 This shift by Timex came at an ideal time for Sinclair . The ZX80 had proved more popular than expected and Sinclair 's existing manufacturer , a small electronics company in St Ives , lacked the resources to deal with the demand . Timex took over production of the ZX80 late in 1980 . The arrangement worked well for both companies and Timex took on the manufacture of the ZX81 , aided by capital investment in its Dundee plant . Sinclair initially planned to produce 10 @,@ 000 ZX81s a month , rising to 30 @,@ 000 a month within a year . However , Timex initially had significant problems in producing enough ZX81s to satisfy demand . As a consequence , it took up to nine weeks for ZX81s to be delivered by mail order . It was not until September 1981 , five months after the launch of the ZX81 , that the delivery times finally came down to the promised twenty @-@ eight days . Those who already owned or had recently ordered the ZX80 were not excluded ; anyone who had ordered a ZX80 in the two weeks before the ZX81 's launch would receive the newer machine , while existing owners were able to upgrade their ZX80s by plugging an extra £ 20 ROM chip into the circuit board . 
