I bought ZX81 on auction a few weeks ago and it flashed back my first contact with this microcomputer when I was 6-7 years old. I bought it as faulty unit, but with few touches it showed the "K" cursor on the screen. I managed to upgrade the internal memory from 1KB to 16KB, played a few demos and games that required 16KB memory pack and it has worked flawlessly since then.
My only concern is that the ULA is getting very warm quickly. I hope it would not fail soon.
My Zeddy below:
Don't worry about the ULA failing - they do get very warm. It's normal. There has been some ... lively debate on whether something should be done. There are 2 sides.
No - just enjoy it, these things are made for playing and no-one can say whether a hot ULA is a problem.
Yes - put a heatsink on it and install a fan!!
There is no definitive winner as yet Me - I say play away!!
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Now as for heatsinks MWWAAAAAA!!!
Almost forgot welcome to the club Claudius and a nice file-sixty keyboard you have there and everything Sir Morris and Lardo said too.