 The reliability of the ZX81 was controversial . W.H. Smith , one of the machine 's key distributors , had a company policy of ordering a third more ZX81s than were actually required for sale , so that it would have enough replacements for faulty machines . Similar problems were reported in the US market , where contemporary reports suggested that only a third of the ZX81s shipped actually worked . However , figures released by Sinclair claimed that only 2 @.@ 4 per cent of pre @-@ assembled machines were returned , although 13 per cent of kits were returned . Clive Sinclair strongly denied any problem with reliability : 
