Laptop Toshiba Satellite that will shut down almost as soon as you plug it into AC. If you run it off the battery, it won’t lock up at all and runs fine until the battery runs out.
You tried a brand new ac adaptor and still has the issue. Tried BIOS updates didn’t help. Doesn’t seem to be overheating. And you didn’t find anything noticibly wrong with the motherboard.
This is one of the most common problems with Toshiba laptops we deal with, is caused by a super capacitor called a Tokin chip. A big one – NEC/TOKIN located on the underside of the laptop mobo in the center of the cpu socket.
You can fix this problem by replacing this super capacitor with four 2.5v tantalum low esr capacitors, the value of each Tantalum must be 330 uf.
also if the laptop freezes while in use or powers down or restarts once windows has loaded the Tokin chip needs to be replaced.
If you do a Youtube search you will see for yourself, on how to replace the Tokin chip with four 2.5v Tantalum smd capacitors at 330uf rating.
If you have the right tools and knowledge you can proceed and have the Toshiba repaired by yourself .
This should be done with a IR Soldering station, but can be also done with a Hot air station or Hot air gun.
If your not good with a soldering iron, then take it to a good repair shop.
It is the only sure fire way to fix the problem ok.