TV repair : 50 "  LED no picture, need help... - Page 1
Forum section: electronics / repair

author: Vince
author_rank: Super Contributor
timestamp: March 12, 2021, 08:01:56 pm
content: Hello world,Just got this TV for repair, a friend of a colleague...Brand : " Continental Edison ", whoever they are never heard of them.Model : " CELED 502723 "Age : looks like it was manufacturered early 2014, so not that old, though I guess with planned obsolescence, anything older than 3 years is considered "old"...Misc : 50 inches size, LED backlight.I promised I would have a look at it, but not going to spend 3 weeks on it, just a decent attempt in the coming days, starting today...A new one is worth 200/250 it seems, so the owner probably is not going to spend more than 50 Euros tops, to fix it. So replacing board is out of the question. Either I can fix it at component level for real cheap and pocket 50 Euros or so, or I give up and scrap the thing.I am asking for your help because I know squat about these modern flat TV things. Don't have a TV at home, gave up on TV in 1998 or so.I don't even know how to operate this TV nor how to interpret the behaviour of the power/status LED on the front panel.Announced symptoms : " I was in the middle of watching something, TV was working just fine as it always did, then all of a sudden screen went black, never to come back...no more TV ! ".Spent a couple hours on, it so far, getting to grips with it.Given the sudden loss of picture, in mid air, I am thinking unliely a didgy cable/connector as it would have manifested itself first with intermittent failures. So am thinking more like a sudden power supply failure, or the LED backlight itself that died, if that's even possible, don't know.So for now I am investigating the power supply route...TV has only two board. A very large and coplex power supply board, and single digital / main board that does everything. That board seems to be a single chip design, around a " Mediatek " ARM CPU.The main board looks like there is some life in it, because the blue status LED a the front, is connected to it, and said LED does react when I press the standby / power off switch on the main board. LED is either lit solid, or blinking, or off. No idea what that means. But presumably that means at least that the CPU is alive / running ??Power supply board : two connectors, one to the back light, another to the main digital board.- Backlight : two pin connector, red and black... easy enough eh. Unplugged the connector to check the backlight for a short.. no short, reads a few megs.Nno idea of voltage ? There is a big electrolytic cap in the area, rated at 250V ! So I guess the backlight voltage must be pretty high, 150 to 200V or something ? I tried to measure it but got zéro volt DC, but strangely, saw 4VoltsAC!So might be the baclight power supply that's kaput ? Or is this 4V AC somehow normal, for some reason, a weird PSU design ??Also, is the backlight supposed to be powered at all times, as soon as the power supply / TV is turned on ? Or does that main digital board have a say in this, and it might turn the backlight on only when it feels like it ?- Main board : connected to the power supply via a 12 in cable. All blakc wire, doesn't help... Looked at the traces on the PSU board. I would make a guess and say that there is a single voltage rail in there, carried by 7 of the pins, and the remaining 5 pins must be some sort of control signals... hence why I a fear that the backlight is under control of the main board rather than being on all the time...I measured that rail at 12.5V, ripple within reason, under 20mV. A quick look at the main board and I would say that there are 2, maybe 3 local DC/DC converters. I guess the biggest of the 3 is for the 3.3V or 5V or whatever, for the ARM CPU, and the other ones for the I/O and audio amp ?!So really, any help will be appreciated !- Any common failure points on these things ?- Is the LED backlight power supply supposed to be on all the time, regardless of the main digital board ? If so then that's marvelous, means this is my problem and I can start digging there.- Any clue what's the meaning of the LED ? I guess I ought to see if I can google the usr manual. Won't even bother searching for a service manual, might as well believe in unicorns.- any suggestions at all really !Checked the power supply board for damaged / burned components, nothing obvious. Checked all the diodes that I could see, all good. No bulged or leaky electrolytic. Don't have an ESR meter just yet. All 11 of them are branded " Elite " which mean the confidence factor is very low.

author: KevinA
author_rank: Regular Contributor
timestamp: March 12, 2021, 08:24:03 pm
content: Always service data avaliable for Vestel.Power supply is a Vestel 17IPS20.Service data for this is available from Electrotanya.Also,https://www.petervis.com/has an article on diagnosing this and repairs.

author: tautech
author_rank: Super Contributor
timestamp: March 12, 2021, 08:32:15 pm
content: First check is for lack of backlight with a strong light shone onto the display to see if a picture is there but not visible due to no backlight.If so, go investigate why the backlight is not active.

