LG TV turns on for a sec then shuts off... - Page 1
Forum section: electronics / repair

author: Lockon Stratos
author_rank: Regular Contributor
timestamp: December 22, 2024, 06:20:16 pm
content: Sup fellow electron hunters?Hoping to get some help with an lg tv. This is what it started doing:https://youtube.com/shorts/KUAGAYy5ino?si=JYt8iiQ_Yxo3BloCData sticker:https://content.section9.me/im/eev/lg_tv.jpgIt turns on for a brief second, emits some abnormal sound then shuts off completely. Even the red led cuts out so i assume the PSU shuts down for some reason. Cant see any cap juice or blow-holes so im pretty much at a loss where to start.Would appreciate any help. Thanks!

author: u666sa
author_rank: Frequent Contributor
timestamp: December 22, 2024, 08:06:05 pm
content: I'm thinking backlight. YES, when backlight is broken it would actually come on and turn off.There is a way to disassemble the screen. You check backlight LEDs individually using a power supply by providing some voltage to them. If at least one LED is burned that means you have to replace all strips, otherwise you'd be diving into it soon one more time. However, each LED can be desoldered and replaced individually.

author: inse
author_rank: Super Contributor
timestamp: December 22, 2024, 08:11:02 pm
content: Spash screen looks quite normal though

author: Lockon Stratos
author_rank: Regular Contributor
timestamp: December 22, 2024, 08:39:56 pm
content: Quote from: u666sa on December 22, 2024, 08:06:05 pmMeasured the 2 strip (2 set of red-black pair coming out) and both measures ~3,1V. So no shorts there. Ill take a look properly tomorrow.Quote from: inse on December 22, 2024, 08:11:02 pmYup, thats what puzzles me. If one of the rails had an issue (either for the logic or for the backlight you would think it effects the image as well.Took out the PSU and now that i have it under some proper strong lighting i think i might have found a clue (or its just some dirt or something):https://content.section9.me/im/eev/lg_tv2.jpg/edithttps://content.section9.me/im/eev/lg_tv3.jpgBlue thingy cracked on the bottom, sneaky little devil. Markings: RR 101K 1kV 70. A MOV maybe? And yes i went medieval on it and cut it out after i noticed the crack, still doing the same thing. The pink resistor right next to it measures 0,148 ohms in both directions, if im not mistaken while decoding that should be a 30,8 ohm resistor. Diode mode shows dead short.....

author: inse
author_rank: Super Contributor
timestamp: December 22, 2024, 11:06:26 pm
content: Its a 100pF 1kV capacitor, probably from a snubber circuit.Not advisable to run the power supply without it.The resistor looks like orange-black-silver-gold to me, which translates to 0.3Ohms 5%

author: Lockon Stratos
author_rank: Regular Contributor
timestamp: December 23, 2024, 06:45:43 am
content: Hard to make it out what that cap does, darned board has a layer of white coating of sorts on top of the solder mask. Removed it for testing purposes to see if that was the culprit, do not plan to run it this way long term.Its actually a 5 stripe sucker, orange-black-gray-gold-green.

author: inse
author_rank: Super Contributor
timestamp: December 23, 2024, 07:22:51 am
content: 30.8 is not from any E series I know of, the green ring might be temperature coefficient or some specific marking for fire resistance e.g.Resistors very rarely fail low ohmic, too.

author: Lockon Stratos
author_rank: Regular Contributor
timestamp: December 23, 2024, 07:32:10 am
content: Maybe LG even custom ordered them with their own markings, who knows. Its a bit hard without a schematic, ill poke around with a multimeter tonight to see if i can trace the short to something else. ATM ill have some errands to run.

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 23, 2024, 12:19:15 pm
content: I'd call that gray stripe silver, looks like the current sense resistor on the PFC's MOSFET so it should be a low value and if it doesn't look damaged should be good (they fail catastrophically when they do).On the other hand if there's a diode in series with your cracked capacitor make sure to check that.Clean the PCB before replacing the capacitor.

author: Lockon Stratos
author_rank: Regular Contributor
timestamp: December 24, 2024, 09:18:00 pm
content: There is no silver option in case of a 5 stripe resistor though, lifted off one leg (desoldered this time) and its 0.38 ohms....Found a faulty zehner (zd9108 marking on the board) diode that is a dead short both directions. Plus a sus diode (d9107 marking on the board) that has a drop of 0,22V in one direction and break in the other. Both in the close vicinity of an ic marked 3s121 sk751a.

author: inse
author_rank: Super Contributor
timestamp: December 24, 2024, 09:20:24 pm
content: Please re-read replies 4, 6 and 8, it’s a four stripe resistor with an additional parameter marking.Did you locate a service manual?Otherwise we have no idea which circuit parts you are talking about…Can you supply a photo ofthe PCB top and bottom side?

author: DavidAlfa
author_rank: Super Contributor
timestamp: December 24, 2024, 09:34:20 pm
content: Normally this is a burnt led, still conducting a bit but causing huge Vf, triggering OVP.

author: Lockon Stratos
author_rank: Regular Contributor
timestamp: December 24, 2024, 11:25:35 pm
content: Quote from: inse on December 24, 2024, 09:20:24 pmOk. Popped off the zehner and diode from the board. Zehner is still a short both directions, the diode also measures the same..... Ill try to dig up some schematics tomorrow, this is going to take a while.Images:https://content.section9.me/im/eev/top.jpghttps://content.section9.me/im/eev/bottom.jpgQuote from: DavidAlfa on December 24, 2024, 09:34:20 pmMaybe, ill know more if i have found some sort of schematics...

author: u666sa
author_rank: Frequent Contributor
timestamp: December 25, 2024, 03:47:46 am
content: Quote from: DavidAlfa on December 24, 2024, 09:34:20 pmThis.

author: Lockon Stratos
author_rank: Regular Contributor
timestamp: December 25, 2024, 08:45:34 pm
content: Ill look into that as well, at least i finally found the correct info for the board i have:https://adjuntos.yoreparo.com/default/0007/06/8787c55ea551c12f230ce656a3db5ae95011a1ca.pdf(I hope the IC didnt get fried, tried mouser and digikey but nothing comes up for that part number.)Ill order the parts then i can test the led strip voltage during operation.....