog anymore). It has a simple triangle wave/square wave generator, controls for Attack, Decay, Pitch and Cutoff, plus a reverb switch. There’s an on-off switch, ext. source input and headphones output. The “keyboard” is a ceramic-like resistive strip and stylus. Where you put the stylus on the strip changes the output sound frequency. The case is heavy plastic, but the bottom plate is just a pre-punched piece of cardboard from the box the kit came in. The synth output is fairly limited as-is, being able to come close to a violin or a bass guitar, but mostly producing only sound effects suitable for 8-bit video games. The knobs and controls are flimsy, and the screw for holding on the bottom plate is too short to hold the plate in place (one of the knobs fell off when I was taking the kit apart, and it rolled somewhere so remote that I can no longer find it).

It’s a very simple kit, compared to the rest of the Otona no Kagaku series – the case, 5 knobs, the resistive strip, the stylus, the circuit board with speaker pre-wired, and 8 screws. There’s also a sheet of stickers if you want to fancy up the knobs and case. Even if you take your time and read the instructions carefully to ensure that you do everything right, it’ll take less than 15 minutes to assemble it.

(I’ve got the 6V DC adapter jack added already. Next is the jack for the noise generator input.)

My first mod was to add a jack for an external 6V DC power adapter. The second was my keyboard, and the third is a simple resistor string. The mook describes a mechanical sequencer (a hand-cranked drum with contacts to various resistive strips), a MIDI converter circuit, tactile bumps for use by the visually impaired, and how to make a wooden case and stylus. Youtube has a number of videos showing additional mods, like adding a piano-type keyboard, a push button bank, a larger resistive strip, a trill function and a noise generator. There’s one video showing a variety of enhancements to the waveform circuits themselves. What I haven’t seen yet, though, is a PC-driven digital controller to replace the ADSR volume controls. I’ve already built the fingertip keyboard circuit.

(From the bottom, minus the cardboard cover plate.

The Ext. Source jack feeds an F/V circuit (which I’m assuming means Frequency to Voltage), so any input source that has a changing frequency can replace the resistive strip. The mook suggests connecting up the Theremin for this, but it’s just as easy to plug in an MP3 player headphone jack (mono plug only), or to use Gakken’s Poulson Wire Recorder. The point is that there’s a lot of unexplored ideas for modding up the other Gakken kits to work in combination with the SX-150 (which I’ve already done for the Theremin).

(The full set-up (so far) – 2 synths, the keyboard, the relay and pots bank, and the theremin hooked up as an external input source. I’ve turned two pencils into fake drum sticks.)

The mook is a wonderful look into the history of synthesizers and the musicians that love them. There are various products from Moog, Roland, etc.; looks at Keith Emerson, Kraftwerk and Devo; and interviews with many Japanese musicians and engineers, including YMO (Yellow Magic Orchestra). There’s a comparison between physical synthesizers and software versions. And, there’s a description of a demo version of a software synth program called MT-1 (Music Track 1). ( And, if you needed suggestions, YMO offers settings for specific sounds for the SX-150 (electric guitar, ambulance siren, cat voice, and synth drums).

One of the guys mentioned in the mook has a few additional mods explained on his website. They include a couple variations on the ribbon controller, some changes to the waveform circuits, and an analog sequencer (same one shown in the mook).

(It’s a little “rats-nesty”, but I’ll get some tie wraps for that later.)

Summary: Of all the Gakken Otona no Kagaku (Adult Science) kits, the SX-150 is the one most amenable to being modded. Out of the box, the sound range is very limited, thin and unsophisticated. There’s no force control, so how hard you tap the resistive strip doesn’t affect the sound out (unlike a piano key). But you can’t expect much for 3000 yen (3300 including tax = $33 USD). Then again, with all of the mod suggestions, and possible sounds you can get when you’re done, $33 is an incredible deal, even if you just look at the entertainment value for reading the mook and the time spent on experimenting with the kit. Sure, you can get better synths for just a few hundred dollars, but the little SX-150 is still a fun toy to play with. Get 3 – they’re small. But only while supplies last.


Of all the Gakken kits I know of, the SX-150 analog synthesizer is the only one that really lets you modify it even before you have the kit in your hands. Because people have been using synths in a hobbyist capacity for well over 20 years, most of the standard circuits and sound modules have long been available on the market, and it’s a simple matter to determine what the SX-150 is missing, and how to add it. A case in point is the MIDI controller. One person posted a home-made controller video on youtube before the SX-150 even hit the shelves back last August.

After going through all of the youtube videos featuring the SX-150 that I could find, I decided that I’d make the fingertip keyboard in advance of buying the synth itself. This was both a good and a bad idea. First, it gave me something to keep busy with for over a week, from sketching up the initial layout to trying to find which parts were available where, to actually building it. Secondly, it broke the bank. I’ve been foregoing snacks and Tully’s coffee in order to redirect the money into buying the Gakken kits. I can generally scrape up $25 a week this way, but I’ve been getting really sleepy during the day, and hungry before I finish work at night. Normally, I’d just buy 1 Gakken kit a month, and the remaining money would go to paying off the “debt” I’d accumulated when buying all of the older Gakken kits prior to them going out of print. I’d gotten my debt down to $90, and then the keyboard circuit single-handedly more than doubled that.

The single biggest cost were the relays. I wanted 12 switches, and I wanted each switch to turn LEDs on and off along with making sounds from the synth. I tried going with a mini 3V relay, but the shops only had 1-pole 3V relays, meaning that I couldn’t use the LEDs with it. The next higher voltage relay I could find was 5V, and there were double-pole double-throw relays in this range, but they cost about $1 more. 12 relays at $3.80 each ran me almost $48 right there. The circuit board was $6, and the 2 terminal blocks were $7 each. All of the other hardware, resistors, and LEDs totaled about $30 all together. At least two of the LEDs burned out for some reason and needed replacing, but at 30 cents each, that’s the least of my worries. The LEDs are only cosmetic, anyway. The potentiometers are what actually drive the synth.

The next biggest expense was the 50 K ohm potentiometers, since I needed 12 of them, and they’re about $2.50 apiece, and the plastic knobs will be another $1.50 each. So that’s another $45-$50 right there. Anyway, I now have a working keyboard and relay bank, sans the synth.

Then again, one of the interesting things about this entire set-up is that I could use the keyboard to drive multiple synths. Program 3 synths to three different sounds, then wire them to 4 each of the pads on the keyboard and route the outputs through a mixer to a single amp. This makes for a decent little drum kit. Unfortunately, the SX-150s are $35 each ($28 from a used book shop via Amazon + $3 for shipping). So, getting 3 of them would put me another 3 weeks into “the hole of no snacks, no lunch and no breakfast on weekends”. Not to mention that the little mixer I’ve been eying is $75 to $98…

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