Having written a C64 emulator before (<a href="https://floooh.github.io/tiny8bit/c64-ui.html" rel="nofollow noreferrer">https://floooh.github.io/tiny8bit/c64-ui.html</a>) I find it incredibly fascinating that some of those hardware errors look very similar to visual artefacts I got in the early stages of the emulator when trying to get the VIC-II timing right :)
One of the most rewarding things I have done in the past few years is learned component-level repair on Commodore 64s.<p>Take a dead 64 showing nothing but a black screen, try the dead test cartridge, follow its lead or start desoldering, socketing and replacing chips.<p>And suddenly you have a working forty year old Commodore 64.