17IPS20 Toshiba 40L3441DG Backlight

[alexszymona]

I got this TV from a friend. It was dead(No power LED). He says it worked just fine until he coudn't turn it on. I measurred the output voltages from the PSU but there nothing.
Then i buyed a new one from eBay. The panel works and the motherboard too.
Then i turn it on with the new power supply the backlight turn on for a second or less and it turns off again, but the image is visible with a flashlight.
I opened the screen to see if there are some damaged LEDs. All LEDs are working(for less than a second).
If i pull the BL_ON-OFF cable out from the connector the backlight turn on everytime i connect it back, but only for a second like before.

Is it possible to fix the TV?

[dick_barton]

Re: 17IPS20 Toshiba 40L3441DG Backlight

Normally that would symptomatic of a led(s) fault.

[alexszymona]

Re: 17IPS20 Toshiba 40L3441DG Backlight

the problem is that every LED is working. I was luckly able to make a picture of the flashing lights.

[dick_barton]

Re: 17IPS20 Toshiba 40L3441DG Backlight

The illumination does not look consistent. Leds can come on, heat up and then go off.

[alexszymona]

Re: 17IPS20 Toshiba 40L3441DG Backlight

The lights on the first and on the last strip are aligned in an other way. The inner strips have the same distance between the leds, but the outer strips have at the begin and the end shorter distances and in the middle longer distances.

[tom66]

Re: 17IPS20 Toshiba 40L3441DG Backlight

It is also worth checking the voltage to the LED strips and make sure it is within the range of (2.75-4.25V) x Number of LEDs as this could indicate a high-resistance LED failure.

[alexszymona]

Re: 17IPS20 Toshiba 40L3441DG Backlight

I connected the red cable from my multimeter to the red cable going to the backlight

When i connect the black cable to the chassis then i get 157V (157V/45 = 3,49V) and it drops slowly to 78V

When i connect the black cable to the black cable for the backlight then i get 108V (108V/45 = 2,4V) and it drops slowly to 29V

I tried to connect the black cable from the backlight to the chassis = same reaction.
But if i reconnect the BL-ON-OFF cable the lights stay on. I read then 130V (130V/45 = 2,89V)
It woudn't be a problem to build a switch into the TV to turn the lights manually on, but is it safe for the LEDs and the powersupply?
And is there a way to fix this? Then not, its ok. I'll use it on my desk, but a properly working TV is always better

[tom66]

Re: 17IPS20 Toshiba 40L3441DG Backlight

It sounds like, since you got the TV to work, that there is a problem with the error detect / fault detect part of the power supply.
You can look at the schematic and the control IC, maybe the controller IC is bad.

[alexszymona]

Re: 17IPS20 Toshiba 40L3441DG Backlight

then i will use a switch. Since this TV will stay on the desk next to me its no problem to turn him on with a switch. Thank you for your help.

[alexszymona]

Re: 17IPS20 Toshiba 40L3441DG Backlight

I connected the ground from the backlight with the chassis and i added a switch to the BL-ON-OFF cable. It works until i plug in any HDMI cable. Didn't tried any other source. then the backlight turns of again and it don't help switching the BL-ON-OFF(if lights up for a second). It also turns of if i go to deep into the settings

[tom66]

Re: 17IPS20 Toshiba 40L3441DG Backlight

That might be due to the backlight level changing with the input setting. You could try forcing the backlight setting to LOW in the menu from the default of AUTO or HIGH.

[alexszymona]

Re: 17IPS20 Toshiba 40L3441DG Backlight

I tried this before and it changed nothing.

[tom66]

Re: 17IPS20 Toshiba 40L3441DG Backlight

I don't know what's causing this effect then, it is an odd one.
It would require a good look over the backlight circuit on the power board.

[alexszymona]

Re: 17IPS20 Toshiba 40L3441DG Backlight

Now it's working. I cut the BL-ON-OFF cable and connected it on the PSU-side to the PWM pin. It doesn't work in some picture modes, but i don't need them.