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No Backlight on LED TV

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downunder:
They only appear to work perfectly, but if one strip is out, you're not going to pick it in the brief time you have, and the diffusion sheets behind the screen tend to equalize the brightness.
As I explained in my last post, when the TV is activated, whatever LED strips are operational will ignite, but those that don't will be detected as a current (I) imbalance and create an error signal to kill the backlights. This wont be instantaneous, hence the brief flash.

What can cause this? An open circuit LED, a shorted LED, faulty joiners (midway) in a LED strip, in fact any defect in the backlighting system. The protect circuit in the common earth return for the LEDs has the capacity to detect it.

AS for measuring the resistance of the strips, you'll get no measurement...that's normal. You can measure individual LEDs like a diode, and your meter voltage may even light the LED, if you can access the terminals.

Bruce

Southerner1959:
Hi Bruce

Sorry, your previous reply was sent while I was writing mine, so hadn't seen that!

Thank you for explaining . . . I can see that makes sense now. The Power board is sending a quick pulse to the LEDs (I can see that on my meter) but turns them off straight away, so guess it is sensing something is wrong. (must be pretty smart if it can detect one LED is out)

I did try by-passing the FET that controls the feed to ONE of the two strips . . . . I fed it from the 75V supply, but through a 35 ohm resistor (for safety) . . .  but the lights didn't come on.

Guess the resistor was too high - the LEDs didn't take any current (ie there was still 75V on the output side of the resistor. Could I get away with feeding it direct from the supply, or does the dimming control circuit also provide current limiting?

EDIT: Just watched the Video you posted the Link to . . . the boards in that set look exactly the same as mine.  So that also confirms this must be the problem.

Sorry to everyone that replied on here . . . I just couldn't see why the picture was fine momentarily if some LEDs had failed . . . but guess the current-sensing must be pretty sensitive!

Looks one hell of a job to get it apart though!  (wonder how many that guy broke before he got the technique sussed?!)

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