Yes! I repaired both a ZX81 and ZX Spectrum 128
:-D
The zx81's video modulator was broken. I replaced it with a composite video signal. Too easy :-D
The keyboard of the ZX spectrum 128 was broken. The 'flat cables' connected to the motherboard seemed to be burned trhough. I cut the connectors from the motherboard, cut the flat cable before the burnt part, soldered threads between the connector and the motherboard, and connected the flat cables in the connector (extra firmly with some tape.
Men, that was some soldering! But it works :-D :-D :-D