author: TheMG
author_rank: Frequent Contributor
timestamp: March 12, 2021, 08:44:15 pm
content: Do you get sound from the TV speakers?Also, take a bright flashlight and point it directly at the screen. Look closely, do you see anything? If it's a backlight issue the LCD screen will likely still display an image, but it will be very difficult to see.Note that the lack of output from the LED driver power supply doesn't necessarily indicate a problem with it. There are a number of protections and any serious problems with the LED backlight itself will cause the LED driver to shut down and output no voltage.Yes, the output voltage to the backlight is often over 100VDC, because the LEDs are connected in series in large numbers.Common faults for LED TVs is with LEDs going open-circuit or short-circuit. At max brightness (default on most TVs) the LEDs are run hard and also at high temperatures, doesn't do any good for reliability. They're supposed to be more reliable and have longer lifespan than CCFL backlight but sadly that is often not the case because they run the LEDs very hard to get the most brightness out of the least number of LEDs. I always advise setting LED TVs to no more than 80% brightness, it's not a very noticeable decrease but should in theory greatly increase the reliability of the backlight. Any component run very close to its maximum ratings will have decreased reliability.

author: Vince
author_rank: Super Contributor
timestamp: March 12, 2021, 09:37:35 pm
content: Thank you to all of you for your help, MUCH appreciated !!!I downloaded the schematics, and also found this article about someone who knows this PSU board quite well it seems !https://masterelectronicsrepair.blogspot.com/2019/02/vestel-17ips20-power-board-circuit.htmlHe says it's a very well known board, found in gazillions of TV sets of all kinds, big brand names as well as obscure ones, high-end devices as well as low end... it's everywhere it seems ! How lucky I am !Guy even specifically talks about the backlight driver. As you said it's protected against shorts AND open circuits, so no way I could have tested it unloaded like I did. No wonder it didn't work. He also says taht the backlight is hard wired to the PSU board : as soon as the TV is powered up, the PSU board powers the LED backlight. The digital board has no say.Speakers ? Don't know... don't have a TV antenna cable to hook up to that TV !But... it has a VGA input, so I connected it to a computer, then looked all over the screen with a torch light at an angle.... couldn't see squat !Then, I looked more closely, real close, and played with the menu switches... and OH ! at some point I saw a very faint little something on the screen, dark brown in colour. It certainly looked like some menu element, some kind of icon with a little text. Played some more and then later I 100% definitely spotted a "folder" icon, with a text string just next to it that read " Gpib" ! YES !!! We have picture, the TV is not dead !!Backlight issue it is !!!Phew !So now the problem is that.. since the driver won't work unless the LED backlight is not shorted or open-circuit... how to tell which one of the two is bad ?! :-/ I tested the backlight with my DMM in diode mode. It's not shorted, I can at least tell this much. But.. is it open circuit ? I don't know.. DMM won't help me here. It reads 5 Meg or so because of course it doesn't put out enough voltage to test these long strings of LEDs...I would have to find a way to power up the LED backlight with an external supply but... I do'nt have a high voltage power supply... just your regular lab power supply with 2 floating 30V outputs. Well I have 2 of them actually, so that would 120V maximum... might be enough to get some life from the LEd's, if they are alive at all.Or maybe I can just power them direclty from the mains, just put a bveefy rectifier diode in series, and a beefy series resistor to limit the current to some safe level so as not to fry the LEDs ?? Will check the schematics to see if I can find how much voltage and current the backlight needs.

author: tautech
author_rank: Super Contributor
timestamp: March 13, 2021, 08:22:17 am
content: I would first download and examine the datasheet for the LED driver IC MP3394 (U500)https://4.bp.blogspot.com/-4TJiYjwWtF8/VzPxsX-XkBI/AAAAAAABV_0/g14WAB9mEY4RrWbxrQ5mE1Ac59fKxYPrACLcB/s1600/VESTEL-2.jpgThere seems some fusible resistors to check on the V_LED+ rail before and after the +24-80 rail that is generated on pin 6 TR300 controlled by IC SG6742 (U300) sourced from +400VDC.Presumably one of the switchers is not working or some protection is kicking in.

