I was looking at a Dell 4500.  The owner rarely shuts this system down, but 
after she did shut it down, the other day, she was unable to get it to turn 
back on.  The power-on light, built into the on-off button, would glow, but 
no other signs of life when the power button was depressed.  When I opened 
the case, I could see that the light on the MB would come on, when the power 
supply was plugged in, indicating stand-by power to the MB, but no fans 
would spin when the power button was pushed.  The diagnostic LEDs at the 
back of the MB showed no activity either.

Testing the power supply with a one of those little power supply testers, 
the power supply seemed ok.  It's fan would spin and all the little lights 
on the tester lit up indicating the various power rails.

I ended up swapping around power supplies, anyway, and I found that a 
different power supply would power up this Dell.  Conversely, the Dell's 
power supply wouldn't power up a known working system.

I know that these inexpensive power supply testers aren't very 
sophisticated, but I was curious as to how this power supply could show up 
as working when tested, but still be unable to boot up a computer.  The only 
guess that I could come up with was that perhaps the power good signal comes 
up too slowly, now, for the motherboard to accept it.  (The tester indicates 
a power good signal.)  Anyone else seen this and/or have any thoughts as to 
what the problem might be?

Thanks,

John Sproule 

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