Laptop powers on, then immediately off, then loops.

I have a background in computers but after trying every trick in the book I can’t seem to solve this problem. After replacing the power connector and upgrading the RAM, the laptop powers up for one second and then shuts off. It will loop and continue to do this until I unplug the charger.

I’ve checked every connector, replaced the CMOS battery, replaced the charger, replaced the battery, tried swapping the old parts back in, nothing seems to work. I’ve tried a hard reset as well multiple times with no luck.

All I get is a power-button light and a click/light from the optical drive and then it’s off and on again and again.

What could be the problem/solution?

I’m starting to wonder if the motherboard is cooked.