A Dreamcast VMU from a real barn find, covered in animal muck. I strip, sanitise and polish it, fix the loose buzzer wire and see if it still works.
The Atari 7800 deserved better controllers. I build a small adapter that lets you use a Sega Mega Drive pad on the 7800, then put it through some games.
The ElCheapoSD from BennVenn puts your Atari Lynx library on an SD card. I set it up, test games on a Lynx II and explain how to avoid bricking it.
A BBC Micro power supply that released its magic blue smoke. I replace the failed Rifa X2 caps and service the whole PSU with the Retro Clinic advanced kit.
Plugging a Mega Drive pad straight into a C64 can kill the CIA chip. I build an open source adapter that makes it safe, and works on the Amiga too.