Mulja tv repair portal
Equipment Repairs => LCD TV forum => Topic started by: Barb777 on June 09, 2019,12:47:58
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Hi, hopefully someone has some idea. I’ve got a panasonic tx-48cx490b.
When I first switch it on it’s great but after around 20-30 minutes the screen goes dark but the sound continues. Shining a light on the screen confirms the picture is still there but the backlight is off. Switching off and on will restart the backlight but it fails after only a few minutes unless it is left to cool down. With a white background there is one dark spot so I know I have one led out.
Anyone got any ideas?
Thanks
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Fairly sure you have backlight LED problem. Even one LED out will cause it to switch backlights off. Perhaps you have more than one LED failing. LED current is monitored and if it falls outside specific parameters, total LED shutdown will occur.
Your first step will be to replace the failed LED you noticed (or the strip its on), then check for other dodgy LEDs. Means you'll have to disassemble the display panel. Bruce
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Thank you. Backlight now out and one led diode out. If I test the strip with 18v and leave it running, will it switch itself off if there’s an open circuit and is there any way to identify a failing led?
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A fixed external supply will run a strip forever, because it is not monitored and have overcurrent protection that are incorporated in the TV's design. It may be that that one faulty LED is the cause of your problem. LEDs can go short as well as open. In your external supply, allow 3V per LED in the strip, eg a strip of 10 LEDs needs 30V supply. It could be that the TV uses 6V (5.6V) LEDs which changes the whole equation.
Do you have a LED tester? Each strip should give the same reading (volts). If one strip is out of kilter with the others, it is suspect.
These testers are not conclusive because they're designed to limit the current in the strip to around 30mA, unlike in a normal operational TV where they use around 300mA, so they are driven much harder and a defective LED is more likely to crack.
My education into the foibles of LED strips seems to be never ending. What happens if you turn the tv on without the display and diffuser sheets. Provided everything else is hooked up, you can operate the LEDs without the display. LEDs will still operate and you may spot another issue with the LEDs. If there are joiners at the centre of each strip, these sometimes have connectivity problems and may have to be bridged out by soldering jumpers across the loins.
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Thanks so much for your reply, that would have been my next question, will the backlight run without the screen. As far as I am aware I only have one led out, I’ve ordered a pack of diodes and I will change this and then try running it before I refit the screen. No, no led tester. I’ve got a multimeter and I’ve tested the leds by wiring two 9v batteries together.
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Yep the backlight will run, but most tvs turn off after a prescribed period if no signal is applied. Let us know how you get on. Bruce
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So far so good, backlights are all lit!
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Just wanted to say thank you so much for your help. It obviously was the backlight as it is now back together and running without going dark!