Friday, June 18, 2010
Gakken SX-150 Modifications
Here's what's got to be one of the best bargain analog synthesizers out there. It's the Gakken SX-150, a stylophone styled analog that can be found on ebay for $40. The machine has to be assembled, but there's no soldering involved. It's just a matter of putting the PCB in the case and connecting the power line, pretty simple stuff really. It's very much a gimmick but since it really is a true analog it's fairly easy to modify it into something useful.
So here's what I added, for reference I'm looking at the board with the PCB text that reads "hechenggi" in the bottom left.
This one is really simple since the cv is tied to the gate, so you only need to wire in a jack and wire the tip to the south side of R7 and the sleeve to ground.
LFO Inputs and Outputs:
Again very simple, you can tap the LFO outputs from the waveform selection switch, to add an LFO input you need to cut the trace between the waveform switch and the south side of R17. You can then add a toggle switch between these points and wire a jack to one side. You will have to add a resistor to ground for the floating side since as soon as the trace is open the tuning changes. With clever use of a two way toggle you can make it so that the internal LFO is disabled when the switch is up and the resistor is tied to ground only when the internal LFO is off.
Disable Pitch Env:
I did this purely for stability, since sometimes the pitch env is still active a little even when the knob was fully off. You need to cut the trace between the south side of R13 and the north pin of the pitch env pot. You can then wire your toggle between these points. Again you'll need a resistor to ground to keep the tuning correct when the pitch env is disabled. Make sure that the resistor to ground isn't connected when the Pitch Env is enabled or you'll loose most of the range on the Pitch Env pot. Exact resistor sizes for this mod and the previous one will vary depending on your power supply settings and control voltage so it's probably best to get a guitar tuner or something similar and try what sizes work best.
Resonance feedback knob:
Fairly simple, wire a 10k pot (as a variable resistor) to either side of R39. If you want to place the pot where I placed mine you will have to remove the power jack and move a capacitor. I extended the legs of the cap as you can see in the following pictures.
Cutoff range adjustment:
I placed a resistor between the north and west pins of the cutoff pot since as stock the cutoff knob only worked for the last 30% of travel. The resistor helped to fix this, again you'll have to experiment with sizes here (if you need to do this). Start with a large resistor and work down until you find a suitable size.
Adjustable power supply:
You can find details on how to make one here, I just made two in one. I would recommend using precision trim pots so you can fine tune the voltage (don't go over 6v though)
Here's the schematic which is a little simplified but could be useful.