How do I spot a blown LED?

[Dang]

I have the notorious Toshiba/Vestel LED Tv with what I suspect is a blown LED, two actually, compared to the rest these two have a crack appearing across the middle, one worse than the other, I presume this means they are blown? there's no blackening that I can see, thanks.

[ReeceyBurger123]

Re: How do I spot a blown LED?

Hi mate do you have a pair of pliers on hand ?

[ReeceyBurger123]

Re: How do I spot a blown LED?

Strange question but you can use them to test the strips, and what size is the tv 40" ?

[Dang]

Re: How do I spot a blown LED?

Pliers I do, how would I use them to test this? and yes, it's the 40".

[Hamie]

Re: How do I spot a blown LED?

Do you mean the plastic cover is cracked?

The plastic is irrelevant and only there to disperse the light.
You can snap the plastic off and they LED underneath will still work.

[ecking767]

Re: How do I spot a blown LED?

you will need to remove the clear white dome, and a visual inspection can be done. then you can use your multimeter set in diode mode to test each led.

When you touch the probes to the led its should light up a little bit.

[Dang]

Re: How do I spot a blown LED?

OK, thank you, so do you have any ideas on the possible issue here? I have no back light but the power supply is outputting the correct voltages, no LEDs show any blackening or tarnishing.

[popeye]

Re: How do I spot a blown LED?

Light Emiting DIODE! how to test?

[Hamie]

Re: How do I spot a blown LED?

Link up 3 x 9 volt batteries in series to the + and - marked at the start of the led strips.

Once you've found out what strip/s are not lighting up, before you crack off the plastic domes check the leads for corrosion bad contacts etc.

If they all look fine you'll need to remove the plastic domes from each LED and use the diode test.

[ReeceyBurger123]

Re: How do I spot a blown LED?

No ! Turn the tv on go to the top strips of led find the negative terminal on the strip use the pliers to bridge that NEGATIVE terminal to the chassis that will fire on the booster for the top row and they should light if not then the top row is out. Next if the top row lights turn the tv off remove the top row of strips then the next row of leds and put them in the top position and repeat the same method if they turn on then they are good if they don't then they are bad. This saves doing all the other time wasting nonsense.

[Dang]

Re: How do I spot a blown LED?

Excellent and fair enough, I'll get back on here later with, hopefully, good news.

[ReeceyBurger123]

Re: How do I spot a blown LED?

Dang my method saves aload of time I will make a youtube video eventually I have repaired 8 of these tvs alone using that method just because the optic is cracked does not mean that the bulb underneath has failed.

[Dang]

Re: How do I spot a blown LED?

Dang my method saves aload of time I will make a youtube video eventually I have repaired 8 of these tvs alone using that method just because the optic is cracked does not mean that the bulb underneath has failed.

OK, seems this back light is in series, will your method still work?

[ReeceyBurger123]

Re: How do I spot a blown LED?

Yes just simply put the one end of the pliers on the negative strip of the bulbs and then to the chassis you must do this on the top strips as that's where the voltage flows to first. Let me know how it goes.

[budm]

Re: How do I spot a blown LED?

You have to be real careful doing that if the circuit was not well designed and does not detect the Vf total change of theLED string to keep the current constant, you can damage the good LED sting, and if it does not use one BIG long string of LED but individual string with its own driver for each string you can damage the booster circuit if it does not shut down fast enough.
When you ground that negative terminal which is connected to the Drain pin of the PWM Controlled MOSFET (current sink), the Source pin is connected to the LED current sensing resistor, the other end of the resistor is then connected to the LED drive Voltage return ground, you will bypass all those parts when you directly connect the Negative pin of the LED string to GND.
You have to be real careful as how the circuits are implemented. See PDF for example.

[ReeceyBurger123]

Re: How do I spot a blown LED?

Thats true Budm but on these vestels it will not affect the bulbs if you bridge the negative terminal, all it will do it let the current from the psu flow through the strips and not from the booster as it wont be using enough current to activate it and it will only light the one strip as it requires all the strips to connected to start it where as if ones short non will light. Where as if you bridge the positive terminal to ground that will draw enough current to trigger the booster and will short the strips out. As you say it depends on how they are connected in these sets the top led strip is the first one to receive voltage then it goes down to the rest as they are wired in series so it the top strip goes out the circuit wont be complete and non will light thats why when you bridge the negative to chassis it completes the circuit for the top row and they will light.

[budm]

Re: How do I spot a blown LED?

But the ANODE of the first string is connected to the Boosted output circuit isn't it (same as the example of the SAMSUNG diagrams), and the negative of the last string is sinked by the MOSFET? I did lot look at the board lay out yet.
All I am saying is to find out how the circuits are implemented first.

[ReeceyBurger123]

Re: How do I spot a blown LED?

Thats not the case with these and no the boost converter will only fire if all the strips light if not it will bypass and just send the 24v output from the psu wiring leds like this is a complete and utter design error and meant to fail like christmas lights one out all out. Im guessing that this is a 40" toshiba with a Samsung panel lta400hm23 most common one to fail but all these vestel led tvs fail in the same way.

[ReeceyBurger123]

Re: How do I spot a blown LED?

Trust me it doesn't not turn the booster on when you do that on others like good lgs and samsungs it may but not on these.

[tom66]

Re: How do I spot a blown LED?

I am sure the boost will turn on, but since it is current-driven boost it will likely hit over-voltage threshold then shut off.