pm sent
When the tv stay on with the removed cable of the inverter board, did you check all the main psu voltages ... normally when capacitor are leaking you will have some ac voltage on top of the dc voltages, sure its an high frequency, it may go up to 100-200khz pwm frequency ...
Just to be sure that the main psu is ok as a good start, normally i recap the psu automatically with good capacitor brand at 105 degree temp, this model is not recent loll
0.5 volts drift are sometime acceptable, depends of the circuit tolerances and structure.
As i said, check voltages impedance when tv is off on the cn800x connectors, when plugged and unplugged ... and i did not see any inverter schematics, it will be hard to help without having it on my bench
The inverter board is essentially an pwm power supply, may outputting up to +/- 600v for cfl lamps, sometime variable because of the contrast / brightness settings, but seems fixed in this tv.
The tl494 may send protection signal because of over current, bad solders, bad capacitors, bad contacts etc ... the two power mosfets near the black transformers may have heated a lot and degraded over time, seen lot of that in lcd monitors, they use the pcb has heat transfert.