Did you ever have this problem?
Computer is dead. No boot screen, no post signals, no LEDs on, no fans running, no
nothing. Is it the motherboard? The CPU? The power supply?

This happened on a standby computer that I use for testing components:
The computer had been running fine. I needed to borrow its AMD Duron 800MHz
CPU to test another motherboard so I removed it. Several days later I replaced the
CPU and the computer was absolutely dead.
My first thought was that I had somehow ruined the CPU by plugging it into the
motherboard that I had been testing or perhaps I had scratched the motherboard while
replacing the CPU heatsink.

I tested the memory and the battery, re-read the instruction manual, checked jumpers,
and I couldn't find anything wrong. Then I turned to the power supply.

On any PC, if the power supply fan won't run, the power supply isn't operating (assuming
the fan isn't burned out). Often a dead fan indicates a dead power supply.
My fan wouldn't run so I thought I had found my problem. I simply connected a standby
power supply, thinking that this would bring my dead computer to life. I was wrong.
My first thought was that I had two dead power supplies; not likely, but possible. But to be sure, I set up a simple power supply test.

On modern computers with ATX power supplies, when the power connector is plugged in, the motherboard socket furnishes a path for a low voltage 'On' signal to one wire going to ground. Without that signal going to ground, the ATX power supply won't operate. To test the power supply without the motherboard connected we have to provide that low voltage signal a path to ground by using a shorting device. Here's how:

First, disconnect the AC power cord then remove the single power supply connector from the motherboard.

An ATX power supply used on modern computers has a single motherboard socket
connector. The older AT power supply connector is split into two parts; you have to plug
in both halves to the motherboard socket - but I'm talking about an ATX supply that has
one single motherboard connector.

The ATX connector contains 20 wires. 7 of them are black grounds (-). Only 1 wire is green; green (+) (low volts) is in position number 14. A black wire beside it, number 13 (-) is in an ideal position to provide ground for this test.

Now that you've unplugged the supply's connector from the motherboard, place a path to
ground between wires 14 (+) and 13 (-). This path to ground simulates the path provided by the motherboard connector. I found that a small paperclip makes a perfect shorting bar for this test.

Bend the paperclip into a U shape, wrap it with electrical tape to prevent shorting it against metal parts. Plug the paperclip ends securely into positions #14 green (+) and #13 black (-), then connect the AC power. The power supply fan should run, indicating that the power supply is operating. Mine didn't. "Ah, ha!" I thought I had confirmed a dead power supply. But wait. We're not finished.

On the workbench, with no devices connected to the output cables, I connected my standby power supply to see if it worked using the same test. It did. So I connected it to the motherboard, hard drive, floppy drive, and CD-Drive, connected the AC line and, "What?" Now it was dead too! So step by step I followed this sequence: I unplugged the AC power, disconnected one drive, reconnected AC and powered up. There was no life in the computer until I got to the last drive: the CD-Drive. With the CD-Drive disconnected, the computer came alive. There was a dead short in the CD-Drive and that was why my power supplies appeared dead when connected to the computer's devices. The supplies wouldn't power up with a dead short connected to a drive power cable - that's good. If the power supply did power up under those conditions it probably would cause instant power supply burn-out.

I had blamed everything else. Myself, motherboard, BIOS, CPU, power supplies and it was simply a shorted CD-Drive. Next time I have a "dead" power supply, I'll remember that it may not be dead at all.

20 pin ATX connector
         (plug view).

There is only one green wire #14 - black #13 is beside it.

  My paper clip shorting bar wrapped in black tape. It plugs  into #13 and #14 holes.

ATX power Supply connector.

Dead Computer Lives Again.

This article is about an ATX motherboard and power supply. If you don't know whether yours is ATX, read its specifications. Also, if you don't know whether it's ATX you probably shouldn't be doing hardware tests anyway unless you don't mind replacing whatever parts you may destroy.

Caution: Diagnosing computer problems is never as simple as 1,2,3. While there are general standards and specifications, there is no guarantee that a manufacturer or assembler followed them.

After 45 years of working with electric/electronic devices the best I can say with assurance is, "This is how the device I have in front of me right now works; yours may be different and the one I work on tomorrow may be different."

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See photos of my hardware testing jig beginning on page 3.

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Copyright © 2005 - 2008 by Howard (Doug) Dickens

Note: I still use several older computers. They work well for my purposes, so there is no need for me to upgrade. I have stopped repairing computers and/or doing research to answer questions. (Age has sneaked up on me.)
I do still answer some Emailed questions but only for items that are covered on this Web site.       May, 2011.
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