Optiplex 755 does not power down. . .amber power light

I am having a strange situation with a Dell Optiplex 755 machine.  This was sitting idle on a shelf for some time (it is part of our reserve loaner PC pool).  Today, I had a need for it.  When I took it off the shelf and plugged it in, it immediately began to churn up even though I did not hit the power button.  Stranger still, when I went to cold boot the PC (by holding the power button in for a few seconds) it would power down only to rev up again once I took my finger off of the button.  Howver, all I get with all this is an amber power light with no POST or loading of Windows that I can see.

The PC is out of warranty, so I cannot contact Dell as I would usually do.

I am thinking that this is either a bad motherboard, power supply, or both?  I would appreciate any additional troubleshooting steps in order to determine.

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The first thing I would try would be the power supply, if a new/good power supply does not work then it is most likely the motherboard.
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
I had a similar issue, power supply was working (ish) but could not supply enough Amps, on power-up and power cdrom, hard disks and motherboard.

replacement power supply, solved the issue for me.
i had this issue .. once it was the power  supply and the other time the board was damaged. look at the capacitors if they are swolen its no good !
most likely is the Power supply that is bad; and both issues can be caused by swollen, or leaking capacitors, as said; here an example :

you can also use the minimum setup to troubleshoot your problem :
http://www.experts-exchange.com/Hardware/Components/Motherboards/A_1945.html                  (Short-overview-of-how-to-troubleshoot-bad-hardware-when-a-pc-does-not-post)

in case the disk went  bad - this is a nice guide also

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AfpSysGrpAuthor Commented:
Sorry I have not had a chance to get back to you all on this. . .i sort of got pushed to the back burner.

The capacitors on the motherboard look fine, so I will proceed with the assumption that it is the power supply and troubleshoot further when I get some time to do so.
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