Can a bad power supply still provide power to the USB ports?

Factual Questions
1

I have a suspected bad power supply on my desktop (had random shutdowns, now nothing happens when I hit the main power switch, already tried jumping the switch to no effect), but the one thing that throws me off is that I noticed that when the power supply switch is on, there is power going to the devices that are plugged into the USB ports. Is it likely that a power supply would fail in such a manner that there’s still power to the USB ports, but the computer otherwise appears stone dead?

2

Last time I cracked an ATX power supply open, you had 3 rails:
12V
5V
3.3V

It’s possible your 5 volt rail is getting good juice but your 3.3V and 12V rails aren’t getting good power. Or vice versa, etc.

I suggest reading this:

I spent 7 years doing tech support for a major electronics manufacturer.
The failure mode you’re describing wouldn’t surprise me if it happened.
Once your power supply gets weird, all bets are off. A bad power supply can simulate a failure in nearly any other part of the device. Also, a bad power supply can randomly kill off other parts of your device.

3

Could also be a motherboard issue.

4

The power supply may be fine, but major components may be disconnected, or otherwise damaged. You can check power at various points on the wiring harness or motherboard with a logic probe or multi-meter. Anyway, if its only the power supply, you lucked out. Replacing that could cost a lot less than a new motherboard.

5

Absolutely and unequivocally, yes.

I’ve had it happen on a couple of occasions and the symptoms you mention are typical of a bad PSU.

6

There should be a -12 in there as well.

You can test an ATX power supply without a motherboard attached. Just short the PS-ON signal to ground and the power supply will turn on. Note that computer power supplies are switching supplies, and may not regulate their voltage very well if they don’t have at least 10 percent or so of their rated max load present.

A quick google search will show you what pin is what on the ATX connector.

7

Sure enough. New power supply fixed it right up.

Thanks!