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There is a voltage regulator in the standard Commodore 64 power supply that fails catastrophically. Specifically, it sends > 5 volts down the 5 volt line.

When this happens, depending on how long it's on it can fry every chip on that line.

That's why everyone advises not to use the original power supplies even for a quick test. Even if it tests 5 volts before you plug it into the computer it can fail at any moment.