Puzzling - Toshiba 49L621U Good backlight

[jason123]

I see many posts about this TV and I'm puzzled by this one.
I got this TV with no backlight. I used a tester and sure enough one string was open.

This TV has 3 strings, each with 4 strips and 7 LEDs each (28 LED per string)

I found 1 bad LED in one string and replaced it, as the strips are not available new.

Tested all the strings with tester. all checks out. Buttoned it up. TV only flashes bright light upon pluging in AC once and that's it.

Opened it up again. Used my home-rigged 200 V setup. Started with current limit of 107 mA all pass. increased to 161 mA - all pass. Now I'm trying to break something so I went to 322 mA - and all pass.

I was testing a string at a time (200 V - 3V*28 = 116V across)

where do I go from here? Any suggestion would be appreciated.

Photos Uploaded
On P803 (Power to LEDs)
VLED+ (referenced to Chassis) goes up to 105 V then down to 65 and slowly discharge
VLED3- and VLED2- goes to ~6 V or so, then discharges to 4.5 V then slowly discharge
VLED1- goes to ~5.8V but almost instantaneously discharges to zero V

On the connector that goes to the main board, all voltages seem present and good
Pin 1 = 15.89 V (16V2)
Pin 2 = 15.89 V (16V2)
pin 3 = 15.89 V (16V2)
Pin 4 = GND
Pin 5 = GND
Pin 6 = 15.89V (16VA)
Pin 7 = GND
Pin 8 = 15.89 V (16V2)
pin 9 = GND
Pin 10 = 5.02 V (5V1)
Pin 11 = 3.32V (ACD)
Pin 12 = 3.98 V (PWON)
Pin 13 = 3.20 V (BLON)
Pin 14 = 5.02 V (5V1)
Pin 15 = 0.899 V (DIM)

[budm]

Re: Puzzling - Toshiba 49L621U Good backlight

Show us good clear pictures of the backside of the TV showing all the board, then good clear straight shots of the whole power supply LED driver top and bottom side.

What Voltage testing have you done so far?

[jason123]

Re: Puzzling - Toshiba 49L621U Good backlight

Show us good clear pictures of the backside of the TV showing all the board, then good clear straight shots of the whole power supply LED driver top and bottom side.

What Voltage testing have you done so far?

Let me know if you need a closeup of anything, or better shots
Thanks

[budm]

Re: Puzzling - Toshiba 49L621U Good backlight

"I was testing a string at a time (200 V - 3V*28 = 116V across)"

Please clarify that.
3V x 28 LEDs = 84 total Vf for the string.
116V across what?

[jason123]

Re: Puzzling - Toshiba 49L621U Good backlight

Hopefully this helps clear it up
200VDC source
Series R 1k or 720 or 360 ohm
Across led string (4 strips- 28 LEDs )

Actual measured voltage is 92 across the LEDs

[Diah]

Re: Puzzling - Toshiba 49L621U Good backlight

again we are at topic LEDs ... they are working together as currents forward not V. !! which type of LED build in the string? i guess yours replacement LED didn't match the CF

[budm]

Re: Puzzling - Toshiba 49L621U Good backlight

Hopefully this helps clear it up
200VDC source
Series R 1k or 720 or 360 ohm
Across led string (4 strips- 28 LEDs )

Actual measured voltage is 92 across the LEDs

92V/28 = 3.28V
As you can see in the graph, the higher the current the higher the Vf as to be expected.

[jason123]

Re: Puzzling - Toshiba 49L621U Good backlight

Yup, makes sense budm

The LED tester is giving very similar voltage reading on the original strips vs. the strip that I changed the one LED on.

So, I can't sleep and naturally what do you do when you can't? you fix TVs
I was searching on youtube and found a video (Toshiba 49" TV, no backlight, 1 Flash only. LED feedback circuit repair) by "bigdog8882" that solved my issue.

Sure enough R585 was reading about 4k instead 100 ohms. Replaced and TV fired right up.

Not sure I can share a link to the video here, but you can see search for it. Thanks for the help folks

[budm]

Re: Puzzling - Toshiba 49L621U Good backlight

Yup, makes sense budm

The LED tester is giving very similar voltage reading on the original strips vs. the strip that I changed the one LED on.

So, I can't sleep and naturally what do you do when you can't? you fix TVs
I was searching on youtube and found a video (Toshiba 49" TV, no backlight, 1 Flash only. LED feedback circuit repair) by "bigdog8882" that solved my issue.

Sure enough R585 was reading about 4k instead 100 ohms. Replaced and TV fired right up.

Not sure I can share a link to the video here, but you can see search for it. Thanks for the help folks

Glad to hear you found the root cause of the problem.
Can you show me where the location of the R585?
BTW, what is the P/N of the LED driver IC?
Also make sure to turn down the backlights level on all input mode to as low as possible to extend the life of the LED's.
You can post Youtube link here.

[jason123]

Re: Puzzling - Toshiba 49L621U Good backlight

Hi budm
The resistor is right to the left of the 2 caps below the FETS
See picture

Here is the video
https://m.youtube.com/watch?v=oFFMpmak2cI

Unfortunately the TV is back with its owner (my 70 year young neighbor) and i no longer have access to get part number. Sorry

[budm]

Re: Puzzling - Toshiba 49L621U Good backlight

Hi budm
The resistor is right to the left of the 2 caps below the FETS
See picture

Here is the video
https://m.youtube.com/watch?v=oFFMpmak2cI

Unfortunately the TV is back with its owner (my 70 year young neighbor) and i no longer have access to get part number. Sorry

So one of the Resistor for the PWM controlled MOSFET for VLED1 (one of the three PWM controlled MOSFET's, One Boost converter) for controlling the constant current is bad..
Thanks for the info.

So the measurement in your post #1 gives us the clue:

Photos Uploaded
On P803 (Power to LEDs)
VLED+ (referenced to Chassis) goes up to 105 V then down to 65 and slowly discharge
VLED3- and VLED2- goes to ~6 V or so, then discharges to 4.5 V then slowly discharge
VLED1- goes to ~5.8V but almost instantaneously discharges to zero V

[jason123]

Re: Puzzling - Toshiba 49L621U Good backlight

I knew that indicated something, but I wasn't sure what