with a led tester the backlight is working fine.

@harunie check the voltage coming from the power board on the Backlight connectors. If your backlight works with the tester but not with the TV it is possible that this is a power board issue (no voltage or low voltage supplied) or a main board issue. The main board tell s the power board to turn the backlight on. Check the voltage on the connector from main board to power board for the BL ON signal. My prime suspect at this time is the power board. If you need further help on this post some good pictures of your boards with your QUESTION. That way we can see what you see. 기존 질문에 이미지 추가하기

BL_ON (L or H): IC9017 #18pin should be ~3V

The BL_ON should change when the TV is turning on. That is because it tries to turn the backlight on. If those do not turn on with that signal you would get an error code blinking. Since the BL_ON is generated by the Main board, you should check that next. Post some more very clear and close pictures of your boards. to see if there is /are fuses on the main board. The voltage on the BL should be relatively high so set your meter to the 300VDC range and then bring it down if need should be.