Hi guys,
I have this set with a backlight issue that i need to resolve rather urgently. The problem is a s follows, Upon pressing standby the tv's picture comes up and backlight is present. I measured about 55V (Picture bright and correct) on each of the LED+ supply with respect to chassis ground. The return voltages i.e the LED- varies from 2.2V to about 1.6V (is that normal?) After running for about 20 seconds the back lights switch off and the LCD is still working (Flashlight test)
I opened up the panel and observed all the LED's they all lit up properly ran for a short while and then switched off all together. I did not notice perhaps a strip going off first nothing. My question is, is the protection circuit so fast that you can't see a bad connection or LED before the LED supply circuit trips?
Anyway, i checked the LED driver IC and got its datasheet and saw that this chip has plenty of protection. Things like LED over volt, LED under volt, Thermal and Open LED protection.
I measured around the IC by soldering a thin wire to a circuit and then starting up the TV and then suddenly the LED+ voltage went to 38V. The LED's are now barely visible but are glowing slightly. I scoped the driver voltage waveform and it seems that the LED driver chip is driving the LED circuit at minimum duty cycle. I dont know what i did wrong. I am 100% sure that i made no short circuit. It feels like the LED driver circuit was faulty and now failed completely. I do have a circuit diagram for this board...
I still have to look at the drive voltage for DIM or ADJ from main board but dont know what to expect there.
As a test i tried to measure on each branch of LED strip what the current draw is and coupled my ammeter in series with one of the branches but the power supply did not like that and shut down the back light voltage immediately. Don't know why it responded like that?
Any help as to where i can start or measure?
Regards
Angelos
I have this set with a backlight issue that i need to resolve rather urgently. The problem is a s follows, Upon pressing standby the tv's picture comes up and backlight is present. I measured about 55V (Picture bright and correct) on each of the LED+ supply with respect to chassis ground. The return voltages i.e the LED- varies from 2.2V to about 1.6V (is that normal?) After running for about 20 seconds the back lights switch off and the LCD is still working (Flashlight test)
I opened up the panel and observed all the LED's they all lit up properly ran for a short while and then switched off all together. I did not notice perhaps a strip going off first nothing. My question is, is the protection circuit so fast that you can't see a bad connection or LED before the LED supply circuit trips?
Anyway, i checked the LED driver IC and got its datasheet and saw that this chip has plenty of protection. Things like LED over volt, LED under volt, Thermal and Open LED protection.
I measured around the IC by soldering a thin wire to a circuit and then starting up the TV and then suddenly the LED+ voltage went to 38V. The LED's are now barely visible but are glowing slightly. I scoped the driver voltage waveform and it seems that the LED driver chip is driving the LED circuit at minimum duty cycle. I dont know what i did wrong. I am 100% sure that i made no short circuit. It feels like the LED driver circuit was faulty and now failed completely. I do have a circuit diagram for this board...
I still have to look at the drive voltage for DIM or ADJ from main board but dont know what to expect there.
As a test i tried to measure on each branch of LED strip what the current draw is and coupled my ammeter in series with one of the branches but the power supply did not like that and shut down the back light voltage immediately. Don't know why it responded like that?
Any help as to where i can start or measure?
Regards
Angelos
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