John wrote:
I think you'll find that the -12V is used only in "legacy" PC design for
ISA busses, or serial ports (which he probably isn't using).

What is the +12 vdc for?

serial ports, fans and disk drives mainly

And nowadays CPUs.... late last year I upgraded a PC motherboard from
800MHz to to 2.4GHz by replacing the motherboard. The new motherboard
had an extra connector to provide 12V to the CPU 9Celeron, IIRC), so I
had to replace the power supply as well. I'm guessing that the CPU had
onchip regulators to provide the other voltages that it required.

Frank