Do you have a DMM (digital multimeter) and know how to use it?
It will be necessary to test the fuses (there are two) with an Ohmmeter to make sure that they’re OK. Do this with the power disconnected from the TV.
If they are then next you will have to perform “live” testing to check if the 5V Standby power signal is being sent to the mainboard from the power board on the 5V_Det. lead. (pin 2 on the cable connector to the mainboard)
Be safety aware as there is exposed lethal AC voltage on the power board when performing live testing. If you don’t know how to test safely when this voltage is present, then don’t do it! Call a reputable, professional TV repair service and ask for a quote.
If there is no 5V standby voltage then there is a problem with the power board.
If there is 5V standby voltage, then check that it is appearing at the ,mainboard at the connector at the end of the cable between the two boards. If it is there also then you have a mainboard problem. It is the mainboard that turns on the Standby power light when the power is connected to the TV.
If you don’t have a DMM then try the following:
Disconnect the power from the Tv and then disconnect the mainboard cable from the power board. make sure that the other power board cables are connected.
Reconnect the power to the TV.
The screen backlight should turn on and stay on. There will be no picture because the mainboard is disconnected.
If they turn on this proves that the power board is mostly OK (not total proof though) and that is may be a mainboard problem. If they don’t turn on it is a power board problem