On the Retrohax web page you can see how to repair your A600 keyboard. The flexible connection to the motherboard can be damaged. Retrohax shows how to repair this connection.
GadgetUK164 made a video about the repair of Amiga diskdrives. In total he got 9 Amiga drives working 100% using the techniques shown and talked about in this video.
Iz8dwf made a video of the repair of a Commodore SX-64 Executive. The problem was a bad Kernel and Chargen ROM. He also modified the user port pins that are reserved for the 9 VAC on a standard C64.