PS4 PRO Faulty Power Supply?

I have a PS4 pro model no. CUH-7116B. Since unplugging the device a few days ago, I now have a problem when the power button is pressed, the console beeps, blue light turns on for around a second, a couple of clicks can be heard then the console immediately switches off. I then press the power button a second time, another beep can be heard but nothing else happens. It then does the same cycle again when power button is pressed.

I have removed the PSU and tested the output. When the power button is pressed, I get 12 volts for half a second, before the voltage slowly drops down to zero. Does this indicate the PSU is faulty? Or is it normal for the voltage to drop away to zero after power button is pressed?

Cheers for any help.