2 LG 55GA7900 blackscreen/no video help!
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:
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?
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8 commentaires
Did you ever figure this one out? I have the exact same tv with the same LD701 flashing light. Would be a shame to toss this tv. Thanks!
par iamdjvoyage
@Clark Gourgue five flashes mean CH05 error which is a power supply error. In the picture I do see a medium cap that is bubbled. that is likely bad. Also check the fuse. lower left bottom.
par stressmaster
I can’t tell if I have any swollen caps but I also don’t seem to be getting any voltage from the bottom left fuse. My meter just bounces from positive to negative. In diode mode I am getting a reading of 1. Are these fuses soldered in? I did notice that I’m getting the proper 3.5, 12 & 24 voltages from the P2301 connectors from the power board to the main board if that makes any difference. I assumed that if the fuses were bad I wouldn’t get anything there. Bummer I can’t post a pic in these comments haha
par iamdjvoyage
@Clark Gourgue
The fuses are soldered in.
The bottom fuse F100 is the main supply fuse. If it were faulty you would have no voltages appearing on the board.
How are you measuring the voltage at the fuse?
You can't measure directly across the fuse as there should be no voltage drop on a fuse. You have to measure between one side of the fuse and earth or the neutral lead on the input and then the other side of the fuse and earth or neutral. If the fuse is good, the readings will be the same i.e. the AC input voltage.
Be safety aware when measuring here as it is lethal AC voltage so your meter should be in >240V AC Volts mode.
Safer way is to disconnect the power from the TV and use an Ohmmeter across the fuse and make sure it measures 0.00Ω
It may be better if you asked your question as a "new question" on ifixit. That way you can add images etc.
Moderator.
par jayeff
Thanks for the reply @jayeff! I'm definitely not an experienced electrical engineer, so I have been trying to keep my voltage testing to a minimum. But, yes, I am getting all the proper voltages being sent to the mainboard after pressing the power button. I can definitely post a new thread, the pictures in this one are exactly the same as mine haha. Even the video with the flashing light.
par iamdjvoyage
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