I'm attempting to fix a laptop for a friend. The power plug from the
power supply has been broken off and lost. Rather than spend $80 for a
new supply I am trying to hard wire (via a quick disconnect) the power
wire to the jack on the computer. I'm skilled with a soldering iron
and I know I can make the necessary connections. My problem is I
don't know how to find out which pins in the jack are positive, etc.
The wire is a four conductor. Ground is easy to find by ohming to the
case but I need to know how to trace the circuit for the other wires.
Can someone please intruct me a bit on this? Many thanks.

rrr