Turn on and off loop

[Engin1]

I have HP Z420 Workstation. Yesterday I turned on PC. First few seconds everthing was normal.  All led , lights and FAN were working. But about 5 second later PC turned off. Led,  lights and FAN stopped.  5 second later PC turned on again. Later turned off. It is constantly turn on and off. I aparted all devices and then reinstated . Nothing changed.

Hard reset did not work. I researced on Internet. Somebody says it is about short circuit. But I couldn't find solve. Please help me. Can't I solve this problem without getting my computer to technical service?

[DGroves]

based on your description it does appear to be power related but try these steps

1. remove all bootable devices (hard drives usb keys dvd/cd drive) and all addin cards.......

you only want the video card installed

does it still cycle on and off? if so the power supply may be defective, test it by disconnecting it from the motherboard (and all other devices), connect power cord to supply . fan on power supply should spin and led light just behing perforated grill should light.  if either fails, supply is defective if both work supply is working  and appears to be good

if power supply passes test, then motherboard may be defective

do you get any beep/light codes on startup? if so post them or read what they mean yourself in the HP z420 service manual

[Engin1]

I did your advices.

I removed all bootable devices and cards. When I turn on PC, Power Supply and Mother board worked and PC didn't turn off. But FAN was very slow. Windows didn't start. There is no beep but Red Light was flashing constantly.

When Power Supply disconnected from the motherboard, I connected to power cord. Led and  FAN worked but first 10-15 second FAN was very slow, later it accelerated. 

There was no beep ever since the first malfunction. Do you have any other suggestions?