Solved, it was the power supply. 
I have an XPS 8700 with original, factory installed parts.  Following a power failure this morning the PC will not power on.  Steps that I have taken to narrow down the fault are listed below with results. 1. Power supply self test.  Pressing the black button on the back of the power supply starts up the fans and provides a green light on the back of the power supply.  The On/Off button on top of the computer is amber/orange while the self test button is pressed.  Please note that it takes a minute or two for the self test to run after the power cable is connected. 2. There is an amber light on the motherboard whenever the power cable is connected. 3. The computer On/Off switch was tested with an ohm meter at the connector to the motherboard.  Switch not connected shows infinity resistance.  Switch depressed shows a near short (<50 ohms).  I was unable to observe a dead short with the button pressed.  That said, my test setup was somewhat unconventional, paper clips in the cable connector with the meter leads held in place with clothes pins (yes, alligator clips would have been better but I'm retired from all this and no longer have them). 4. Power to the HDD, Optical drive was removed along with the video card and connections to the USB jacks.  This was to eliminate those devices as a possible impact on system power.  Said devices have been reinstalled or plugged back in. 5. The RTCRST jumper is three pins. I moved the jumper from the factory installed top two pins and placed in on the bottom two pins (again - 3 pins total).  The jumper was returned to the original position after a couple of minutes with and without the system power cable attached.  The same thing was done the the PSWD jumper.   6. A new CMOS battery was installed. At this point I am suspecting a failed motherboard provided that I can trust the validity of the power supply self test. Observations, thoughts and advice from your experience is welcome. Thank you!
XPS 8700, not turning on, #7