Need help trying to fix this TV.  - Page 1
Forum section: electronics / repair

author: cvriv
author_rank: Frequent Contributor
timestamp: January 16, 2022, 02:21:42 pm
content: I have this TV that is not working correctly. It was given to me because of this reason. The crazy thing is that when i connected it to my computer about two nights ago it worked fine. The next morning I went out and bought a mount for it and after it was mounted and connected... It stopped working.It appears that everything works fine, it powers up and theres audio, but no image. If i shine a flashlight close to the screen, i can see the image, so i know its a backlight issue.I already disassembled the TV and Im staring at the LED strips. I want to test them but dont know how. I want to make sure they work before accusing the power board even though im thinking its something on the power supply board hopefully not the main mother board.I cant find any info about these strips. When i power the TV on they flash real quick and then turn off. I want to test these strips but down know how. I believe each strip is in parallel because thats how LED strips are usaually. I applied 12v and even 24v accross the strips and nothing happens. It can be more than that right???Any ideas?

author: cvriv
author_rank: Frequent Contributor
timestamp: January 16, 2022, 02:23:49 pm
content: Some more pics. Other than the input 120v, i can only see 12v and 5v on the board. All of the LEDs cant be blown. That would be nuts.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 16, 2022, 02:43:12 pm
content: If the LEDs come on for a split second when you start the TV - the LEDs themselves are probably OK.You could try to capture a video of the startup, and freeze the video at the right moment, and check that they are all lit!More likely, the problem is with the circuitry that drives the backlight...

author: cvriv
author_rank: Frequent Contributor
timestamp: January 16, 2022, 03:17:49 pm
content: *Hold my beer*I was actually doing what you suggested while i waited for a reply. It seems all light but 4 of them. So i have to find some replacements, but what now? There are two boards... The power supply board and the main board. Have no idea what to start checking. I mean i know i have 120v as an input. I checked the one obvious fuse and its good.

author: keymaster
author_rank: Regular Contributor
timestamp: January 16, 2022, 07:44:46 pm
content: 5 led are out , not 4 .Some power board will stop giving power to the leds, if a lot of the leds stop working .Replace the 5 broken before anything else.

author: wraper
author_rank: Unknown
timestamp: January 16, 2022, 07:52:43 pm
content: You need to replace all of the LED strips (not individual LEDs), working LEDs are at death's door anyway. Many TVs overdrive LEDs to claim better brightness spec, and it causes premature LED failure due to overheating and thermal cycling. Intermittent failure is due to LED driver protection being on the brink of kicking in or some LED intermittently going open circuit.

author: m k
author_rank: Super Contributor
timestamp: January 17, 2022, 11:51:11 am
content: How are those powers, leds have their own second stage power and LNB has completely own one?DC connector then has BL_On/Off and PWM.

author: wraper
author_rank: Unknown
timestamp: January 17, 2022, 11:56:20 am
content: Quote from: cvriv on January 16, 2022, 02:21:42 pmThen why you did not mention a TV model?https://www.aliexpress.com/item/4000427048071.html

author: SilverSolder
author_rank: Super Contributor
timestamp: January 17, 2022, 01:43:58 pm
content: Quote from: wraper on January 17, 2022, 11:56:20 amHow did you work out which model it is from those pictures? [impressed!]

author: TheMG
author_rank: Frequent Contributor
timestamp: January 17, 2022, 02:26:16 pm
content: I dont think anyone mentioned this yet but the LED strips in TVs are usually series, not parallel. So you need quite a high forward voltage to light them up.There is a constant-current boost regulator which power the backlight. Usually the circuit will have protections and shut down if the LED forward voltage is not within the expected range.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 17, 2022, 02:35:47 pm
content: Quote from: TheMG on January 17, 2022, 02:26:16 pmIf they are in series, how do they keep working with "blown" LEDs? - do the LEDs blow to short circuit?

author: wraper
author_rank: Unknown
timestamp: January 17, 2022, 03:39:45 pm
content: Quote from: SilverSolder on January 17, 2022, 02:35:47 pmThey go short or they may have zener diodes in parallel.

author: CaptDon
author_rank: Super Contributor
timestamp: January 17, 2022, 04:31:40 pm
content: TAKE NOTICE of one very important fact. Every series strip has ONE failed LED. This often indicates the power regulator went ape shit and the weakest LED in each string failed. Indeed, try the replacement of the failed LED's and see if it powers up correctly or blows out one more LED in each strip. If it works briefly and self destructs again you have a power supply current regulator problem.

