Understand digital circuit with 74xx series

[indianajo]

Bingo!
So far the problem seems to be solved. The device responds to all buttons and to my midi foot controller.
A faulty Z80ADART IC was the problem.

Congratulations.
I would have done a lot more staring at the various inputs for maybe voltage levels (not 1, not zero) when the output data froze up, but skipping that and going staight to parts replacement has done you well, this time.