There are many ways to prevent the original PSU on the Commodore 64 from destroying your beloved computer. Its a well documented fact that the design of the PSU is somewhat flawed. The most dreaded issue is when the regulator used to regulate the 5vdc fails in a way that will harm your circuits. Here is one of the cheapest way I know to fix the 5v issue on a PSU of this type. You can eighter use the well known and reliable but power consuming LM7805, or you can use a less power hungry modern switch mode solution like the UBEC. I found that this particular model works fine on the C64 psu. This is what I use for this project. On the left there is a LM7805 kit which works great, but this time I will be using the UBEC. The tool on the left I use to carefully pry open the bottom lid.
This is how it looks on the inside of the PSU. The red square shows where the problematic regulator is. I usually just snip the legs of it and desolder the legs from the PCB. Make sure to unplug the unit from the mains! 🙂
Normally one would also swap out the capacitor, but this one seems to be in good shape, so I will leave it alone. I can always go back and exchange it later. This cap shows normal ESR and are within 5% of the rated capacitance, so I see no point in putting in a new one. I used the very reliable Peak Atlas ESR 70+ to measure the cap.
The deed is done. I used the legs of the capacitor for the input, and I used the traces that leads to the original 5v output wires for the UBEC output. I also used some hot glue to seat the wires, even if its not necessary. Below you can see the legs from the original Sanken SI-3052 regulator. (the original schematics from Commodore falsely claim its a transistor.)