author: Vince
author_rank: Super Contributor
timestamp: March 13, 2021, 01:46:49 pm
content: OK, progressing.- That board is well known because so widespread and so... shitty, keeps failing.- 99% of the time the problem is... backlight, backlight, backlight....... or sometimes, backlight. So plan for now is to concentrate on the driver rater than the LED panel itself, especially since I have no immediate, easy way of testing the latter.- There are many variations of this board.- Schematic is very messay, confusing and "overloaded" at first... then realized it's a fantasy schematic : no board actually implements all that stuff. Schematics shows the sum of all possible variations/options, and you have to pick and choose a small portion of that stuff only, not all of it. Board layout is consistent with that : lots of unpopulated footprints, and even in places, several footprints stacked on top of one another..- In my case, the top side of the board, where the big through-hole components are... I would say 50% of the components are "missing", unpopulated footprints. On the underside where all the small SMD stuff is, it's more like two thirds missing !so the actually schematic of my board is way simpler than what the schematics shows.- It's not perfect though : some components I DO have on the board.. are nowhere to be found in the schematic... but overall, the basic topology is the same, so the schematics are still a huge help, no doubt about that !!!- Fusible resistors : lots of them. Half of them have been replaced by 0 ohm SMD resistor jumpers. The other half, which are actually implemented, are easy to spot with their package that's different from anything else on the board. They all test good, dead short.. eventhough the schematic pretends they are supposed to be 50 ohms ?!...So for example, schematic shows 4 rectifier diodes in parallel for the boost circuit... I have only one single diode doing the job. Same everywhere you see 3 or 4 or 6 or more similar components in parallel, be it resistors or caps or diodes... in real life I have only one component, 2 or 3 at the most.Voltage rating for the big electrolytic caps : schematic shows 50V ratings everywhere in the LED driver circuitry.... but I have a 250V one.found a video on youtube where a guy is used to repairing lots of these very board, again 95% of the time, backlight issue. Says a working driver puts out 180V at startup then settles a bit lower at 160V. Often one would measure 115V, indicative of a faulty driver, not an LED problem.So now I am concentrating on the power board, and only on the LED driver part of it for now. Will download datasheet for the driver IC, and start measuring voltages... is the IC power good ? Is LED power rail good ? Is the chip even enabled ?I disconnected the LED from the board, so I can work on the stand alone board on the bench so much easier than when it's connected to that huge TV thing. Of course the driver IC will sense an open LED fault condition and most likely cut power altogether, won't switch the gate of the FET. But I don't care for now, this still allows me to check the basics that I just mentioned. If the basics are OK, then might reconnect the board to the LED/ TV and troubleshoot further...Anyway, looking good so far :- TV is not dead, runs fine had has picture...just no backlight.- Most likely a board issue rather than the backlight itself, so I do stand a chance of repairing it at component level.- We have schematics for that board, and some online info on the web from people having been there on that very issue, many times...So it's worth spending a few more hours on the thing, might actually be fixable...

author: KevinA
author_rank: Regular Contributor
timestamp: March 13, 2021, 06:53:02 pm
content: 17IPS20, no backlight.Plenty of repair kits on the market,http://www.ohmsupplies.co.uk/epages/62027733.sf/en_GB/?ObjectPath=/Shops/62027733/Products/KIT414and they contain all of the unobtanium parts.

author: Vince
author_rank: Super Contributor
timestamp: March 13, 2021, 08:44:38 pm
content: Thanks for the link, interesting. Wow, a company whose sole business model is selling parts kits for TV sets ! Says a lot about the reliability of modern TVs....Not going to buy any kit though, as I don't know what I need to buy since it's not precisely diagnoseds yet, and even if it was, I certainly would not buy such a kit since it's absurdly expensive for what it contains. Just a handful of dirt cheap bog standard parts, at least in the case of that board. Only part on in that backlight circuit that might have been troublesome to find potentially, is the driver IC ofr course, but had a quick look and it's readily available, for a small fraction of the price that this kit company sells it for...Besides, this repair can only be economically viable if I can fix it by replacing nothing but needs to be replaced, for a song. Otherwise I will eat my very slim margin and basically I will have worked hours and hours on the thing.. for free. I don't work on week-end and evenings for free... would rather work on my own stuff and also work on my house that needs so much done...Anyway, I started measuring voltages here and there in that backlight circuit. Again, power board alone, no LED connected, no main board either. Just the power board plugged to the mains nothing more.I am rather puzzled at my findings, this is one strange/sick puppy ?!Here goes :With the LED disconnected, the driver is supposed, from what I gathered, to detect an failed open LED, and cut power. But what I found instead was :- At the output of the regulator, I have life ! I get 157 Volts ! Exactly the voltage level that I am supposed to get !Weird !!! So maybe my particular version of the board does not implement this fault detection ?? Sounds unlikely, so I probed some more to see if I could witness activity from the driver.... and, there is NONE ! I probed pin 16, where the IC is supposed to output its internal 5.8V voltage reference, an dI get... ZERO. Then probed the gate output....ZERO, no activity. Then checked pin 15, supply pin for the chip. There is 12V. Good enough for the chip to work (requires anything between 5V up to 28V). Then probed its Enable pin... ZERO again (need above 1.8V to be enabled).WHAT ? The chip is not running, not even being asked to, yet I get the required 160V at the output ? By what miracle ! It supposed to be step the voltage UP... how can it be stepped up, to 160V, if the driver is not even running ?! Help me !!!!Probed some more... MOSFET is indeed not switching anything. The 160V is coming straight from upstream, just passing through the step-up converter unaffected. The supply is labelled as " +24-80V ", so I was expecting.. I don't know, 24V.. .or 80V... confusing net name !But at the least I was expecting way less than 160V, otherwise there would be no need for a step UPPPPP driver....But I don't get 24 or 80V, I get 157V ! How can that be ?! The schematic is not rocket science eh ? The 2 secondaries of the transformer are in series, the a diode for half wave rectification, a smoothing cap, then a low pass LC filter, then into the step-up converter...So.... is the transformer kaput ? Is it fine but there is a problem in the circuitry that drives its primary winding ?!.......Without a service manual, it's hard to tell, but I don't see how getting 160V at the input of the converter makes any sense....Misc : found a major discrepancy. On the schematic the negative lead of the LED goes straight into the driver chip inputs directly, but on my board there is a MOSFET in between. Not sure of its purpose (since the driver IC is there to cut power if need be), but of course if it's not turning on, it would explain why I have 160V at the output of the converter / positive terminal of the LED.... yet zero voltage across the LED. Could not figure out exactly where the gate of the MOSFET goes.. PCB layout is a mess with all these unpopulated footprints, and zero ohm jumpers everywhere to reconfigure the board.As for the chips driving 4 independent LED "strings", none of that on my board either. Only two pins on the backlight connector, so one string. All 4 string inputs of the driver IC have been connected together with zero ohm jumpers.As for the driver enable pin...schematic is clear. It's driven by the main digital board... so either I reconnect the digital board, or I solder a wire to force the Enable input, high.So, it's very mysterious and weird.... not sure what to do next.

