Laptop won't power on

When I press the power button to turn on a dv9600 laptop, nothing happens. The only led that is lit, is the power connector led. First of all I tried with another AC adapter, but again nothing happens. I have disassembled the laptop to check if the 19V reach the motherboard correctly and indeed there are 19V on the motherboard.

How could I check if it is a motherboard problem or something else, eg the power button board?