Hi Al, LGs often suffer from main board not supplying INV-ON signal to power supply to turn on the backlights. So locate the INV-ON pin on the power supply connector and see if indeed 3.5V is present. If not, a cook up should solve the problem.
Be wary of LED testers - they only supply about 30mA to the LED strips which in a working set use around 300mA. So a faulty LED may not present at such low current. But almost certain to be the main board. Some of them, especially the LW series, do not respond well to cooking. Bruce