Watching the retrocomputing enthusiasts, apart from obvious things like water damage, it seems that the first thing that one always has to check before attempting to power up is capacitors. A generalism true for all old electronics, rather than Sinclair-specific.

And always assume the C64 power supply will fry the computer. It fails over time by raising the output voltage.

Don't just plug it in! The power supply and/or VRM can fail in ways that deliver bad voltages to components. You might want to watch some of Lee's[1] videos first on how to bring up a ZX81 safely, or ask in his discord community for more help.

[1] https://www.youtube.com/@MoreFunMakingIt/videos

That is why my Timex 2068 has never left the box since around 1990.

Better admire for what it was and use an emulator instead.

Thanks for the tip! If/when I do I'll try to use a bench power supply so I can put a current limit on it.