Chosen Solution
Hi everyone, I currently have a LG 55GA7900 that has turned into a black screen with no video or sound. So I decided to take off the back cover and see whats going on. There are 2 boards; power supply board and main board. I hooked up the electricity and I noticed the “LD701” light flashes 5 times and then repeats again. The LD701 bulb is on the power supply board. The other thing that happens when I turn it on is that the logo (LG) on the bottom center of the tv frame slowly glows and fades. I have also tested the standby voltage and its correct (3.5V). I found a manual online for the TV but I cant seem to find anything related to trouble shooting the LD701 flashes (I know 6 indicate back light issue from reading forums). I have attached pictures of the boards. I dont see any bulging capacitors.
The pdf manual (service manual) can be found here: LG+55GA7900-UA+LA37G.pdf I have also attached the multi-meter I bought (cheapest one). Is this sufficient to test the capacitors?
I also checked if the issue is the back light by flashing onto the screen but its truly black. Thanks! Update (03/04/2021)
here is the flashing im seeing. Still not luck to find the issue. Ive measured the voltage while its connected to several capacitors. 450V big fat capacitor measures 400V. The 100V capacitors measure 31V and 61V (labeled in picture below). I also checked the 2 fuses labeled below. Does this mean the power board is OK and its probably the main board thats faulty?
Hi @melsawy The service manual actually gives you a good indicator on possible faults if you start measuring voltages as per page 62. If you can’t narrow it down, I would skip to page 100 and 108, and measure the voltages on the highlighted connector. Also what you need to make sure that you measure voltages that are steady and not fluctuating. Unfortunately your meter is not capable of measuring capacitance so the only measurement you can do is to see if the caps are charging up to the voltage of the power rails. You might also have shorted lights on your LED strips which you can test with your multimeter using diode mode.
I had the same issue described by the OP on the same model TV. I removed the main logic board and used a heat gun at about 500* for a few minutes to “reflow” the CPU. After cooling, I reassembled the TV and it started working again. It’s been over a year and it still works perfectly. Hope this helps as it may be a common issue.
