[Michael] loves this old organ of his, but recently he wondered if it would be possible to add MIDI out without altering its original functionality. With a bit of research and more than a bit of hard work he accomplished his goal.
The nice thing about working on a quality piece of hardware like this is the resources you can find regarding how they work (which we bet is tailored for how to repair them when they break). [Michael] found a website with plenty of info on the circuit boards and how they work. From this he was able to locate a few chips which stream serial data regarding which keys have been pressed. Bingo!
Once he located the three signals he was after he built a board to translate them to the MIDI protocol. His circuit is based around an ATtiny2313. It is supported by a liner voltage regulator circuit as well as a buffer chip which converts the incoming signals to the 5V levels needed. His home etched board is clean and well mounted, and the success of the project can be heard in the clip after the jump.