Sceptre X505BV - Logo appears for 1 sec then backlight shuts off, LEDs tested good

[events.b]

Hello, this TV currently has me stumped. As the title states, when I power on the unit, the backlight comes on for 1 second then shuts off. The TV is still on, and I can see the video if I shine a flashlight on the screen.

Using the LED backlight tester I was able to confirm all LEDs are illuminating and seem to be working correctly when independently powered by this tester, so that leads me to believe there is something wrong with the power supply board.

Perhaps the great minds here can help me figure it out, as I don't see any thing obviously wrong with it. The backlight circuit seems to be controlled by an op-amp (U14). The part number for this op-amp is 10358. The hot side of the board is controlled by a FAN7930C PFC controller. I have not found any shorts on either side so far and the capacitors look to be ok.

The backlight circuit is located on the top left of the board (top view) or top right of the board (bottom view).

Thank you for your time.

[neilc6]

Re: Sceptre X505BV - Logo appears for 1 sec then backlight shuts off, LEDs tested goo

Your LED tester only puts out about 20mA while your TV runs at a couple hundred mA. You have intermittent LEDs in the string. Their voltages will be higher or lower or they may be slow to turn on. You can also record the flash and go frame by frame to see whats not lighting up.

[events.b]

Re: Sceptre X505BV - Logo appears for 1 sec then backlight shuts off, LEDs tested goo

So, you think the tester is giving me a false negative? That's disappointing to hear that it is a flawed test. I will try your suggestion and record a video to see if that gives any clues, thanks for the tip.

[budm]

Re: Sceptre X505BV - Logo appears for 1 sec then backlight shuts off, LEDs tested goo

"Using the LED backlight tester I was able to confirm all LEDs are illuminating" Did you take apart the panel to see that all LED lights up?
What Voltage does the LED tester show on the reading for each LED string?

[events.b]

Re: Sceptre X505BV - Logo appears for 1 sec then backlight shuts off, LEDs tested goo

Yes, I did take the panel apart because I too initially suspected faulty LEDs. The tester was showing about 79 volts I think, on each array. The TV has two separate LED arrays, one on each side. I illuminated each array separately with the tester and they both showed the same voltage. This is what caused me to step back and second guess myself, because I would expect one array to have a different operating voltage if it had bad LEDs. Each and every LED did in fact come on, although not at full brightness like neilc6 mentioned due to the current limit of the tester.

[budm]

Re: Sceptre X505BV - Logo appears for 1 sec then backlight shuts off, LEDs tested goo

I see.
So is the PFC boosted Voltage on the main cap staying at 380 ~ 400V after the LED shutdown?
And how about the BL-ON and PWM dim or ADIM control signals? Are they still present when the LED shutdown?
Bad LED can fail when driven at high current.
When test LED with tester at small drive current the bad LED may cause the Voltage reading on the LED tester to not be stable, the Voltage may vary up and down.

[events.b]

Re: Sceptre X505BV - Logo appears for 1 sec then backlight shuts off, LEDs tested goo

Hello, after the LED shutdown, the voltages are as follows:

The 400V caps near the LED backlight connector are at 0.4V
The signal labeled "BK" is 4.5V
The signal labeled "ADJ" is 4.1V

[events.b]

Re: Sceptre X505BV - Logo appears for 1 sec then backlight shuts off, LEDs tested goo

Update: One of the optoisolators for the backlight current feedback I found to be open circuit. I replaced the optoisolator but still no change. I suspect the op-amp that drives the optoisolator is also bad.

[events.b]

Re: Sceptre X505BV - Logo appears for 1 sec then backlight shuts off, LEDs tested goo

Update: The op-amp (U14) was also bad. After replacing this op-amp and the optoisolator, the TV is working again.