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I support a refurbished Dell Dimension 3000 PC (c. 2004) which occasionally will not boot: no POST or video signal, but the fans will begin blowing and an LED on the mainboard lights up. I believe the hard disk also spins up.

I discovered, after trial and error, the PC will boot after I switch the power supply from 115V to 230V and then back to 115V.

It seems we cannot trust the power supply anymore, but I am curious if someone might provide insight as to why this might occur, from an understanding of the power supply itself? What happens in the power supply when I switch the voltage back and forth to allow the PC to boot?

Everything seems normal while running the PC except for a noisy rear cooling fan.

I believe the power supply came with the Dell, for what it is worth: Dell HP-P2507FW, serial number CN-0K2946-47890-49L-7061, REV A00.

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Reset to default
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It is all about the power supply. Over the years of usage the circuit which produces the PowerOK signal, which tells the motherboard that "all DC levels are okay, you can boot up now", can be broken. That circuit hangs up in some circumstances. By switching the source voltage you actually reset the supply, so that specific circuit.

See this link: http://pinouts.ru/Power/atxpower_pinout.shtml

enter image description here

(Click to enlarge)

The PowerOK circuit is at the bottom center of this power supply scheme example)

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