Getting Started - Casio SA-1
This project on based on the Casio SA series of keyboards. I completed 5 sound modifications and added a ¼” audio jack. Project Intermediate (some electronic knowledge needed).
Take out the circuit board from the case. You will see two large semiconductors, one has a crystal with 3 pins next to it. I first took out the speaker so there was room for the new electronics and then soldered two leads to my new 1/4” jack. This will allow me to connect to a mixer and sample the unit at a later stage
Power Crash - The CPU needs a stable regulated voltage to run normally. If CPU does not have enough power to run correctly, but it has enough to not stop, this causes glitches in the sound. I used the recommended 470K pot and after removing the power socket soldered in the new pot between the two points. NOTE: if you do not solder the pot but remove the power the unit does not work. Outcome: worked to too sensitive maybe try a pot around 270K.
5th Switch - When you play a key this mod will play a second note. Easy m
od as it requires one on/off/on switch wired to 3 pins on the main sound chip. Outcome: worked!
Glitch randomiser - The CPU constantly scans all the keys and buttons andthis mod takes the final amplified audio signal out of pin 5 of the amplifier chip and injects it back into the scanning matrix via a 1Meg pot. The CPU sees this amplified single and does not know what to do hence you get a glitch effect by moving the 1 meg pot. Two things to solder a switch and a pot. Outcome: good glitch effect and worked.
Filter / Feedback - The modification takes the audio output from the amplifier and injects it back to the CPU. Potentiometer used is a 1K pot, but a trimmer is also fitted so there is still some resistance between the input and output of the amplifier, otherwise it would feedback all the time. Outcome: works a treat although you need to spend some time settting up the trimmer.
Pitch shifting - The CPU runs at a fixed speed of 22Mhz which it needs to create the internal 
sounds out of its Rom and to run the main program that scans the keyboard. This is the most complicated mod as it requires you to de-solder the existing crystal, install a switch, a new and existing crystal plus two capacitors. Outcome: old crystal worked but could not get my new crystal to work. I did use the wrong capacitors so need to rework and try again.
Final product: Circuit Bent Casio SA-1
Watch it on my Youtube Channel
Full project details on http://www.casperelectronics.com/