Fancy repairing your own Spectrum? Then this handy diagnosis ROM written by Brendan Alford and others is what you need!

This EPROM drops directly into a Spectrum 128K, +2 or +3 - in a 16K or 48K Spectrum the board needs a slight modification to accept an EPROM.

More details of the excellent ROM can be found here: https://github.com/brendanalford/zx-diagnostics/wiki/Firmware