 Timex produced a second version of the ZX81 in the form of the TS1500 , essentially an Americanised ZX81 launched in August 1983 . It dispensed with the membrane keyboard and used a case similar to that of the ZX Spectrum , incorporating 16 KB of on @-@ board memory . It was effectively a stopgap between the ZX81 and Spectrum . However , it was unsuccessful due to increased competition from rival US machines and the after @-@ effects of Timex 's botched marketing of the TS1000 . Although the TS1000 had initially been a great success , Timex failed to provide the essential RAM pack upgrades to the market for two or three months after it launched the TS1000 . Consumers would take the machine home , plug it in and find that it would not do anything useful due to the lack of memory . 
