The Power Mac G4 Cube is a very unique Apple computer, with its special and compact desktop (for the time at least) "cube" form factor. It features easy access to its internal components for (some) upgrades, but mostly, this accessibility is used to show off to your friends and family. Unfortunately, it ended up being a commercial failure due to lack of specific market focus and higher price compared to other Apple products of its time with similar or higher specs. Thus, it had a very short life span, being produced only between July 2000 and July 2001 what today lead to a somewhat rare and collectible Mac.
The Cube, like most Apple products, is a solid machine and if you are lucky enough to find a reasonably priced one, most units tend to be working to this day. Among the non-working ones available, chances are that it's because of its power supply (PSU) being dead. Sadly, the Cube PSU is custom made for the Cube, using a proprietary connector cable - making it very rare / expensive to find a plug and play replacement.
Thankfully, you are lucky enough to come across this article! :)
If you have a Cube with a dead PSU, you can use this tutorial to replace it by a modern PSU preserving the original cable, power connector and overall OEM case of the original Apple made PSU.
First, you will need to open the OEM PSU to remove (unsolder) the original cable and power connector plate. I used this guide (from iFixit user Darie H.) to open the OEM Apple PSU. Be patient as it is not easy to get to internals of the original PSU.
Once you open the original PSU, you will be able to desolder both side plates: one with the connector and one with the power plug:
With the original cable in hands, connect it to a bench PSU adjusted to 28V DC to test the Cube and ensure it works. Here is the pinout for the power cable: (A and B are +28VDC, C and D are the negative of the +28VDC and the outer shell is ground):
Note that in my test above, the Cube is also powering the Apple Studio Display through the ADC connector. I wanted to check the current drawn with the display connected and without to order an appropriate 28VDC power supply.