Badcaps.net Forum
Go Back   Badcaps Forums > Troubleshooting Hardware & Devices and Electronics Theory > Troubleshooting TVs and Video Sources
Register FAQ Calendar Search Today's Posts Mark Forums Read

 
Thread Tools Display Modes
Old 02-08-2021, 11:03 PM   #1
events.b
Member
 
Join Date: Oct 2010
City & State: Florida
My Country: USA
Line Voltage: 120VAC 60Hz
I'm a: Hobbyist Tech
Posts: 23
Question Sceptre X505BV - Logo appears for 1 sec then backlight shuts off, LEDs tested good

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.
Attached Images
File Type: jpg PXL_20210209_042922988.jpg (986.3 KB, 16 views)
File Type: jpg PXL_20210209_042940827.jpg (1.07 MB, 11 views)
File Type: jpg PXL_20210209_042955332.jpg (992.5 KB, 7 views)
events.b is offline   Reply With Quote
Old 02-09-2021, 12:37 AM   #2
neilc6
Badcaps Veteran
 
Join Date: Nov 2015
City & State: BC
My Country: Canada
I'm a: Knowledge Seeker
Posts: 1,126
Default 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.
neilc6 is online now   Reply With Quote
Old 02-09-2021, 10:03 AM   #3
events.b
Member
 
Join Date: Oct 2010
City & State: Florida
My Country: USA
Line Voltage: 120VAC 60Hz
I'm a: Hobbyist Tech
Posts: 23
Default 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.
events.b is offline   Reply With Quote
Old 02-09-2021, 12:26 PM   #4
budm
Badcaps Veteran
 
budm's Avatar
 
Join Date: Feb 2010
City & State: S.F. Bay area
My Country: USA
Line Voltage: 120V 60Hz
I'm a: Knowledge Seeker
Posts: 39,203
Default 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?
__________________
Never stop learning
Basic LCD TV and Monitor troubleshooting guides.
http://www.badcaps.net/forum/showthr...956#post305956

Voltage Regulator (LDO) testing:
http://www.badcaps.net/forum/showthr...999#post300999

Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl

Tear down pictures : Hit the ">" Show Albums and stories" on the left side
http://s807.photobucket.com/user/budm/library/

TV Factory reset codes listing:
http://www.badcaps.net/forum/showthread.php?t=24809
budm is offline   Reply With Quote
Old 02-09-2021, 12:33 PM   #5
events.b
Member
 
Join Date: Oct 2010
City & State: Florida
My Country: USA
Line Voltage: 120VAC 60Hz
I'm a: Hobbyist Tech
Posts: 23
Default 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.
events.b is offline   Reply With Quote
Old 02-09-2021, 12:56 PM   #6
budm
Badcaps Veteran
 
budm's Avatar
 
Join Date: Feb 2010
City & State: S.F. Bay area
My Country: USA
Line Voltage: 120V 60Hz
I'm a: Knowledge Seeker
Posts: 39,203
Default 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.

Last edited by budm; 02-09-2021 at 01:01 PM..
budm is offline   Reply With Quote
Old 02-10-2021, 11:28 PM   #7
events.b
Member
 
Join Date: Oct 2010
City & State: Florida
My Country: USA
Line Voltage: 120VAC 60Hz
I'm a: Hobbyist Tech
Posts: 23
Default 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 is offline   Reply With Quote
Old 03-23-2021, 11:47 AM   #8
events.b
Member
 
Join Date: Oct 2010
City & State: Florida
My Country: USA
Line Voltage: 120VAC 60Hz
I'm a: Hobbyist Tech
Posts: 23
Default 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 is offline   Reply With Quote
Old 06-07-2021, 12:20 AM   #9
events.b
Member
 
Join Date: Oct 2010
City & State: Florida
My Country: USA
Line Voltage: 120VAC 60Hz
I'm a: Hobbyist Tech
Posts: 23
Default 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.
events.b is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump



Badcaps.net Technical Forums 2003 - 2021
Powered by vBulletin ®
Copyright ©2000 - 2021, Jelsoft Enterprises Ltd.
All times are GMT -6. The time now is 01:31 PM.
Did you find this forum helpful?