Here are some mods of my Gakken SX-150 analog synth.
I have added resonance feedback and drone knobs. CV/Gate inputs and 1/4 inch audio output. LFO blue LED and external power based on 5V power regulator connected to the battery terminals.
Remove batteries and unscrew the board. Drill hole for the resonance pot between decay pot and power switch. Install 1kΩ pot and solder leads to the ends of R39 smd resistor on the PCB.
5V power regulator is added on the other side of the PCB, it’s middle (GND) and right (OUT) legs fits to the battery wire holes. We also need to move capacitor to the side, to make room for resonance pot.
Drill holes for the CV/Gate-in and Line-out sockets. Solder Line-out tip and sleeve to the speaker terminals.
CV and Gate sleeve to the ground (you can find couple of empty pads with the GND on the board). Gate tip to the bottom end of R12. CV tip to the stylus terminal (PAD3).
Drill 5 mm hole above carbon touch panel for LFO blue LED, solder to the ground and positive side to the empty pad between R21 and R20 resistors on the board.
Drone pot fits perfectly in the speaker socket, but speaker needs to go, we will use line out any way 🙂
Drill hole next to the drone knob for DC power socket, use hot glue to attach it to the side. Solder wires to the 5V power regulator pins. We are not using any stability capacitors with the regulator, there is no room for them, it will work just fine.
You can use 100kΩ pot for drone knob, with 22nF capacitor between ground and any pot lead, to make some interesting glide effect when you touch stylus to the pot shaft! Pot shaft must be grounded also.
Solder drone pot middle lead to the stylus terminal, opposite leads to the R51 left side and R52 right side smd resistors.
This is how it sounds like.
UPDATE! In additions to this mod I need a device to change voltage of CV/Gate, to fine tune signals coming from different sequencers.
This is small box with 100K stereo pot, and two mini jack sockets and two mini jack out leads.
One of the out leads is combined CV and Gate signals in one stereo mini jack. This means that there is CV signal on the tip, Gate signal in the middle ring and bottom ring is the Ground. I can use that to connect modded devices using only one CV/Gate socket, like in my Monotron Duo 🙂