Any update on your ESQ-1 debug? I got one non-working for cheap and is in the same state, display self test mode only. Will follow your steps so far in debugging mine, thanks for documenting all this! Seems these mainboards are more prone to issues than many other vintage keyboards I've worked on...

Wondering how your rebuild came along? I'm considering a beat up JD-800 for restoration, but worried of running into 'dead-ends' like hard to find parts..