A few months back I dragged out my sister's old Yamaha PSR-77 keyboard and popped it open to see how it functioned. I've been interested in MIDI for some time now and thought about maybe adding a midi in/out function with an aftermarket board. I ended up soldering a ribbon connector to the existing keyboard matrix connector and closing it up so I could work on it later.
I didn't touch it again until yesterday when I popped it open again and after 5 minutes of looking over the logic board realized this would be the perfect project to start my hopefully long and exciting career in Electronic engineering.
The innards of this device are pretty simple after a close inspection. There's two main boards the first of which is a logic board with the uC and what I believe to be either a memory chip or a dedicated chip for the keyboard scanning. It also holds the power and audio amplification circuits which will me a simple task to replace with my own.
The other main board is the input and display board which has a matrix of dome-switch buttons and a matrix to control the 5 LEDs and the 2 digit 7 segment display. I spent the majority of last night with the continuity mode of my DMM mapping out where all the connectors go and writing them down and making a matrix map. The other part was spent trying to find the pin-out of the display from the Japanese data sheet.
I'm hoping to replace the logic, power, and amp circuits with some of my own design adding proper MIDI in and MIDI out to it along with a few new features and modes. If the mystery chip ends up being a ROM with all the original voices and songs then I'll happily recycle it in a OEM mode.
This post was mainly to indicate where I'm at and I plan on posting pictures and data for it over the next week or two and fleshing out what I exactly want to do with it in the long run.