My yoga 900 wont turn on and the charge light not lit.

"Hi @nick3470

If by ‘sweet spot” you meant that the charger had to be connected just so for it to work, perhaps the DC-In jack is the problem

Try replacing it and check if that resolves the problem for you.

If you have a Lenovo 900-13ISK2 here’s the hardware maintenance manual that may be of some help.

Go to p.46 to view the necessary pre-requisite steps and then the procedure to replace the DC-In cable. The link for the cable is only to give an idea of the cost of the part and what it looks like. There are other suppliers that my suit you better. Just search for 5C10K48429 to get results"

"To fix it I replaced the mother board. That's no separate or daughter board I could take apart. It's all hard soldered into the main motherboard.

Also, the I/O board was fine."

Well I am sad to say, if you did not use an OEM or charger with the same voltage you probably fried your laptop. Your system board will need to be replaced and more than likely your SSD if it was fried. If you have enough money it is cheaper to replace it entirely than to pay someone to replace the entire board.