author: tautech
author_rank: Super Contributor
timestamp: March 13, 2021, 09:36:03 pm
content: Quote from: Vince on March 13, 2021, 08:44:38 pmAs I interpret the +24-80V it's an secondary side SMPS rail, HF AC 24V and -80V then fed to the LED supply circuitry for boosting and rectifying. L500/501 apply the boost I believe.Just using a scope probe nearby can show if the switching circuits are working by the EMI they emit.Might have that wrong but some probing around should show how it works.

author: Vince
author_rank: Super Contributor
timestamp: March 13, 2021, 09:53:04 pm
content: Wow.... sick indeed it is ! Looks like with TV I opened a huge can of worms !My fears seem to be confirmed. Problem looks indeed to be on the transforme and primary side rather than the secondary side / driver circuitry.I mean, played some more with it :- Reconnected backlight panel and main board.Result :- At first no change whatsoever, I still get my 157 Votls, and still zero across the panel itself.. that MOSFET in the return path is not switching on, and/or the driver IC is not.- I can hear a weird noise coming from the transformer ?! .... then aftr a few minutes listening to that noise, suddenly the backlight voltage started climbing ! FAST ! Within a few seconds I saw 240 Volts and still climbing ! Scared the shit out of me, feared some component would blow in my face or something ! So I pulled the power plug in a hurry !- Waited a minute for the HV caps to discharge... then I powered it back. Again I got my trusty 157 Volts.... but after a few minutes, like 3 or 5 tops I don't know didn't time it... suddenly the transformer stops making noise, becomes quiet, and at the same time the backlight voltage drop to zero !!! Measured voltage across the primary windings of the transformer.... I get 340V... ie there is not switching going on on the primary side !Probleùm is definitely on the primary side of things then...Problem is that I do'nt know if the transformer is still good, and how much damage has been done to the secondary side of things, teh driver IC and LED panel ! So it's a whole can of worms... this could become very messy very quick. I could try to fix the switching stuff on the primary side, but then would have maybe to replace tons of stuff on the secondary side, and just spend lots of time and money working and thing.... to get what... 50 Euros pocket money if the owner feels generous ? Minus 50 EUros worth of parts and mulitiple ordesr / shipping... maybe if I am lucky I will make 5 or 10 Euros profit ? For several days worth of work ? On my spare time when I have so many other things to do for ME ?!Just not worth the trouble !I am frustrated because I like to get the final word on everything but... at some point I need to be reasonable. It's best to declare this TV scrap and salvage the electronics from it, as a " payment ".I will play with the thing some more but not for long... its fate is pretty much set in stone by now

author: Vince
author_rank: Super Contributor
timestamp: March 13, 2021, 10:34:21 pm
content: Scope probe tip (pun intended ?!...) worked well Tautech !I got the best result by touching the ferrite core of the transformer. Scope picked huge amounts of 50Hz hum of course, but if you speed the time base it kinda disappears and you can clearly distinguish the switching noise from the power supply. After a couple minutes only, switching noise disappeared, as did the (audible) noise from the transformer, and backlight voltage disappeared too, all in unison.So there is definitely something wrong with the switching circuitry on the primary side of that transformer, or so it seems.Don't know how to trouble shoot these things.What could make it "die" after a couple minutes... then come back after cycling the power (just long enough for the HV caps to drain, a few seconds) ?Hmm stupid me.. I guess all I need to do is power it up, wait for it to die, then NOT pull the plug. Let the primary energized, 400V DC every where so I can get zapped... and probe around the switching circuitry see what I can find.... VERY CAREFULLY !Now might not be the best of times for such a risky endeavour... it's 23H30 and I am tired.. that's when you do mistakes and here they might have catastrophic consequences.... so I will leave it at that for this evening, and come back to it tomorrow...Anyway, making progress none the less....

author: tautech
author_rank: Super Contributor
timestamp: March 13, 2021, 11:17:45 pm
content: Quote from: Vince on March 13, 2021, 10:34:21 pmAh la Near field probe.I can even tune a chainsaw to peak RPM doing this.Check the VCC IC rail bulk caps. Typically ~22uF for SMPS IC's. Either gone high ESR or low uF, sometimes both.

