Another tip when checking voltage from suspect power supplies:
Set your meter on AC and check for "ripple" on the DC. A modern digital multi meter should show zero volts when set to low AC voltage settings whilst testing DC rails. I have seen ripples as low as 2mV cause problems. Ripple is an indication of inadequate filtering. If the supply used to be good but has developed ripple then the most likely culprits are ageing electrolytic capacitors.

<edit> Umm, small caveat here: You will always see some ripple as power supplies aren't perfect, but you should expect less than 2mV.

The 12 Volt rails are less critical than the 5V and 3.3V. The CPU will also have additional power regulation and filtering on the motherboard. As a result, power problems will be more evident in other circuitry; like cards in slots, IO devices, video cards, RAM etc..

Last edited by Lawrie; 10/24/10 07:26 PM.
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