Power problem with a PCG-5J5M

Hello.

A friend brought over this laptop in case I can help but unfortunately I am a software person.
This will go to some shop for fixing but I'd like some opinions on what could be wrong, in case I missed something obvious.

The whole thing happened after a coffee spill on it.
It stopped working, taken to the dealer, cleaned, worked sporadically after that, but the last few months it's almost dead but doesn't know it.

When I connect it to the mains, it tries to start up.
The power light flashes once with a whir of the disk, off again, repeats in 3-4 seconds. Never even gets to the BIOS screen flash.
This goes on forever. My friend says that trying it MANY times, it works sometimes. Well, half a day of it, didn't do anything.

I removed, in turn, the two memory chips, and tried using one at a time on different slots, but nothing.
No disassembly manual to try and remove the hard disk.

Trying without the battery connected, same thing.
Leaving the battery on in case it charges and then trying with the battery on but no mains power, it gave the same results and a bit later just a dead system.

I suppose the above takes out the possibility that it's a bad connector on the power plug side.

This leaves something in the main unit that causes the power disconnection.
A resistor of some sort, fuse, or whatever. I'm getting out of my depth here.

Does it ring a bell for any of you? Did I miss checking something (that doesn't require breaking the laptop apart)?

Thank you.