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Question DetailsAsked on 12/14/2017

Is my backlight problem on my LG tv the power supply or led strips

LG 47ln5750 uh led tv. Screen went black instantly. Flashlight test reveals a picture but no backlight. I have 166 volts on the led output leads on power supply board on power up but rapidly falls off to 83 volts within 15 secs on both the led output pins.Not sure what the voltage is supposed to be. Suspect the powersupply as opposed to leds. Which is it?

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LOTS of articles/blogs on the web about this, especially with the 47" models like you have, so sounds like a design/manufacturing defect whether or not the company admits it. I would go to the www.cpsc.gov website and put in your make and model numnber to see if there has been a recall on yours already.


I would also google about whether LG is doing a stealth recall on your model. A "stealth recall" is where the fault is not inherently hazardous, so a government (CPSC) mandated safety recall is not necessary and it has not gotten to the point where massive public pressure has forced a voluntary recall, but the manufacturer will give a substantial percentage refund or sometimes a total replacement of the item to people who seriously complain and maybe threaten filing a complaint with government agency, so the company does a "customer courtesy" refund or repair or replacement for the defect even if not under warranty still.


Obviously, works best if just a bit out of warranty, does not work of course if the screen is physically damaged from the outside, and can take a bit of digging around to find the department with the authority to do a refund/replacement. Normal customer service rarely can authoprize this, you usually have to find the corporate customer satisfaction VP or such name's office to contact about it. And of course, the squeeky wheel gets the grease - you may have to escalate the issue up through the company till dealing with it fairly is easier/cheaper for them than having you continue to contact them about it. And of course, if there is a safety aspect to the defect, then you have cause to report it to the CPSC if the company does not replace it for you, and they do NOT like CPSC attention because a few complaints about a hazard can end up with them getting recall orders from the CPSC on many millions of $ of product. And they HAVE to reply in writing to the CPSC and to the customer on all properly filed CPSC complaints.


Couple of solutions that were found to work for your described symptoms -


1) in some cases (sounds like rare) if the LG logo comes on initially then the screen blacks out, if you wait about 2 minutes (one source said 99 or 100 seconds, other 120 seconds) and it turns itself back on with the logo again (apparently with four dashes instead of the time showing), a firmware update/reload apprently fixes it. Webpage to select your model and download firmware update (obviously has to be uploaded from a connected computer) here:


http://www.lg.com/us/support-product/...


2) in most cases, sounds like it is a bad backlight LED strip causing the LED Driver to shut down to protect the TV - hence the blackout. There are some blogs on how to force the board to drive the LED strips to determine which is bad (though it evidently fries the entire strip with the bad LED portion doing so, so you then have to replace the entire bad strip, not just a single block of LED's). You can find a lot of YouTubes and blogs on the subject and on troubleshooting by googling this search phrase or similar -


LG 47ln5750 uh led tv LED driver voltage


3) Blog describing troubleshooting and repair for your model and problem here when it is a bad backlight strip - specifically talking about the voltage drop from about 160 some down to the 70's or low 80's VDC, which appears to be a protection feature cutting in, here:


https://www.badcaps.net/forum/showthr...


If you were checking the PSU (power supply unit) with the LED strips connected, then the power dropoff to 83 bolts could indicate the unit is detecting a bad LED and cutting back the power as part of the self-protection feature. Sounds like from a couple of the blogs that if you disconnect the PSU from the LED / video driver board (sounds like VDI and LED driver are on same board) then the PSU would have a constant output on those pins around 165V. Of course, you have to be using an electronics rated VOM to test this or you risk the VOM acting as enough of a load to possibly damage the PSU or video board - you need an electronics-designed VOM which has very little effect on the circuit being tested when checking electronics, as a regular electrical one can damage components by allowing excessive current through the meter during the checking process, or by putting too much power from the VOM into the circuitry.


There was one blog talking about disconnecting the LED strips from the VDI connectors, then doing a very short contact time between certain numbered output pins on the VDI to each LED strip one by one to see if the LED strip comes on (sounds like there are 4 strips on the backlighting in your unit) - that can tell which is/are bad. I would do some more googling for instructions on that to be sure you do not burn out some units which are sitll good or overload the VDI or PSU by connecting to a bad strip - obviously this should be a very short contact time at a minimum.

Answered 11 months ago by LCD




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