TOSHIBA 48U7653DB powers up, no backlight

[finntricky]

Hi
i have the above tv, that i got as spares/repairs, yes it boots find but no back lights, can see pic with bright light,
so i knew some of the LED's have blown, stripped it down and found 7 LED's out, replaced and now all working, but the power board now seams at fault, only getting 79 volts for each set of LED's, that is not enough for a set of 3 strips that shows needing 92 volts. there are 2 sets of 3 strips in the set.
thinking it was a board issue, i ordered another from EBAY, and tryed that, but still no back light and only 79 volts for each channel, same as my board, so i now need help. board is 17IPS20, and yes i know there is so many versions of this board, and can't seem to find any info for pin outs and volts.
took some pics, but they are not great, will see if i can get better latter.
by the way, the pic with the board number and the LED connector plug, you will notice that a black neg wire is missing, i have used ground of the chaise as could not get a reading from it when testing the POS conection next to it, but grounding the black wire gives me a reading of volts.

[CapLeaker]

Re: TOSHIBA 48U7653DB powers up, no backlight

the backlight leds are current controlled, not voltage controlled. When you turn the TV on, do the backlights come on for a second, then turn off again, or are they not turning on at all? How much DCV do you have across the main 450V filter capacitor with the TV running?

[finntricky]

Re: TOSHIBA 48U7653DB powers up, no backlight

393 volts, led's did flash once, when turned on, but not doing that now,

[yokoono]

Re: TOSHIBA 48U7653DB powers up, no backlight

You have 2 red and 2 black wires. The red wires go to + power for backlight. The 2 blacks DO NOT GO TO MINUS (GROUND) but to the driver or 2 mosfets. You cut a black wire and laid it on the ground. You left an arm of the driver without the burden. It notices a problem and gives the processor command to turn off the backlight.
Glue the black wire back in and test. Measure the voltage on the strip between the red wire and the metal housing of the TV.
Put the measuring instrument on the red wire and turn on the TV. See what is the maximum voltage read by the instrument.
Example
https://www.youtube.com/watch?v=2pyuXwNkBMA
https://www.youtube.com/watch?v=6hQOiN4-XPk

[finntricky]

Re: TOSHIBA 48U7653DB powers up, no backlight

You have 2 red and 2 black wires. The red wires go to + power for backlight. The 2 blacks DO NOT GO TO MINUS (GROUND) but to the driver or 2 mosfets. You cut a black wire and laid it on the ground. You left an arm of the driver without the burden. It notices a problem and gives the processor command to turn off the backlight.
Glue the black wire back in and test. Measure the voltage on the strip between the red wire and the metal housing of the TV.
Put the measuring instrument on the red wire and turn on the TV. See what is the maximum voltage read by the instrument.
Example
https://www.youtube.com/watch?v=2pyuXwNkBMA
https://www.youtube.com/watch?v=6hQOiN4-XPk

already said volts are 79v, red wire is + and orange wire is - and the other 2 wires are brown + black -, same volt with LED's pluged in, or unpluged if i put my probe on brown + and black - i get no reading, but if i put probe on red + and orange - i get a reading of 79 volts, hence why i removed the black wire and put to ground and then have a reading of 79 volts from brown +

[budm]

Re: TOSHIBA 48U7653DB powers up, no backlight

Do not ground the black wire of the LED to the chassis, the LED return wire is not connected to ground directly, it goes to ground via PWM controlled MOSFET and current sensing resistor.
I need to see good clear focus straight shots of the whole top and bottom side of YOUR power supply board.
Put the black wire back in place, them put black probe of the meter to the chassis, keep red probe on the Red wire plus then turn the TV on to see how high the Voltage jumps up to and wait 10 seconds to see how low it goes back down to then turn the TV off, do the same on the brown plus, we are trying to see what the Voltage at the + wires of the LED when TV is first turned on.

[finntricky]

Re: TOSHIBA 48U7653DB powers up, no backlight

Do not ground the black wire of the LED to the chassis, the LED return wire is not connected to ground directly, it goes to ground via PWM controlled MOSFET and current sensing resistor.
I need to see good clear focus straight shots of the whole top and bottom side of YOUR power supply board.
Put the black wire back in place, them put black probe of the meter to the chassis, keep red probe on the Red wire plus then turn the TV on to see how high the Voltage jumps up to and wait 10 seconds to see how low it goes back down to then turn the TV off, do the same on the brown plus, we are trying to see what the Voltage at the + wires of the LED when TV is first turned on.

goes to 79 volts, and no more, just sits there at that voltage untill turned off, and then volts drop

[budm]

Re: TOSHIBA 48U7653DB powers up, no backlight

It sound like the BL-ON is not present to turn on the boost converter circuit for the backlights or the boost converter has failed, still need pictures as requested.
BTW, you did put black wire back in place, correct?

[finntricky]

Re: TOSHIBA 48U7653DB powers up, no backlight

It sound like the BL-ON is not present to turn on the boost converter circuit for the backlights or the boost converter has failed, still need pictures as requested.
BTW, you did put black wire back in place, correct?

yep, wire back in plug, here are pics

[finntricky]

Re: TOSHIBA 48U7653DB powers up, no backlight

been doing a bit of testing around the LED controler, found 2 caps showing short and 1 diode,
uploaded a pic, just got to find out if all faulty or just 1

[finntricky]

Re: TOSHIBA 48U7653DB powers up, no backlight

been doing a bit of testing around the LED controler, found 2 caps showing short and 1 diode,
uploaded a pic, just got to find out if all faulty or just 1

turns out, that all are fine when tested of the board, it can only be the chip that is the problem ?
what do you think ?

[CapLeaker]

Re: TOSHIBA 48U7653DB powers up, no backlight

take the ic off the board and see if the short disappears.

[yokoono]

Re: TOSHIBA 48U7653DB powers up, no backlight

It sound like the BL-ON is not present to turn on the boost converter circuit for the backlights or the boost converter has failed, still need pictures as requested.
BTW, you did put black wire back in place, correct?

Did you do what he writes here? Measure, compared to the ground, if there is voltage on the BL ON pin when you turn on the TV. About 3V.

[finntricky]

Re: TOSHIBA 48U7653DB powers up, no backlight

all fixed, up and running, changed LED controler IC MP 3394s and MDD 14N25