Retro Thing have a great post about Grant Searle, a keen computer collector , who has written instructions on how to build your very own piece of history… the ZX81.
Grants project does come with a warning though:
“I must point out this is not a job for the absolute beginner. You will need to blow your own EPROM image and it will probably be a distinct advantage to have access to an oscilloscope in case it doesn’t work.”
So if you’re keen to relive those early days. The mighty 1K of RAM. The dodgy 16K expansion module which, after an innocent nudge, would reset the computer and wipe away the hours spent typing in that program you found in ZX81 Magazine. Be my guest.