Hi @francy66 ,
Is the red power standby light on or off when the TV is turned off by the remote or the power button on the TV?
There may be a problem with the cable connection between the mainboard and the power board or a problem with the power board or the mainboard. Further testing would be required to determine where it is.
Sorry to be so vague but when power is supplied to the TV the power board sends a voltage signal to the mainboard (standby voltage) that indicates to the mainboard that there is power available. It is the mainboard that turns on the red standby power light and it also maintains a signal to the power board not to turn on full power output i.e. stay in standby mode until the TV is turned on
If there is no signal on the PS on/off lead at the power board for whatever reason then the power board turns on out of standby and the backlights turn on full intensity. (Look for the PS On/Off lead at the mainboard - power board cable connector on the power board)
Be safety aware when working in the back of a TV when the power is connected as there is exposed lethal AC voltage on the power board. Disconnect the power from the TV unless testing for the various voltages