820-3115, not stable

[yobad]

I have 820-3115, who was refurbished by someone and sold, I think it was water damaged before,
the board isn't stable, sometimes it beeps, but after swapping the ram it boots, sometimes it flashes quick white squares on the screen.
sometimes it won't boot but it makes the sound of the HDD and DVD(motors sound) and before it ending, it stops,


I don't know from where start the troubleshooting,

 

[2informaticos]

This could be waste of time, if someone else messed with it before.
Does the board always stay on, but not booting?
Graphics artifacts could mean RAM, or CPU issue.

 

[MOMOH]

The 820-3115 are quite common with faulty RAM slot, I suppose you can start by troubleshooting which of the slot is faulty, usually its the bottom one. A little tap when the machine is on could reveal that as well, as that would make the machine to freeze when you do so. I suggest you start from the ram slot.