I'd just like to expand on a couple of points in lvd's excellent answer.
You might get by with just using a jumper wire to short the reset pin to ground for a brief moment after you've powered up the CPU. It's worked for me, but if you're having problems it's best to build a proper reset circuit.
Many CPUs have a minimum length for the reset ...
There are many issues here.
As it is already said in comments, decoupling capacitor is a must!
555 (non-CMOS) timer output is very much like the output of TTL ICs, however Z80 requires a firm logic one. When feeding Z80 clock pin from a TTL output, you should use pullup resistor of 200..500 Ohm.
NMOS Z80 uses dynamic logic, that means it has some minimal ...
I have no definitive information, but I highly doubt it. Why? Moore's Law.
In short, the very same technology that is (gradually) enabling "artificial intelligence" (or more specifically, machine learning) is also making those "old systems" totally, utterly obsolete.
Think about it: The incredibly inexpensive computers like Raspberry Pi ($35, I think less ...