I just only want to send notes to myself. Z80 CPU clock must be 5Vpp and pure TTL levels sometimes are too low for good and stable CPU work.
Even this old CPU is very stable at 5Vpp clock, meanwhile 3.3Vpp was too low. Some CPU versions are working fine, but some are very unstable. The 3V CPLD output was good only for the CMOS version, meanwhile NMOS was not working- only 2..3 CPU cycles.
This information is not written in all available datasheets.