Complex sentence: The TK82C was compatible with the ZX81, but it had not the ULA chip from Ferranti, instead it was manufactured with a dozen of TTL integrated circuits, what resulted in large power consumption and the computer became very hot in few minutes.
Split into simpler sentences: The TK82C was compatible with the ZX81, but it did not have the ULA chip from Ferranti. Instead it was manufactured with a dozen of TTL integrated circuits, which resulted in large power consumption--the computer became very hot within few minutes.