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The Commodore 64 had two separate +5 V power supplies: one supplied by the external PSU brick and a second one generated on-board from the 9 VAC via a 7805 regulator. This second +5 V power supply was called "CAN +5 V" and according to the schematic it supplied the VIC-II, a system clock chip (U31) and perhaps other circuits I've not noticed.

Why was this separate CAN +5 V supply used, and what does "CAN" mean?

For bonus points, if you can explain the effect of this on the system ground, that would be interesting.

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This is done to stop HF noise from spreading through the VCC rail. The VIC and the 8701 clock generator (U31) run at much higher speeds than the rest of the C-64, and thereby generate more HF noise than the other components.

My understanding is that it's called "CAN +5V" because it supplies the components in the shielded metal "can" on board. [1] Note that the RF modulator -- the other HF component on the board -- also sits in a metal can, and also includes its own regulator (bottom left portion of the modulator schematic on p. 39 of the service manual you linked to).

The bridge rectifier used in the generation of the +5V CAN supply ties the lower end of the AC input to one diode drop above GND. This is not a problem with the original Commodore power bricks, where both supplies (+5V DC and 9V AC) are floating, with no reference to an external ground (like PE). Even when you attach an external device (floppy drive, monitor, printer, whatever) that has its ground tied to PE, the ground of the 5V DC supply will be tied to that ground as well, but the 9V AC will still float.

If you're thinking about building your own supply, ensure that its AC output also floats with respect to the 5V DC.

[1] I remember this explanation from back then, but I can't give a source. I checked a couple of books from the time, and they all use the term without an explanation.

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    Note that the RF modulator -- the other HF component on the board -- also sits in a metal can, and also includes its own regulator (bottom left portion of the modulator schematic on p. 39 of the service manual you linked to).
    – Michael Graf
    Apr 8, 2020 at 11:27
  • 'VIC' = Video Interface Chip II, by MOS Technology
    – smci
    Apr 8, 2020 at 20:35
  • Regarding the ground, one example of a potential issue is that there have been suggestions to use separate external power supplies to generate 9 VAC and 5 VDC. Given that the two 5 VDC grounds seem to be linked, are there any precautions one should take when doing this?
    – cjs
    Apr 8, 2020 at 22:03
  • Ah, I see what you mean. With the original Commodore power bricks, both supplies (+5V DC and 9V AC) are floating, with no reference to an external ground (like PE). However, as soon as you attach an external device (floppy drive, monitor, printer, whatever) that has its ground tied to PE, the ground of the 5V DC supply will be tied to that ground as well. Therefore, the 5V DC supply must either be floating, or have its ground tied to PE. The bridge rectifier then ties the lower end of the AC input to one diode drop above GND. You should therefore use a floating 9V AC supply
    – Michael Graf
    Apr 8, 2020 at 22:29
  • But don't worry, if they use a transformer, they're all floating by design.
    – Michael Graf
    Apr 8, 2020 at 22:31

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