Does this set have a 17PW20 PSU? If so check the ‘Backlight ON’ 3.2V supply at PL802 pin 12.
The 3.2V is an enable signal from the CPU to the inverter PCB (via the PSU), to switch on
the backlights. If this is low, the backlights will switch off. Try replacing Q841 BC858B, D861 BAT254, C904 1uF 16V.