Toshiba 40L2400VQ No backlight but LEDs are fine

[buchman]

Hi,
I have a Toshiba LED tv model number 40L2400VQ.
I don't receive the correct voltage at the output to the LEDs.

I disassembled the front panel and checked all leds and they are fine. Then I checked the voltage to the leds on the power board and it flactuate in milivolts.
Tested all caps with esr meter and they are fine.
The diodes are fine and the resistors too.
Where should I concentrate on?
Thanks in advance.

[Repair_dane]

Re: No backlight but LEDs are fine

If you can't find the error there is always the possibility to install a CA-288 board and run the LEDs that way

[sigmeyer]

Re: No backlight but LEDs are fine

Check the output transistor on the bottom of the board for shorts ,also you should be getting voltage at 2 rectifying diodes after the transformer on secondary side no matter if the led driver is working, if everything else fine then its the pwm ic is probably bad.

[alfatv]

Re: No backlight but LEDs are fine

Try to measure voltage across LEDs while TV is turning on

[budm]

Re: No backlight but LEDs are fine

Hi,
I have a Toshiba LED tv model number 40L2400VQ.
I don't receive the correct voltage at the output to the LEDs.

I disassembled the front panel and checked all leds and they are fine. Then I checked the voltage to the leds on the power board and it flactuate in milivolts.
Tested all caps with esr meter and they are fine.
The diodes are fine and the resistors too.
Where should I concentrate on?
Thanks in advance.

1) TV OFF, All the boards and connectors are back in place.
2) Keep DCV (set t0 200VDC scale) meter Black probe on the chassis.
3) Red probe on the 'ANODE' pin of the LED connector (LED connector in place).
4) Turn on the TV while watching the meter to see what Voltage it jumps up to, then turn off the TV
5) Red probe on the 'CATHODE' pin of the LED connector (LED connector in place) then turn on the TV and see what Voltage it jumps up to.

BTW, what is the history of the TV?

[buchman]

Re: No backlight but LEDs are fine

Thanks for your reply guys.
Well, when the red probe is on the ANODE pin then the voltage jump to 150V and after a second to 200V and then fluctuate between 180-200V.

When the red probe is on CATHODE pin then the voltage rises to around 80V and then fluctuate between 60-80V.

I bought this TV for about 15 dollars and the guy said that there is a sound but no picture. I once fixed a TV like that by replacing a few LEDs and I thought that this time would be the same but when I opened and checked the LEDs, they were all fine.

[buchman]

Re: No backlight but LEDs are fine

Try to measure voltage across LEDs while TV is turning on

The voltage flactuate very fast between 70V to -100V.

What does that mean when the voltage goes to a negative zone?

[angelos]

Re: No backlight but LEDs are fine

It sounds to me like a backlight strip issue. Remember that if there is a break anywhere in the series LED circuit like a loose connection the tv's circuitry picks that up and shuts down the voltage.. I had that the plug where the voltage connects on the strip was loose.
Another abnormality is that you could have the led current shorting to chassis through a stray path. I had carbon build up once that caused this kind of issue. What I would do in that case is loosen all the strips and let them lie on a book (insulative) and then start up and see if the backlights come up. If they do then for sure you have a short to ground somewhere in the strip circuit.

[angelos]

Re: No backlight but LEDs are fine

If you have a LED tester you can bias each strip one by one and note at which voltage the strip drives. If there is one that has a different voltage from the others then you can check for a fault on that strip.

[buchman]

Re: No backlight but LEDs are fine

I checked each LED strip with a power supply and they all work fine

[R_J]

Re: No backlight but LEDs are fine

I checked each LED strip with a power supply and they all work fine

Did EACH led light up at the same brightness as the others in the strip? Did you have the lcd screen removed when you tested the strips with the power supply?
Remember that the TV power supply led circuit closely monitors the current and will shut down when it sees too much current drawn by the strip (maybe one shorted led)

[buchman]

Re: No backlight but LEDs are fine

Did EACH led light up at the same brightness as the others in the strip? Did you have the lcd screen removed when you tested the strips with the power supply?
Remember that the TV power supply led circuit closely monitors the current and will shut down when it sees too much current drawn by the strip (maybe one shorted led)

Yes I removed the LCD screen. Seems like all leds light up the same brightness

[Diah]

Re: No backlight but LEDs are fine

LED units for backlight.. never examined if they light on or not... you should test each one with digital meter at diode test to exam the forward currents ziner diode which built in the LED not shorted or open.

[buchman]

Re: No backlight but LEDs are fine

Just checked with the LED plug off and the result is the same.

[R_J]

Re: No backlight but LEDs are fine

Just checked with the LED plug off and the result is the same.

The VOLTAGE reading is the same with the led plug connected or not connected? that means there is an open in the led string.

[buchman]

Re: No backlight but LEDs are fine

The VOLTAGE reading is the same with the led plug connected or not connected? that means there is an open in the led string.

I checked the voltage on the connector that is located on the power supply board with the LED plug on and off and the voltage on that connector that located on the power supply board is still fluctuate.