I haven't a real floppy to test with, but i'm pretty sure the issue is in the logic that activate the FDC controller.
I found another error in the schematic (it missed the IC112 pin 1,2 and 3, IC109b in felge1966's schema, it wasn't "spare" after all! ;).
But i soldered some flying wires to correct the schema, without progress: i noticed that when i press a key in TA the /CS line of the upd765A go down, and floppy led flashes, and that seems a bad symptom to me