Toshiba 50L1350UC No Picture

[Crawl71]

Good evening Forum People. I just got finished fixing my plasma and now my led has died. Done some searching and some testing and I rushed out and replaced the t-con board thinking that was the problem. Mistake. I have sound but no picture. Right now it's too light out to do the flashlight test so I will post that later. Here's the test results.

No back light; voltages with TV on are:
L2- 15.3VDC
L2+ 76.9VDC
L1- .3VDC
L1+ 76.9VDC

Cable unplugged
L2+ 130v down to 76v

PWM: .2V
5.1V: 5.1V
BL_On/Off: 3.2V
Power_TV: 3.2V
AC Detect: 3.5V
5.1V: 5.1V
GND: 0V
12V_LNB: 0V
GND: 0V
12V_AUD: 12.4V
GND: 0V
GND: 0V
12Vcc: 12.4V
12Vcc: 12.4V
12Vcc: 12.4V

Pwr board: PK101W00501

As always any help or suggestions are appreciated.

[attainteddragon]

Re: Toshiba 50L1350UC No Picture

throw some pics up for reference, but it looks like led failure to me.

does the backlights come on for a moment before it shuts down?

[Crawl71]

Re: Toshiba 50L1350UC No Picture

Couple of photos attached.

Backlights flash on for a second and then nothing.

[budm]

Re: Toshiba 50L1350UC No Picture

You have open circuit in the LED L1 string, you can verify that swapping the set of L2 LED string wires with L1 LED set of wires to see if reading will follow.

[Hobby 1]

Re: Toshiba 50L1350UC No Picture

I recently picked up this same model with the same problem and almost the same voltage measurements. I have lots of experience with the older CCFL LCD tv's and have fixed dozens of them including replacing lamps but this is my first LED backlight one so I need some advice.
The backlight flashes for a second and then goes out.

No back light; voltages with TV on are:
L2- 12.41VDC, using chassis as gnd.
L2+ 76.8VDC, using chassis as gnd.
L1- 11.09VDC, using chassis as gnd.
L1+ 76.8VDC, using chassis as gnd.

PWM: 3.2 and drops to .2V
5.1V: 5.16V
BL_On/Off: 3.24V
Power_TV: 3.31V
AC Detect: 3.53V
5.1V: 5.16V
GND: 0V
12V_LNB: 0V
GND: 0V
12V_AUD: 12.57V
GND: 0V
GND: 0V
12Vcc: 12.56V
12Vcc: 12.56V
12Vcc: 12.56V

L1+ goes to around 92v at startup and L2+ goes to 132v at startup. Both drop to 76v.
12v LNB is always 0V

I have the same boards as posted earlier, no visible damaged components. Do I tear down to backlight assembly to get at the LED strips? Is there a way to test the LEDS before doing this?

[budm]

Re: Toshiba 50L1350UC No Picture

"L1+ goes to around 92v at startup and L2+ goes to 132v at startup. Both drop to 76v"

You have problem in L2 LED string.

[Hobby 1]

Re: Toshiba 50L1350UC No Picture

Can you give some more info on testing and repair? Tear it down to the LEDs? Am I looking for an open LED?

[budm]

Re: Toshiba 50L1350UC No Picture

You are looking for open circuit in the L2 string.
The 76V is feeding Voltage to the Boost converter, when it has no load the Voltage will go up to over 130Vdc due to no load so the protection circuit kicks in and drops the Voltage back down to 76VDC feeding Voltage.
BTW, you can have L1+ and L1- connected and Leave the L2- not connected to see if you see the backlights flashes on or not when first turn on the TV, then remove L1 - and reconnect L2- and then turn the TV On again to see if the backlights flash ON for seconds or not.

[neilc6]

Re: Toshiba 50L1350UC No Picture

LED panel is very fragile and will crack if bent. There is another thread in here with someone who removed the frame with the panel facing down.

https://youtu.be/T2FboV-1PJg

[Hobby 1]

Re: Toshiba 50L1350UC No Picture

Thanks for the info. I will order a LED backlight tester so I can test the strip down to the defective LED. I have removed LCD panels up to 46 inches before without breaking them to replace CCFL tubes. Does anyone know what LED to order as spares? Replace them all or just the defective one(s)? I've seen both types of repairs done.

[Hobby 1]

Re: Toshiba 50L1350UC No Picture

I disconnected the strings one at a time. Only have a flash with L1 connected. L2 is the bad one. I am learning. Thanks guys.

[neilc6]

Re: Toshiba 50L1350UC No Picture

You can test LEDs individually with a DMM in diode test mode. If there aren't test points, you will need to scrape the soldermask to get at the copper.

Replacing the strip is the easiest. You can replace just the defective LED but need to match the specs as close as possible. Reecey has a "stacking" method where he cuts off the section off LED strip and stacks it on top.

[Hobby 1]

Re: Toshiba 50L1350UC No Picture

I watched his video. I will decide what to do when I open up the panel. I don't have any LED strips from other sets yet. I have to make space for all the large parts first in case I have to keep it open and wait for parts. Thanks again for the info.