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 17, 2021, 12:23:57 am
content: French Wikipedia page on Continental Edison:https://fr.wikipedia.org/wiki/Continental_EdisonSo they've been American, French, Turkish, Chinese, and now "French" again but still using Turkish electronics.Before deciding the TV isn't worth the bother I'd try lighting the LED string with external power, 120V may (or not) be a bit short.I wouldn't recommend rectified mains without isolation, you could use a couple of small transformer's chained as step-down, step-back-up.Aim for about 150V Vf and 10mA test current don't get the polarity wrong LEDs don't like it.If you do get backlight PSU is defective of course. Often enough the LED's are the trouble though.Supply's being constant current you can get away with shorting a couple of bad ones for testing before replacement.Edit:Doing some audio repair I thought I'd mention this as I've run into it once again...The above applies for the Continental Edison TV's.In the 70's-80's in France you'd also find Continental Edison Hi-Fi which ISTR was the Thomson (Houston/Hotchkiss/Brandt) group.I'm not sure how much was actually designed and built by Thomson but for sure I've seen (Polish made) Unitra amplifiers and other Japanese designs rebranded as Continental Edison.

author: Vince
author_rank: Super Contributor
timestamp: March 17, 2021, 09:30:06 pm
content: Quote from: tautech on March 13, 2021, 11:17:45 pmAh yes, an ESR meter I must get ! ESR meter I must get, ESR test I get I must !I need so many things for my lab, but a basic ESR meter is cheap so no excuse, and particularly useful since I seem to spend my time fixing old stuff...I could say that I have other priorities and all that, which is true but.... this evening alone I bought half a dozen more old broken 5/10 Euros CD players ! Of course I don't need them... could but a little less of them and that would finance the ESR meter, I guess... hum...It's all ShakalNokturn's fault ! It is !!! Because of him I have now become addicted to these old Philips CD players, I am now so fond of the swing arm thing, and their DAC everyone keeps talking about... and I like their industrial design, I feel in tune with them, somehow...I bought also some non-Philips ones as well, broken of course, just so I can get some fun and experience fixing these things. The more the merrier... I found a cute looking Toshiba and Radiola and Technics (which somehow has a SWING ARM in it, eh ?! Thought it was a Philips patent ?! ), even an ancient but somehow cool looking Thomson player ! 7 Euros, hard to beat.. Also a nice Telefunken but IIRC guy would not ship, too bad. I am shocked at how expensive the old Philips unit go for ! Most players go for 5 to 20 Euros, 30 tops... the old Philips can go up to 150, even for broken ones !A bit crazy....Anyway, sorry for the digression, and sorry Shakal, it's not really your fault.. or maybe just a little, but that's fine, I am enjoying it...

