Commodore intended the computer to live a maximum of 5-6 years and so the components chosen reflected this - no thought went into considering what happens past this point. Now that it's 30 years later we know that the 5v regulator has a tendency to short out input and output voltage, meaning that those sensitive 5v chips will intermittenly, or sometimes even permanently, end up getting something like two or three times that... enough to melt that precious silicon! Your options are either to get a brand new power supply or combine your old one with a C64 power saver for as long as that'll still be around, running unprotected is simply foolish these days!