Hi Kevin ..
The main reason for chip failure was that nasty brick psu which at best was crudely regulated..
The big capacitor inside undergoes ESR changes ..the regulator device is also suspect .
Th result is that the supply lines suffer high levels of ripple and sometimes voltage creep.
If left on a few hours ..those precious chips fail ..PLA and SID are particularly in the firing line..
along with SRAM chips.
If I wanted to preserve a C64s functionality..then I would take time to chage all the electrolytic caps ..and get one of BWACKS C64 saver devices...as these will protect the C64.
..or better still ..get a modern PSU as I have ..
These have overvolts and overload protection built in.
rgds
VS
https://www.youtube.com/watch?v=aug2MOIgfHk