Toshiba 50l1350u low power to LED

[Gundulf]

Hello guys. My toshiba 50l1350u tv has a problem. I can shine a flashlight at it and get to see the display. So i thought i had dead LED's and took it apart and realized the LED's are all functional(powered each strip(12). So it became evident i had no power to drive them. Should have checked that first anyways i pluged all back and read 73 volts on the Led outputs. it has 6 strips of leds divided in 2. Both outputs read 73 volts. With leds plugged in. At night i turned the tv and was able to operate it(just the menu) with the remote without the need of a flashlight, because the leds appear to turn on with the 73 volts just too dim to be of any use. I guess im supposed to have about 150v+ on these outputs, so something is wrong with my power supply. Everything looks well, nothing is blown, no bulged cap, no burnt smell. The Tv maybe had 1 month of use( i installed it on a clients house guest room and really saw not much use) But i have no idea where to look for a culprit. Any light will be most welcomed.


thats a picture of the board in question.

[budm]

Re: Toshiba 50l1350u low power to LED

Are you getting the BL-ON signal to turn on the LED Voltage booster circuit?
The VLED per chart as printed on the board is 110VDC with load.
So if you leave the black probe on the chassis and red probe on the L+ and then turn on the TV, what dcv do you get? Does it jump up pass 110VDC and then drops down to around 70VDC?

[Gundulf]

Re: Toshiba 50l1350u low power to LED

hello Budm Thanks for your reply, I read .3 volts with the set off on both L+
and they jump to a stable 72 volts. I see no voltage spike while turning it on(beside it rising to 72v) I tried verifying the voltage out of the transformer but my voltmeter could not find the reading not even on the 1kv setting. I have no idea about that though. Its evident some voltage is coming out of it since i can get the 72 volts. Thanks again and sorry for the late reply

[dick_barton]

Re: Toshiba 50l1350u low power to LED

If you disconnect the leds and repeat the above test. The voltage should rise fast and fall just as fast so if you have min/max on your meter it will capture it.

[Gundulf]

Re: Toshiba 50l1350u low power to LED

If you disconnect the leds and repeat the above test. The voltage should rise fast and fall just as fast so if you have min/max on your meter it will capture it.

Okay did this and still got the voltage rise up to 72 volts. It really doesnt rise that fast as i could read 35 volts 50 volts and then 72 volts. My meter is very cheap so no max min function.

[dick_barton]

Re: Toshiba 50l1350u low power to LED

Looks like its a possible led driver fault. Do you have a nice sharp picture of the back of the power supply to try and identify what IC is used to drive the leds.

How many leds per strip?

[Gundulf]

Re: Toshiba 50l1350u low power to LED



Thanks for the reply Dick, when i took it apart i saw 2 led channels(hence only have 2 L+) there are 12 strips(6 rows 2 strips per), each strip has 5 led so its 60 leds total. they did power on when i used a 22 volt dc power supply on each strip. I hope these are the chips you need to see?

[dick_barton]

Re: Toshiba 50l1350u low power to LED

Can you check out the two mosfets using your meter while I try to find info on the OB3372KP

[dick_barton]

Re: Toshiba 50l1350u low power to LED

Here's a manual which may be of assistance in showing how the OB3372KP works

[Gundulf]

Re: Toshiba 50l1350u low power to LED

spent a few minutes trying to find the datasheet of that chip and it proved to be a tough cookie xD haha anyhow ill try to check the mosfets but im trying to post a picture with s(what i believe) are some blown resistors. While checking on that Driver i kinda found 2 pins that the solder looked different than the rest. And sure enough there seem to have been something wrong on these channels. But since the camera i have is so terrible its taking me some time to put it all together xD

[dick_barton]

Re: Toshiba 50l1350u low power to LED

Good luck

[Gundulf]

Re: Toshiba 50l1350u low power to LED

Thanks again for the help
this is the image i made up. Tried different ways to capture the buses and the failures i found.


Im not an expert at electronics but they seem bad to me? have to remove them to test them right?

[budm]

Re: Toshiba 50l1350u low power to LED

I cannot read the PCBA numbers as printed on the two barcode sticker, can you take closed up pictures of the the barcode. I also need good clear straight shot of the whole bottom side of the board so I can see how the circuits are made, shopjimmy does not have pictures of your board.
BTW, those 3 resistors (RL22~24, 1R?0 marking, 1 something Ohm) are low Ohm resistors connected in parallel, so the device connected to those resistors may have shorted out and damaged the resistors.

[dick_barton]

Re: Toshiba 50l1350u low power to LED

RL19, 21, 22,23,24 look as though they have a mark on top, possible a burn mark. Using your ohm meter what do they measure.

[Gundulf]

Re: Toshiba 50l1350u low power to LED

Is faster if i just give you the numbers
Customer Barcode Labelthe large sticker)
03-PK101W0100I-VDAFL-DO9-0302-100

Smaller sticker :
B178-B01 D09 REV.C
2D 00015 54611 01815
yes there must be something burnt. There are 2 circuits that drive the the 2 different led channels. The mirror circuit has its 3 resistors good. So its something on that specific circuit.


I read about 12k ohms on the rl 22,23,24 resistors 16k on the rl 21 (nearest to the chip) 15.6k on rl 19
that is mounted on the board.
interestingly enough i read 16.6 ohms on the 3 paralleled mirror circuit resistors.

[budm]

Re: Toshiba 50l1350u low power to LED

The two indentical circuits are the Voltage booster section to boost up the Voltage from 72VDC to 110VDC for the LED backlights.
As you can see, those resistors are connected to the Source pin of the Power MOSFETs Mounted on the heatsink, QL11, QL12), so you need to test the resistance between the Source and the Drain pin of those MOSFETs to see if it shows low resistance.
BTW, the pictures should be higher res, I have a hard time tracing the circuits.

[dick_barton]

Re: Toshiba 50l1350u low power to LED

I'm sure that RL22, 23, 24 are 1R00, (1 ohm)
RL21 = 4700 ohms (4K7).
RL19 possibly 2200 ohms (2K2)

[Gundulf]

Re: Toshiba 50l1350u low power to LED

The two indentical circuits are the Voltage booster section to boost up the Voltage from 72VDC to 110VDC for the LED backlights.
As you can see, those resistors are connected to the Source pin of the Power MOSFETs Mounted on the heatsink, QL11, QL12), so you need to test the resistance between the Source and the Drain pin of those MOSFETs to see if it shows low resistance.
BTW, the pictures should be higher res, I have a hard time tracing the circuits.

haha Well im sending you the best pictures my crappy iphone can take. Luck would had it that my motorola with superb camera died on me a few days ago. Anyhow we are thinking alike while you where telling me this i was tracing all the burnt components and came to a suspect : Field Effect Transistor. P0920ATF


I traced the circuit source in red, where they split on the 2 channels and marked where the burnt resistors are with the pink bus/boxes.
i need to test that P0920ATF

[budm]

Re: Toshiba 50l1350u low power to LED

Yep, two boost converter circuits, you can see two inductors (LL1, LL2), two Power MOSFETs (driven by the OB3372), Two boost DiodeS DL2, DL4.
SMD MOSFET QL5 (3?), QL1 (PWM driven) are the return (Cathode) lines for the LED string returns (two set of LED strings).
If you can find the application notes circuit of the OB IC, then circuit will probably be close to what you have on your board. The OB IC may be damaged when the MOSFET went out.

[dick_barton]

Re: Toshiba 50l1350u low power to LED

P0920ATF data sheet