author: wraper
author_rank: Unknown
timestamp: January 17, 2022, 07:08:50 pm
content: Quote from: CaptDon on January 17, 2022, 04:31:40 pmNearly impossible to happen. The issue is factory overdriven LEDs.

author: CaptDon
author_rank: Super Contributor
timestamp: January 17, 2022, 08:31:32 pm
content: Well, as a matter of fact it can happen. I just had a 4ft LED fixture made up of one long string of LED's but connected as 4 parallel strings and each string having around 20 SMD cool white 4000k LED's. There was no equalization between the strings, all 4 in parallel. BUZZ...POOF.....SMOKE light goes out. Every string had one bad LED. I replaced them, the power supply looked o.k., no visibly fried parts. Powered it up, ran about 30 seconds perfectly. BUZZ.....POOF.....SMOKE. Cut the nice cord off and shit canned the rest of it. Sure enough, one more visibly fried LED in each string. I know it was a general illumination fixture and this guy is fixing a t.v. but one bad led in each string looked suspiciously like my LED failure. My unit appeared to have a current buck regulator. Voltage be what it may it looks like it was designed to regulate current with as few parts as possible. Guess something drifted off tolerance and over currented the strings. Maybe a thermal intermittent since it worked 30 seconds and blew up again.

author: wraper
author_rank: Unknown
timestamp: January 17, 2022, 08:59:05 pm
content: Quote from: CaptDon on January 17, 2022, 08:31:32 pmThe difference is that your thing emitted a magic smoke and did not work for prolonged time with failure present.

author: BrokenYugo
author_rank: Super Contributor
timestamp: January 18, 2022, 02:00:40 am
content: CaptDon's failure mode seems plausible, but I also somewhat doubt it's the case here. I have to imagine TV backlight drivers are designed a bit better than that, and if it were really hammering the strings with some fault you'd probably see more burn out while it is being tested and diagnosed.As suggested earlier, the controller is probably just taking offense to the compliance voltage going out of range. The parts are cheap enough and failure common enough I'd probably just throw a new set of strips at it and call it a day, keep the backlight control away from maximum if you don't want to repeat the job. If one insists on diagnosing properly, check the forward voltage of the individual LEDs (some have multiple dies in series in one package), and tack diodes into the series strings to make up for the shorted ones, should stay lit then if nothing else is wrong. Just be careful as this runs at fairly high voltages, some are up around 300V.

author: CaptDon
author_rank: Super Contributor
timestamp: January 18, 2022, 02:44:28 pm
content: I examined the power board of my general illumination fixture. The board was still in the trash bin by my bench. It suddenly became apparent. Looking at the power board circuitry it seems to only sense current with no regard to voltage. All 4 of my series strings were connected in parallel. So assuming one LED went open circuit and that strip was basically 'gone' the power board would have caused a 33% power increase to the remaining strips and when the next strip failed, double the power to the remaining two. So it isn't any wonder all 4 of my series strings had one failed open LED. And it all probably started with just one failed LED. Does seem strange that the O.P. has one bad LED in each of his strips. So a question to the O.P., Did the thing slowly demise one LED at a time, or one 'strip' at a time? Or did it go dark all at once?

author: SilverSolder
author_rank: Super Contributor
timestamp: January 18, 2022, 04:05:50 pm
content: Quote from: CaptDon on January 18, 2022, 02:44:28 pmIsn't there a limit to how much voltage it can provide? - e.g. a doubling of output voltage if half the strips fail, seems a lot...

author: CaptDon
author_rank: Super Contributor
timestamp: January 18, 2022, 04:32:30 pm
content: No need to double the voltage, you should know that. LED's like any diode have a very sharp increase in current with very little voltage increase!! At 20 LED's at 4.2v each that is about 84 volts. I bet if you raised the voltage to 90 you could easily double the current. With a rectified 120vac and filter cap the buck current regulator had around 150vdc to work with!!

author: SilverSolder
author_rank: Super Contributor
timestamp: January 18, 2022, 04:36:36 pm
content: Quote from: CaptDon on January 18, 2022, 04:32:30 pmDooh, yeah, I see what you're saying - the voltage remains relatively constant because of the parallel diode strips... when a LED goes open, the 300mA is now forced though 3 strips instead of 4, so each strip dissipates more power (at roughly the same voltage).I guess the design shortcoming here was the idea of connecting the strips in parallel instead of giving them a pass transistor each...

author: wraper
author_rank: Unknown
timestamp: January 18, 2022, 04:45:59 pm
content: This TV has an individual transistor for each LED strip though.