 Other than what Clarke described as " a few remarkably poor programs on cassette " Sinclair released only two official peripherals for the ZX81 , a 16 KB RAM pack ( actually the same one previously released for the ZX80 , but rebadged ) and the ZX Printer , both of which plugged into the edge connector at the rear of the ZX81 . They retailed at a launch price of £ 49 @.@ 95 each but both had notable flaws . The RAM pack was top @-@ heavy and was supported only by the edge connector . It had a habit of falling out of its socket at crucial points and crashing the ZX81 , losing anything that the user had typed in . Users turned to using sticky lumps of chewing gum , double @-@ sided tape or Blu @-@ Tack to cure what became known as the " RAM pack wobble " problem . The ZX Printer was a tiny spark printer that used two electrically charged styli to burn away the surface of aluminium @-@ coated paper to reveal the black underlay . It worked reasonably well at first but its output deteriorated rapidly after a time . 