author: Vince
author_rank: Super Contributor
timestamp: March 17, 2021, 09:54:02 pm
content: Quote from: shakalnokturn on March 17, 2021, 12:23:57 amHi Shakal, thanks for stopping by ! I didn't expect you to spot this thread among the millions on this forum, lucky me !Yes, this manufacturer is all over the place.. that's the world economy in action I guess !QuoteYep, did that yesterday even though I did not post about it. I wanted to be sure that the LED pane l works, before I waste time trying to fix the power board..Since the board somehow outputs the 160V that it's supposed to.. even though it produces that voltage in a defective way tha tneed to be fixed... I though might as well use that defective power supply, eh...So I wired teh cathode of the LED panel straight to ground, so as to bypass the non-working driver IC, and that MOSFET that they put in the return path, somehow. Of course I adde a beefy power resistor in series to limit the current to something half reasonable. Plunged my hand in my box of salvaged power resistors, and it came out of it... a 10Watt ceramic packed , 3,600 ohms resistor. At 160V that would be 45mA or so, not tha much for that kind LED I guess. Of course with the massive voltage drop from the LEDs, the current would be way less than that.. but I though i should still be able to see some light out of it.. and if not, I could just pick another resistor with a lower value, and do ti again.so I did that and.... nothing. Not a sausage. No light, and zero voltage drop across my powe resistor, meaning there was zero current flowing through it... so I guess the LED panel is open circuit ? End of the repair then... thanks for your participation...QuoteYes. From what I saw, on this board, it starts 180+ V then settles at 160V. When it detect an open circuit fault condition, it drops the voltage to 115 V or so. So I guess that means that at this voltage the LED panel is not supposed to light at all.QuoteYes sure ! I was extra careful and triple checked before applying power !QuoteFrom my experience above, I fear you are right... so the power board still behaves weirdly so needs fixing as wellQuoteYou sound like it's trivial and common to replace LEDs in these panels ?I though i would be impossible..though there would be thousands LEDs spread all over the surface. But found a video where the guy replaced an LED in these big TVs. I was amazed... there are only 2 strips of LEDs, in parallel, each strip bearing only 10 LEDs or so !It looked very easy to work on them.... the impossible part is to remove the LCD / glass without damaging it !!!!!!Woudl need to find a video where the guy show in detail eactly how he proceeds to lift the glass/ LCD panel without damaging it...If I can find that, I might be tempted to give it a try... just for fun, to see if I can do it.. the more experience the better.If I can get access to the LED strips, at least I would be able to make a 100% reliable assessment of their condition...What I don't get, maybe you can enlighten me is.... WHY do we need such a high voltage to begin with ?!I mean.. panel is made of 2 or 3 whatever strips, wired in parallel. So as far as voltage drop is concerned, we can just consider a single strip. EAch strip is made of say 10 white LED in series. White LED drops 3+ Volts or so, I gather. So that's about 30V .... 30V, not 160 !!! So in theory a current limited 40V supply should be enough to light them up !I just don't get it.... but I want to understand, as always...Anyway, 11PM, time to go to bed...

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 18, 2021, 12:23:27 am
content: CDM mania !Well I'm pleased to have a couple of dozens of them already, it seems all the others around will end at your place.The pickups were actually used in quite a few other brands, outside of the Philips group Technics are the easier ones to come by.Just wait till you get started on the CDM0 (/1) models then you're in real trouble.Oh, may I point you towards the V2000 VCR's too...?Fantastic obsolete technology!Speaking of VCR's: Sooner or later you'll run into a broken nylon drawer pulley on a Philips CD deck. A substitute can be knocked together from parts of a Philips Turbo Drive VCR.Now you know you have at least a dead LED in your TV, unless the driver has been killing them (which I doubt) I bet there's nothing wrong with the LED supply in the end and it's just the open string fault detection you've been witnessing.Getting into the screen isn't that hard, you need a little patience, calm and a large clean surface to work on.The glass itself isn't that fragile as it's reinforced with the plastic polarizers on both sides you can even allow it to bend more than you'd expect.The awkward bits are the COF's (line/column decoders) which are very fragile, and not contaminating the inside of the screen with fingerprints or dust.LEDs are widely stacked as a series string for simplicity.Variations in manufacturing and Murphy can lead you to having a noticeable difference in string Vf, once strings are paralleled and driven close to the limit the string with lowest Vf is going to be driven harder and while overall current can be controlled individual string current isn't. The low Vf string is likely to have reduced lifetime or meet thermal runaway problems.The way to drive a LED is constant current, in order to avoid multiple current sources it's simpler to have one long string in series even if that means high voltages.Anyway if you do get in there to discover only one LED is open, depending on the backlight spreading and position of dead LED the simplest fix may be to just jumper it in place.I've done it before without noticeable difference on the picture. (Don't forget... It's going to be used to watch crap anyway...)

author: Vince
author_rank: Super Contributor
timestamp: March 18, 2021, 03:59:56 pm
content: VCRs ?! No no no, not my cup of tea ! When my dad bought one when I was a kid, I was thrilled to see such of high-tech piece of machinery at home, wow..... then after a few days playing with it, I got bored with the thing. I must have recorded one movie (" Les dieux sont tombés sur la tête " .. I remember it vividly) . Then I recorded ONE episode of my favorite car program, was the launch of the Lamborghini DIablo in 1990... 2 days later my mom erased it by mistake when she carelessly overwrote it with something else... and that's about it ! Lost interest real quick.I have zero VHS tape to put in a VCR, and not even e TV to hook it to !At least CD players I can play my old disks on them, I can actually use them...Anyway, thanks for your input. Looks like you managed to convince me to have a go at getting tot he LED strips ! I will have to ask the owner of the TV first, just in case I do end damaging the glass or LCD layer... hopefully he won't care at all since the TV is basically useless to him, has nothing to lose.. .either I manage to fix it, or it gets scrapped. But it's his TV not mine, so I must ask...So the strips are in series then, not parallel ! Sure explain the 160V then, and the fact I have only 2 leads coming out of the LED panel...The driver IC is actually capable of driving 4 LED strings, and balance the current between them. The power board is even equipped to do that, right up to the LED connector. But since there is only one string on that particular panel, they unpopulated the 3 other inputs, and instead added a few zero jumper resistors so the current of the single string, is spread into the 4 inputs of the driver IC.So if I understand you well, your lay the TV screen, face down on the bench, and you lift the metallic case hosting the LEDs, upwards ? Not the other way around ? Will try to find some videos on youtube showing how people do it exactly... need to get as much info and illustration as possible, since I will have only one go at this !

