Brought an Arduino Leonardo a week back with thoughts of making USB Spectrum (then fitting Raspberry pi). Had been looking for non-fixable broken rubber key spectrum to use as USB keyboard for a year or so, but kept fixing broken ones (ok only 3 but now I have 4 working and no donors).
Various articles on using Spectrum keyboard to create a USB keyboard so I’ll link to those whilst doing this. Here is the main one : http://www.instructables.com/id/Convert-a-ZX82-Spectrum-keyboard-into-an-expandabl/?ALLSTEPS
Keyboard membrane connectors arrived today so I don’t need to butcher a working motherboard, or try and solder wires onto the keyboard membrane directly.
Here are the bits – the Arduino Leonardo and the 8 and 5 segment connectors. Various resistors etc from bits and pieces drawers.