Laptop won't start up

Hi guys im doing a love job for a friend and need a little help

Problem: Laptop would start up for 10-20 seconds then shut down.
I did the basic stuff, remove ram, HDD and try again.
No go. So i stripped it down, had a look around, re seated the heat sink fan with new paste.

Now I hold the power button on, light come on as normal but shuts down straight away. Lights only on for 1-2 seconds (while holding power button). I can hear the fan spin and the dvd rom spin.

Notes: This is running off a power cord not the battery (has no battery)
I was thinking it might be a faulty power adapter, any ideas?
 
Hi, your saying when you try and start it that you are holding down the power button, try letting go of it and see if that makes any difference, just push it and let go.

Is there anything appearing on screen?

Remove any disc that might be in the CD drive.
 
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