your HP 400 sff G4 uses the newest weirdware spec in pc power supplies.........it's a 12v only supply all other necessary voltages are created/regulated on the motherboard and it's also a custom size (non atx spec)
this has advantages in that the supply is cheaper, and most likely more reliable due to lower parts count, it's also more efficient
only thing you might be able to do at this time is to see if a 3rd party 12v power supply can be made to fit inside your current power supply case or if the replacement supply is enclosed, will it fit in the space the original used
note that not all 12v supplies can be used in your PC the power regulation/ripple must match or exceed the current specs this is why those 12v led power adapters will not work, and will damage your system
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