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Backlight problem on lcd tv

phlox

May 19, 2012
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Hey guys

I got a small problem with an lcd tv, the thing worked fine, then stopped working and when it gets switched on the light flashes, so i open it up and found a blown transistor or what ever its called, its got 4 leg en small black square body just like a transistor, well anyway i got that fixed, but the backlights wont go on still, there is image on the display as i have connected the ccfl tubes from the tv to another inverter from ccfl pc lights and that works, but the tv inverter self isnt working,i get volts up to the input of the inverter transformer but no volts out to the ccfl tubes, is the inverter transformer probably blown adn how would i test it?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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The inverter board is probably an item you can purchase.

Since it seems you've isolated it as the fault, and because those inverter transformers may be quite hard to source, I'd be investigating a board replacement.
 

phlox

May 19, 2012
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No backlight on lcd tv

Unfortunately the inverter circuit and power supply are on board, dont know where ill get a replacement board , i was thinking of using a lcd computer monitors inverter transformers if that might work?
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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Have you tried lcdparts.net? I got a transformer from them that fixed my inverter board. They also sell replacement boards.
 

phlox

May 19, 2012
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lcd tv backlight problem

hey guys, so far no luck, got a inverter transformer from another lcd screen and same problem, now ive got a question, might it be the inverter driver mosfet?
The reason for asking is on the benq lcd pc screen ita has got 4 inverter tramsformers as well as 4 driver mosfets basically right behind the transformers, and well 2 of the backlights didnt go on but 2 did, found out it was the drivers mosfets on those 2, replaced them and all 4 backlight ccfl tubes lit up again....well except it still goes off after about 3 seconds,but thats another prject im working on,any help on that would be greatly appreciated.

So for the lcd tv which is the actual problem,how would i know if its the driver mosfet and where is the driver mosfet? I've attached 2 images, of what i think is the driver mosfet, i dont know.

But on the mosfet it states : " FML G22 " with the little diode symbol and 07 09.

Ive also read about a "pico fuse" but the board doesnt seem to have one on.

thanks so far for the help
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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That device on a heatsink is a diode.

Have you checked all the DC rails from this power supply.

Beware that everything inside those thick lines can kill you while the device is switched on (or even plugged in) and even if it's not, it may be able to give you one hell of a boot.

Can we see the other side of this board?
 

phlox

May 19, 2012
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lcd tv backlight

how do you mean th DC Rails?...ill see if i can post a pic bit later
 

phlox

May 19, 2012
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i did check the volts going to the transformers, and i get about 24V dc going to the primary input of the transformers, but no volts going out
 

markm6164

Jan 22, 2012
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Is there a driver ic on the other side of the board for the ccfl tube? This will drive the mosfets to power the inverters. We need a clear of the other side of the board.
 

GreenGiant

Feb 9, 2012
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from past experience try finding the same model TV on craigslist/ebay that someone cracked the screen on, then pull the board from that and put it in yours, you have to make sure that they are compatible though

A friend of mine scored a 50" LG, 1080P, 240Hz, 3D ready TV for $220 total that way
 

phlox

May 19, 2012
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lcd tv backlight problem

attached is two pics of the bottom side of the power supply board, i made a circle around the component which i now suspect is the driver mosfet?.....the board had 2 and one was blown and caused the screen to power on and power off again in sequence, removed it and tv came on and got picture but no backlight
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Have you tried replacing it?

It seems unlikely they placed two of these on the board if it would operate with one.

What was the part number of the one you removed?

Looks more like a regulator than a mosfet, but without more information it's hard to be sure.
 

phlox

May 19, 2012
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lcd tv backllight

both were exactly the same, uhm ill check the part number bit later, dont have it with me, but it did have two exactly the same ones on, and it looks to me like the power for the inverters come from them , or there abouts, i might be wrong, i followed the pcd tracks.
 

phlox

May 19, 2012
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pcb not pcd sorry, oh and i dnt have a spare so didnt try replacing it yet
 

phlox

May 19, 2012
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okay the "mosfets'' or regulators are the following stu 404d(7bd68) and the IC as far as i can make out, the writing is bit faded : u2003,xk 7n1
 

phlox

May 19, 2012
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i just checked the ic and from some of the pins that goes to the output of the inverter transformer(followed the schematic diagram in link above) theres either no volts or only about 2.7v........so most likely the IC?
 

markm6164

Jan 22, 2012
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You need the check your voltages on the power rails as stated before and also your voltages at the ic first. The driver ic will send a pwm signal to the gate of the mosfets which is best measuring with a scope. The driver ic will have over voltage protection circuits and feed back circuit to regulate the power the the ccfl tube and will shut down if it detects over voltage usually it lights the ccfl for a few seconds then it shuts down. This might be what's happening. I haven't looked at the service manual yet or googled the driver ic because I'm on my phone.
 
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