Ok, then the Backlight on command from the micro, OR...the power supply isn't turning on its backlight power section. We don't know which one yet.
If you have a voltmeter, we can check the TV's Backlight on command. That would tell us if the power or main board is the problem.
Another option is to disconnect the plug /cable between the two boards, install a jumper on the Power on and Backlight on wires on the power board and see if the TV screen lights up.
Are you willing to remove the TV's back cover, and possibly remove a giant metal box (schematics show it on the 40 inch TV) to get to the power supply and main board, and either -
1] Use a voltmeter to check the power supply voltages or
2] Use a couple pieces of solid wire like you find on a resistor or capacitor leg, and jumper the standby 5 volt line to the power on and backlight on pin holes?
Let me show you a picture of that --
It's not complicated to do, but you do need either a voltmeter or maybe a electronic hobbyist store in your area.
I will tell you this. I live in the USA, and we have Radio Shack stores in practically every little town here. They originate from the UK. In the UK they were named TANDY back in the 60's and 70's. I've asked several customers if they exist, but with no clear cut answer to that question.
Does Tandy or Radio Shack....or even "The Shack" exist? The last one is what Radio Shack claimed they were renaming their stores. It hasn't happened, and it's been about 2-3 years since they mentioned the renaming.
They have those alligator jumper wires I use all the time, plus resistors and capacitors you can use as solid wires.