DIY

Gakken SX-150 Mark II

This odd little synth may be all but forgotten. However, with this DIY modification you could bring it back to life.

The Gakken SX-150 was first introduced in 2008 as a kit in the Japanese magazine Science for Adults. Once assembled, the SX150 could be played with a stylus and parameters like cutoff, LFO, and pitch envelope could be adjusted. Later, the SX150 Mark II was released, pre-assembled. In this version new functions, like cutoff resonance was added. Out of the box, the SX150 Mark II sounds pretty good for such an inexpensive synth. It sounds much like a 555 timer chip synth. However, if you run it through some effects, it sounds very nice and is capable of some extremely low bass tones. Soon after I got mine, I came across a mod to add cv/gate connections to it. This was a very easy modification and it adds so much functionality because it allows it to be played with a sequencer or keyboard with cv/gate outputs, instead of the stylus.

What you will need for this mod is: 2-1/8” mono or stereo jacks that are panel mounted.

Insulated wire.

Soldering iron and solder. Electric drill and bit large enough for the jacks.

First, you will need to take the back off of the Gakken. There are 4 screws, one on each corner. Once you have the back off, decide where you want your jacks to go. I chose the side simply because of the free space inside the case. Next, drill two holes far enough apart to fit both jacks side by side. You can go ahead and mount the jacks in the case at this point, which is what I did. But, I think now I would solder the wires on first and then mount the jacks. Either way works. Next, cut a few short sections of wire and solder them to the jacks. Solder one wire between the ground connectors and one between the tip or ring connectors. Then solder a longer wire to one of the ground connectors and another wire to one of the ring connectors. Next, connect the loose end of the ground wire to the ground terminal on the Gakkenʼs circuit board. There is a screw holding a wire and ring connector there. I just hooked my ground wire under the screw head and tightened it back up. Then take the loose end of the other wire and solder it to the battery terminal. Now, you can hook up your cv and gate patch cables and give it a try. It doesnʼt matter which one you use for cv and gate, they work either way. Once you have it connected to your sequencer or keyboard, you will probably have to tune it, using the tuning knob on the bottom of the Gakken. Another tip I found is that you can use the LFO depth to adjust the pitch, similar to an octave knob. The SX-150 Mark II is a fun little synth and if you add this mod and run it through some effects, it can be a valuable addition to your rig.