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.

CV/Gate in:

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.


  1. can you upload a more detailed schematic?

  2. there's one on the net, its kind of simple but perhaps it's some help. I'll add it to the post.

  3. Fantastic!
    Ive bought 2 of these to build in to a little synth/sampler box that im working on.
    I have been trying to decrypt googles translation on various Japanese gakken mod sites and often it feels like i am blind in the dark.
    This is very informative and i am really going to use this.
    The thing that am missing now is understandable instructions how to get external audio in through the filter.
    Do you know how to do this?
    Could you please tell me how?

  4. if you look at the schematic you can see a single line from the VCO to the VCF, so it should be a matter of intercepting that to a switch jack so that the VCO is connected when the jack is not plugged in and when it is plugged in its conencted to the tip of the jack, the sleeve of the jack would be tied to ground. How well this will work you'll have to try, let us know how you get on.

  5. Hmm,Do you mean before or after the capacitor C6?
    Do you think that the signal has to be amplified or could it be possible to connect a guitar or something like that directly in to the jack?

    I am sorry that i have to bother you with questions like this and i am very thankful of your help.


  6. Hey man. This post completely inspired me, so thanks.
    I have been doing all the mechanical parts to the mod, like drilling out the case, wiring up the switches, jacks and knobs, and have moved the cap and power jack. Now ready for the PCB work.
    I added a knob and two extra switches, for LFO depth, LFO to filter, and pitch env re-trigger, respectively. The last two I came across on Dutch and Japanese sites, so I'm going to try them, but I have not come across anything to do the LFO depth - How would you go about this?
    Also, What range of resistor sizes did you go with for env-stability, LFO-disable and fixing cutoff range (dodgy stock also)? Very new to hacking electronics.
    Last Q, promise. And a silly one: which line gets resistor to ground for disabled pitch env, R13 side, or knob side?
    Thanks again for post - really opened my mind. Afforable analogue - who knew?!

  7. @Salvor

    the lfo depth would be easy enough to do if you intercept the lfo output and placed an attenuator in line. This is as simple as a potentiometer and a resistor to ground, you could adapt this circuit.

    I can't remember which resistor sizes i used, i think the best bet would be to just try what works best with your unit, since the units most likely vary from piece to piece my resistor sizes might not be perfect for yours anyway.

    From the post "You need to cut the trace between the south side of R13 and the north pin of the pitch env pot." so just flip the gakken face down with knobs at the top and it should make sense.

  8. My envelope generator died, I went through all the conections in that part of the circuit, the pitch knob still works,but attack and decay are basically dead, would you think that the 555 timer crapped out, what would you recommend examining?

    1. Hello Barry,
      I had the same problem. Did you solve yours ?
      How did you manage ?

  9. @Barry use a scope and trace back from where the env enters the vcf, changing the 555 will be a tad tricky since its smd, so you'll want to be sure thats the problem first.

  10. Hello again. Just about finished mods and they all worked nicely - so thankyou. The resonance is particularly evil.
    The mods I got from elsewhere were a bit weird, but pleasantly so - I have no idea what they actually do, but the LFO to cutoff seems to carry the pitch env over too?;(
    Also, not yet done LFO depth, as I want to find out if its possible to make it affect pitch/cutoff more aggressively.
    Anyway, a successful first hack thanks to good sources. I won't be able to check CV ins until I get a midi-CV converter, which is why I'm writing. What do you think of Kenton gear, namely the Solo mkII? It'll just be used to sequence the Gakken and later the monotron from crap gear - won't be able to afford any Roland relics in the near.

    BTW did you check out this hackage?:

  11. i would go with silent way since the gakken isnt goign to track 1voct very well withotu some calibration routine

  12. Oh mate, I see what you mean. The Kenton is nice, but the scaling system is too great to get even a semitone across two notes. I guess Silent Way can really vary the voltage.
    Before I get on the program with Silent-Way/DAW-software/interface, do you think a trimmer on the CV input will help? Or will this mess up the gate?
    Also, the note-off messages aren't turning off the VCO, does Silent Way fix this? I have to put in really low notes to retrigger the envelope.
    The Kenton does work well with the Monotron, though, but still requires manual tuning.
    Anyway, I can't get the filter input to work with Gakken LFO outs, but the Gakken outs work for modulating the Monotrons' pitch input nicely. The Kenton has an LFO, and that sort of works on the Monotrons' filter, but I have to play around with all kinds of parameters.
    Anyway, I haven't run either of these units through any processors or effects yet, and the fatness of analogue (over "soft synths" or digi-emulation) is, even with these budget boxes, priceless. Thanks again.

  13. Thanks for the info. If you are interested, I may need a little help. I am building a semi-modular synth out of 3 of these little guys. There's a picture here: The touch-strip is a soft pot and controls all 3. Having a hard time getting them all in tune though. How vital is proper grounding for tuning?

  14. Sorry about that... wrong link. This one goes to the entire photo album which explains what mods I made to it so far. Input or ideas from anyone is welcome:

  15. for the cutoff range adjustment i have used a 28k resistor. I am using the stock battery setup.

    Check out more of my bends at:

  16. I also love the Gakken. I tried to make some mods to my Gakken also recently. The LFO depht mod is working great. But the LFO to FILTER seems not to work. I am not sure about the right LFO to FILTER connection. I tried to connect the right pin of R16 trough 150K ohm resistor to the left pin of R31 (according to Mod7 pic) and also tryed to connect the right pin of R16 directly to the right pin of CutOff pot (according to the flickr article). In both cases I only get lowered freq OSC sound, but can not hear any LFO influence - any CutOff changes. Whats the right, working LFO to Filter schematics, can you give me some advice please? Thanks very much for any info:-)
    I probably also have some mistake in the LFO sawtooth wave to ramp UP and DOWN mod. I can not hear any difference when I switch LFO wave from sawtooth to rampUP or rampDown. I did the mod only trough 22K resistor and 2 diodes, some idea please, what can be wrong? Should there be some audiable difference between the sawtooth and rampUP/DOWN LFO waves (I hope so)?
    Thanks a lot again.

  17. Any thoughts on adding a keyboard? I saw one on YouTube and wondered how it was done.

  18. Great blog post (and rest of your blog, I might add). I'll be "hackin' da gakken" as soon as I get to it. Would it be too much hassle for you to clarify the following points? (1) Are the cutoff range adjustment hack coordinates with respect to the solder side (back) of the PCB? (2) Did you wire the resonance pot (a)in series with, (b)parallel to or (c)instead of R39? Thanks, and Cheers!

  19. Have you considered testing out modifications for the MKII? I have one and the PCB looks almost nothing like the MKI and I'd love to have both CV Pitch and CV Gate. I'm working on having a better (stereo) output so my left ear can get a bit of a break.

  20. i might if i had one but i dont :)

  21. Would wiring a cv jack into a Mark II be essentially the same process even though the board is different? You'd still wire to the south side of R7 and all that?

  22. These modifications are great! How the learning process worked out for me by reading the descriptions of where the mods are to be made turned out to reveal SO MUCH more than I would have hoped to learn by just going by the schematics and automatically modifying it strictly by copying what I saw in the plan. This is something I want to keep in mind for learning electronics. With PCBs having many layers, it's not always so obvious just where you are in the schematic, so these descriptions that you've gone to the trouble to type have been of phenomenal value to me. Next for mine is to mount to a panel and throw it in the eurorack, maybe even go ahead and add the voltage regulators to plug into the buss. Thank you SO MUCH!

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