This is a digital synth that was sort of Korg's answer to the DX7. Another early software based synth with some specialized chips to perform the computing tasks. The 707 is kind of the strap on version.
Other than soldering the board involved I don't have a lot of hints at this time. Due to the low value of the units at the time I scrapped the one that I've gotten in.
Let me know anything you need. I have a scrap unit that had something wrong in one of the processors it appeared. SO chances are most of the stuff is just fine. Contact