author: Vince
author_rank: Super Contributor
timestamp: March 18, 2021, 06:10:07 pm
content: Yeahhhhhh..... just got the green flag from the owner : permission to give the LED panel a go, doesn't mind if I ruin the screen as nothing to lose ! White card !So will have a crack at it this week-end for sure, I am all excited....

author: quadtech
author_rank: Regular Contributor
timestamp: March 19, 2021, 07:15:17 am
content: It's not too hard to remove the panel and check the LEDs. A backlight tester like this is very useful.It will be good to use Suction cups to remove the LCD panel -

author: Vince
author_rank: Super Contributor
timestamp: March 19, 2021, 05:44:59 pm
content: I am puzzled... the second video, other than being an advertisement to flog their suction cups, makes it look like the screen is NOT bonded to the case but rather simply laying on it, and just needs to be lifted ?! No need to pry the screen off of the case ?! Sounds too easy... I am dubious... will look for some more videos before I attempt anything on "my" TV...

author: Vince
author_rank: Super Contributor
timestamp: March 19, 2021, 06:03:04 pm
content: Just found this video :Guy doesn't talk but the picture is enough to understand so that's fine.He shows the process from start to finish, in one take, no cut / editing, and he does it like I thought I would be best doing it : laying the screen face down, onto a sheet. This way it's fully supported all over its surface, no stress on it.I am baffled but... it looks indeed like the screen is NOT bonded to the edges of the metal case ?! It's simply held in place by the plastic bezel that snaps onto the case ! Eh ?! Screen comes out in no time, with zero fuss !What was I worrying about then ! It's a million times easier and less risky than I thought it would be !LED strips here I come, watch out !

author: Vince
author_rank: Super Contributor
timestamp: March 19, 2021, 07:25:25 pm
content: DONE !!!Took only a few minutes to do, I am stunned !The LCD screen actually fell off bvy itself BEFORE I was even ready/ expected it ! I had the screen standing up, on its longer side, as I was unclipping the inner bezel... as soon as I had finished unclipping it, the LCD panel parted with the metal case !Luckily it looks like it didn't get damaged in the process. I hurried and went to wash my hands, and grabbed the screen carefully by its edges, using paper towels. This way I could handle it without touching it. Then tried to find a place to lay it flat and clean... not enough space for that... best I could find was the kitchen table that's covered with freshly washed clothes... will have to do for nowIt's not fully supported as you can see, but it's not bending all that much.. as you said ShakalNokturn, it's quite flexible...Then I removed the polarizer / diffuser.... a stck of two pieces. 30 seconds later I had already forgotten which around its supposed to go ! What side of the stack is facing outward or inward.... will worry about that later...Then had access to the inside of the metal case. There is white light reflector.. made simply from glossy paper ?! Glued to the case, everywhere.. scared the shit out of me as it could get ripped of ay any moment ! Somehow it came of undamaged.. mostly, but there are kinks to it here and there, sorry.... will try to do better next time... if there is ever a next time that is...Once that's removed... I can now see the infamous LED strips !! Not in a youtube video... but rather in real !Now we're talking !!!So let's check all these diodes now...As you can see, the LEDS are arranged as 7 vertical strips of 9 LEDs each. The strips look like they are arrange all in series, though not sure because that's 63 LEDs at 3.3V each, that makes for a 2087 Volte drop ! So 160V is nowhere near enough to light them up ! So maybe 160V is the driver IC holdingback to protect the things... I saw in that video they said 180V start up,; and 160V when crusing, and 115V when an open-circuit condition / fault is detected.... maybe that was for a 40" TV can't remember now... mine is 50 each so more LED's, so 160V could be a plausible voltage when an open condition is detected, and the we need a much high votlage to turn them on.... which would explain why I briefly say like 220 or 240V at power up, at times ! That would be normal then...OK so looks like Shakal might be right, as always... looks like the most likely scenario now is an open circuit in the LED strips, and the power board backing to 160V as a safety precaution.Let's inspect these LEDs closer now...

