Funny you should post this today. Yesterday I spent more time than I care to admit to, looking for a matching PSU for a C128. I recently received a 1541 FDD and wanted to test it.
Now, I don't use the C128 that often. But do you think I could find the Commodore PSU for it...
I found one C64 PSU, two different A500 Amiga PSUs, two different A600 Amiga PSUs and a A1200 Amiga PSU.
Anyway, before I found a C128 PSU, I rediscovered my box with two C64C computers. So I tested the +5V from the C64 PSU that I had found. It measured 5.3V. So I hooked it up and soon had the C64C loading and saving to a new floppy disk (I had also been looking for my box of new 5.25" disks)
In the time when I took a break from my search, I searched the Internet to see what Commodore fans had to say about the various PSUs and what you can swap between the different computers. Well, not much opportunity for mixing and matching, what with different connectors and PSU outputs / computer power requirements
The easiest way to replace a C64 or C128 PSU, is to build a 9V mains transformer and a modern +5V SMPSU together in a new case.
I have only briefly looked at the schematics for the C64 and C128. But I believe the main logic is supplied from the +5V line and that the 9V AC feeds the audio circuit supplies, plus the +9V DC or +12V DC for the SID.Mark