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+5VSB Power Supply Failure?

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Hi forum,
First of all, I just want to say thanks to all of the active forum community members for your helpful insight and contributions. I peruse this site from time to time to troubleshoot and find answers to various problems I've had.
My current issue is as follows.
I have a PC which I built from scratch about a three and a half years ago.
All of the sudden, it stopped powering up. When I flip the power switch, the fan in the power supply does not spin.
However, the green LED on the motherboard does light up.
Because of this, I presumed that the problem was the power supply, naturally.
I called Ultra products support to access my warranty information. They told me that my lifetime warranty was expired, as I had never registered the product (That's really neither here nor there, but may be useful information to those of you looking to buy a power supply-or having recently bought a power supply made by Ultra).
They did tell me that the power supply could be fine and it may be just receiving an incorrect signal from the motherboard. This is something I had never heard of before. Further, they told me that testing the PSU with a multi-meter does not provide an accurate test of the PSU either, and that a PSU tester is to be used. I'm not an advanced hardware technician, but I always thought that a multi-meter would read the power supply in the same way that a specifically designed tester would.
So, in any case, I ordered a power supply tester from Hong Kong and tested the power supply. With the 24-pin and 6-pin connector cables connected, it simply reads +5VSB. This, by my understanding is the standby power voltage. The HDD, SATA and other connectors don't register anything.

I'm not even positive if this tester works, as I've got nothing else to test with it for a baseline, right at the moment.
But, it seems like for some reason, the power supply is only outputing a standby level.

Could this be an issue with the power supply or even the wiring in my home?

The support guy from Ultra told me that the power supply needs to receive a signal back from the motherboard in order to output the correct voltage, as per various advanced power management protocols.
I don't really want to get a new power supply if I don't need to, as my budget is pretty tight right now. But, of course, needing to drop a bunch of money into a new motherboard would be worse.

If anyone is able offer their advice or guidance in this situation, it would be strongly encouraged and largely appreciated.

Here are the system specifications:

Asus M3N78 PRO Motherboard - Socket AM2+, Geforce 8300, ATX, HDMI, SATA, Gbit LAN, Hybrid SLI
AMD Phenom 9500 Processor HD9500WCGDBOX - 2.20GHz, 4 x 512KB Cache, 1800MHz (3600 MT/s) FSB, Agena, Quad-Core, Retail, Socket AM2+, Processor with Fan
OCZ SLI-Ready Dual Channel 4096MB PC6400 DDR2 800MHz Memory (2x2048MB)
Sparkle SFPX94GT1024U2 GeForce 9400 GT Video Card - 1GB DDR2, DVI, VGA, PCI-Express 2.0
Ultra LS500 Lifetime Series 500W Power Supply - ATX, SATA-Ready, PCI-Express, Lifetime Warranty
WD Caviar 500GB Serial ATA HD 7200/8MB/SATA-3G (x2)

Thanks again. I look forward to members' input on this matter.
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Hello jacobt

Please see my canned text below regarding PSUs, some of which you are already aware of BTW;

As a PSU puts out various voltages +3.3V, +5V and +12V it may appear that the PSU is working correctly but it is not, any significant drop of any output can prevent the system from booting up, the other scenario is a significant increase in the output which can be worse as it can fry one or more major components such as the MB, CPU, Ram, add on video card etc.
Also please be aware that there are no user replaceable parts in a PSU so a bad one should be disposed of in a responsible manner and any type of conclusive testing will need to be done by a suitably trained Tech who has the required testing equipment and the relevant knowledge as to how to use it.

The support guy from Ultra told me that the power supply needs to receive a signal back from the motherboard in order to output the correct voltage,

That would be the Power Good Signal http://powersupply88...wer-supply.html
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