Help identify positive, negative and ground on a LCD screen PCB
Assuming that power supply is the original, the symbol to the right of the OUTPUT current shows that the outer pin is - and the center pin is +. The diagram is shown looking head-on with the barrel jack. The + is shown touching the center. This is the most common configuration, but should never be assumed!



Zoomed in and color-coded:


Looking at the PCB after the plastic female jack was removed, it looks like pin 1 is - (ground), as it has the shape of the arm that pinches the outside of the male barrel jack. Pin 3 is +, and pin 2 is a switch that is physically disconnected once the barrel jack is inserted. The switch is used to detect if the plug is in, but that feature may or may not be used by the device.