PSOne voltage drops to zero when power switch is turned on

As the title states, I have an otherwise very clean psone (slim)- when I read the voltage at the psu barrel plug on the board and on the psu side of the power switch, I see 7.7 V or so. when the switch is set ON, the voltage drops to zero at the psu side of the switch, but is still reading 7.7 V at the barrel plug.
Is there something in line between the psu and the switch that is faulty?

I am new to electronics generally but have a multimeter and soldering experience.