Just got an LCD TV screen from a friend for repair. It had beed sent for repair earlier, came back fixed and then broke again within days. The screen is a Toshiba 40L1333B, which was produced after 2012.
The problem: The backlight of the 40L1333B screen does not come on. After a good amount of investigation I noticed that someone else had been in there to replace 2 of the connections between the LED boards. The backlight LEDs are mounted on a PCB strip which has board to board interconnectors. I checked all the connectors and noticed that they had very inconsistent resistance values anywhere between 0.25 Ohm (which is what they should be), some kOhm and even half a MOhm. The LEDs and boards are wired in series, so if there is a bad connection anywhere between any of the boards, all of them turn off. I really did not expect that the connectors would fail before any cheap or underspec’ed electrolytic capacitors or in fact one of the LEDs. However, the cheap caps on the main power supply board are perfectly fine.
- Check that the “Backlight On” signal (~3.3V) is sent from the control board to the power supply board and that 24V are supplied to the LED supply board, which is annoyingly located behind the LCD screen. The power supply PCB is a vestel 17PW07-2, schematics can be found here. The Backlight on/off pins (labelled BL_ON_OFF_1 and BL_ON_OFF) can be probed at the two larger connectors (bottom right corner of schematic).
- Fix the board to board interconnects.
- Check for broken LEDs.
The solution: The backlight on signal was present, 24V were supplied to the LED board. After removing the LCD I noticed that the LEDs would momentarily come on before turning off again, which made me worry that I am looking at more than just faulty connectors. Anyway, the faulty connections were so obvious that no matter what else might be wrong, the mod wires would be required. So I continued with that and voila, the screen turned on. Problem solved. My opinion is that interconnects are engineered to fail.