dell 780 power up problem

Why Won't my Desktop Computer Turn On?:  http://dell.to/1aBMEQ1

When you press the power button, the power LED, CAPS lock, Scroll lock and Num lock LEDs blink amber or in a particular pattern. The PC may also emit beep codes that can help identify error on a specific hardware device.

• LEDs blinking (usually in a pattern and/or amber in color
• Beep codes heard on the PC (continuous or pattern)
• Display is blank (black) with blinking LEDs or beep codes
• Fan noise is heard but LEDs blinking or beep codes are heard
• PC is stuck at the Dell logo during POST

thanks for your response but the above video did not resolve this startup problem, I get a green diagnostic light all the time no matter what but the tower will not power up. The only led pattern I get is as described in my OP.

thankyou for this further feedback. The two green led's that flash together are on the front of the tower at the bottom. I don't think this is a motherboard failure because I can start the tower and boot into windows if I hold the psu diagnostic button in and press the power button.

The Green LED on the back is NOT an overall Diagnostic LED.  It only indicates that Power is Available not that Power supply is Good.   "there's two LED's flashing together " is not information.    WHICH LEDs? In addition to the Floppy and CDROM there are 4 Diagnostic LEDs, Network Activity LED, Hard Drive Activity LED Etc.

One and Two Blinking with Solid Amber Power Button is Motherboard Failure.

There are 4 diagnostic LED's and the Power Button also has indications. The one that Looks Like a Cylinder is the Hard Drive.   Not powering on is not the same as NOT POSTING.  This is not the psychic friends network. Ms Cleo also does not take diagnostic calls.  There are also Beep codes or not.You also did not say what SIZE the unit is or what OS etc.

Please tell us which green LEDs are blinking.  They should be numbered 1, 2, 3, 4.  Tell us what other LEDs are on or blinking and what color they are.  

Without complete information, no one can really successfully help you.

the form factor is SFF.. The two tiny leds that blink green on and off at the bottom/front of the case are not numbered.

Again: when I press the power button nothing happens, there's no fan, no beeps - the green HD light flashes once and then the other two leds next to it start blinking. If I press and hold this rear PSU button and then press the front power button the dell powers up properly and works.