author: Vince
author_rank: Super Contributor
timestamp: March 19, 2021, 09:00:00 pm
content: Alright, I am done testing all the strips and individual LEDs..... result is NOT GOOD !See annotated picture below (inverted, so top of the pic is actually bottom of screen, and left is right, hmm...).I tested the LEDs using my lab power supply, current limited to minimum (about 40mA from memory, can't go lower than that). Added my beefy 3.600ohms resistor in series, which means that even with the supply cranked all the way to 30Volts, I would get at best, barely 10mA, not even that. Somehow it was plenty enough to light the LEDs.Results in detail :- Strip #1 : lights up, one shorted LED, first from the top.- Strip #2 : lights up, two shorted LEDs, #2 and #4 from the top.- Strip #3 : does NOT light up, one OPEN LED, first from the bottom.- Strip #4 : GOOD- Strip #5 : GOOD- Strip #6 : GOOD- Strip #7 : lights up, one shorted LED, #3 from the top.So, I have one open LED, which explains why it does not work, of course...But I also have FOUR shorted ones !Is it worth repairing ? ........knowing I would get only 50 Euros for all the parts and many hours spent on it ? Don't know...My thoughts right now :1) I find it highly unlikely that 4 shorted diodes and an open one, happened all at the same time ! Don't know what you think, but I find more likely that first, we got one shorted diode... then later, another one, then later again, a third one, then a fourth one.. and somehow the owner did not think too much of it since he kept using the TV anyway ! So maybe I could just leave these shorted diode alone ?....Then, one day, one of the diodes went open circuit and all of a sudden no picture at all ! This time the owner had no choice but to notice it !2) First, I could short the open diode and reconnect the power board to the LED strips to see if the backlight works now...3) If it does and the power board is not at fault, then I could put the diffuser back in place, to see how (un)even the lighting really is. If it's bearable of not. Previous owner apparently thought it must have been ?!4) ... if it's really bad, then study how economically viable a repair could be achieved ? I gather that you can buy individual strips. But how cheap can one get the, including shipping, and how to be sure that the colour temperature will match the existing strips sufficiently well.... hmmm... or maybe it's an industry standard and they are all interchangeable ? Or it just doesn't matter a rats ass and the owner won't even be able to tell the difference in temperature from one LED to the next ? I saw a video where the guy cuts a strip to retrieve individual LEDs to replace the bad ones. Looks easy enough to do, I could attempt this I think.5) I still have the problem that the power board cuts the backlight after a couple minutes...so that will need fixing as well...https://www.youtube.com/watch?app=desktop&v=RxwdC7V5Ue0

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 19, 2021, 09:38:26 pm
content: I'd start by shorting the open LED and testing the whole TV as is just in case you damaged the LCD when it slipped.If backlight is working try to scope the startup current on the LEDs, see if there is overshoot that could be serial-killing them.If you choose to stick with dead LEDs (your theory of failure sounds reasonable) I'd at least swap the positions of two strips to avoid having too many dead LEDs in the same area.You can find used spare strips for about 10€ but then you have to go to the trouble of swapping individual LEDs. A LED a little mismatched is still going to be better than no LED.If buying individual LEDs check the footprint first (including thermal path) and go for cold white.

author: Vince
author_rank: Super Contributor
timestamp: March 19, 2021, 11:57:04 pm
content: Thanks for all the good info as always, everything noted...Repair is over, LCD is kaput ! Nothing to do with the way I handled it though, so not feeling guilty : look at the bottom right corner, the star shaped pattern of the damage.. this screen clearly suffered a severe head on impact ! Somebody hit it hard with whatever... not my fault sorry bob !I don't know, maybe the owner hit it by accident once it got decommissioned and maybe lying on the ground... where it was vulnerable. Or my friend damaged it when he put it in his car to bring it to me... I don't know.I flexed the LCD on purpose while the TV was displaying a picture, and it didn't care at all, was just fine. So clearly they are not made of sugar if you handle them with reasonable care. A little flexing is okay... but here it's clearly a big, severe head on impact, not flexing, nor even a fall on the edge.So I won't make any money from this one ! Hopefully I will get to keep the electronics as a compensation for the time I spent on it. Will keep the LED strips as well, might be handy to fix another backlight on some other TV one day.Somehow the 6 dead LEDs did not make for a noticeably uneven backlight, I didn't see anything wrong with the picture on the screen. Weird. 6 LEDs kaput, 3 in the same area to boost... and still watchable. Good to know...Swapping strips to spread the dead LEDs as you said, would make it even less of a problem.Anyway, learned a lot from this first TV repair, nothing beats hands-on experience...So quite happy. Next time I am presented with an LCD TV repair I will not refuse it, I would give it a go for sure.No money for me but good valuable experience, and some scrap boards and spare LEDs. Not so bad. Didn't spent that much time on it overall, so no big deal...

author: tautech
author_rank: Super Contributor
timestamp: March 20, 2021, 12:16:22 am
content: Thanks for sharing Vince.

author: Vince
author_rank: Super Contributor
timestamp: March 20, 2021, 05:39:31 pm
content: You're most Welcome M. Tautech, glad it was not all a total waste of bandwidth !Good or bad, any experience is useful and builds knowledge and skills...