toshiba 50L1350U no backlight

[sdinelli]

did some troubleshooting. i have picture and sound but no back light
the power supply is obviously bad bc i dont have all my voltages
this is a LED LCD tv and i found 68V going to the LED's
Is it safe to say there's definitely a blown LED?

has anyone messed with this model and replacing or jumping bad LEDs?
dont want to spend money or time repairing power supply if tv is shot.

Can provide pics of the board and voltages if necessary

Thanks

[ReeceyBurger123]

Re: toshiba 50L1350U no backlight

I would probably put money on a shorted led or bad driver, have you checked the blon voltage ?

[sdinelli]

Re: toshiba 50L1350U no backlight

yes sir, here are my voltages

PWM - 3.25V on start then to 232mV ( with LED unplugged. plugged in screen flashes once)
5.1V - no wire
bl_on/off - 3.236V
power_tv - 3.31V
AC - Detect - 3.48V
5.1V - 5.13V
GND
12V_LNB - no wire
GND
12v_AUD - 12.47V
GND
12Vcc - 12.46

So it actually seems all my voltages are correct

Also, i was wrong its 76V going to the LED connections

[newtothis]

Re: toshiba 50L1350U no backlight

Watch the voltage going to the LEDS when you turn the set on... Does it go high then settle back at 76v?

[sdinelli]

Re: toshiba 50L1350U no backlight

Watch the voltage going to the LEDS when you turn the set on... Does it go high then settle back at 76v?

actually i get 77V as soon as i plug the plug into the outlet, then it slowly lowers to zero, then kicks back upto 77V when i press power putton and the BL comes on.
Then slowly declines when i press power off again.

I found a bad diode measuring .200 and a rising voltage with reverse leads
and 4 that look a little low at .389. < is this too low?

Also, found a few questionable resistors.
I havent gotten a chance to test anything out of circuit tho.

[budm]

Re: toshiba 50L1350U no backlight

Is this your board?
http://www.shopjimmy.com/toshiba-750...upply-unit.htm
If it is the board, check the DCV between L1+ (ANODE of string 1) & Chassis, L2+ (ANODE of string 2) & chassis.
There are two strings of LEDs and two LED drivers circuits.
I will not be surprised if it has bad LEDs.

Please up load the pictures of the whole back side of the TV so we can see all the board.

[sdinelli]

Re: toshiba 50L1350U no backlight

Is this your board?
http://www.shopjimmy.com/toshiba-750...upply-unit.htm

Please up load the pictures of the whole back side of the TV so we can see all the board.

yep thats it,
i may try to repair first and see if i can get that backlight voltage back to normal. is there a way to see if theres a shortage in the LED's?

i dont want to put the time and money into fixing if LEd's are bad.

[budm]

Re: toshiba 50L1350U no backlight

I updated the thread.
You did not see if the backlights even flash on for seconds?

[sdinelli]

Re: toshiba 50L1350U no backlight

thank you sir,
and yes BL flash's on start up for a split second.

unfortunately im headed out of the house, i wont be able to play with this any more today .
thanks for all your help. will update and add pics when i can.

[budm]

Re: toshiba 50L1350U no backlight

I just notice this 'PWM - 3.25V on start then to 232mV"
Try removing just that PWM wire and see if the backlights will stay on.

[dcaron614]

Re: toshiba 50L1350U no backlight

I just had an issue with this same TV/model .. mine would flash the back lights for a hot second then nothing, you could see the image with a flash light. I tested the PSB and L1+ and L2+ were only giving 77.5v. I brought a used PSB of ebay for $31 and replaced the board and TV powered right up... back lights work fine now. I retested the L1+ and L2+ and now they are pushing out 117v

[sdinelli]

Re: toshiba 50L1350U no backlight

so after testing a bunch of stuff. i found 2 bad resistors
i have trouble figuring out the value. i think its 5.38M ohms?
does that look right to you?
They measured .6 ohms

Can anyone recommend a better or equivalent replacement?
why do you think they went bad?






two bad resistors are on the lower left by diodes and caps

[budm]

Re: toshiba 50L1350U no backlight

Red, Yellow, Silver? That will be 0.24 Ohms resistor. They are resistors for the Source pin of the Power MOSFETs of the LED driver circuit.
So did you remove the PWM signal wire from the connector as I suggested?

[sdinelli]

Re: toshiba 50L1350U no backlight

Red, Yellow, Silver? That will be 2.4 Ohms resistor. They are resistors for the Source pin of the Power MOSFETs of the LED driver circuit.
So did you remove the PWM signal wire from the connector as I suggested?

what about the rest of the colors? gold and green
and i didnt know silver can be the 3rd band. maybe its grey?
im using the resistor calculator on digikey.
do you think those 2 mosfets are bad?

http://www.digikey.com/en/resources/...or-code-5-band

i didnt remove the pmw connector
i did test L1+ and L2+
both 77V

[sdinelli]

Re: toshiba 50L1350U no backlight

I just had an issue with this same TV/model .. mine would flash the back lights for a hot second then nothing, you could see the image with a flash light. I tested the PSB and L1+ and L2+ were only giving 77.5v. I brought a used PSB of ebay for $31 and replaced the board and TV powered right up... back lights work fine now. I retested the L1+ and L2+ and now they are pushing out 117v

i dont understand why LEd's need so much voltage.

[budm]

Re: toshiba 50L1350U no backlight

what about the rest of the colors? gold and green
and i didnt know silver can be the 3rd band. maybe its grey?
im using the resistor calculator on digikey.
do you think those 2 mosfets are bad?

http://www.digikey.com/en/resources/...or-code-5-band

i didnt remove the pmw connector
i did test L1+ and L2+
both 77V

My typo, it is 0.24 Ohms which is typical value for the Source resistor. The PWM Voltage should not drop down like that, the female pin on the the connector for the PWM line can be removed for the connector body.

[sdinelli]

Re: toshiba 50L1350U no backlight

also, if i remove the pmw connector, what should i look for?

[budm]

Re: toshiba 50L1350U no backlight

i dont understand why LEd's need so much voltage.

LEDs are connected in series, each LED requires about 3~3.5, so for 50" TV there will be many LEDs connected in series just like Christmas lights.

[budm]

Re: toshiba 50L1350U no backlight

also, if i remove the pmw connector, what should i look for?

To see if the backlights will stay on.

[sdinelli]

Re: toshiba 50L1350U no backlight

To see if the backlights will stay on.

ok ill put these two resistors back in and pull that wire, and report back.
thanks.
so without the pwm, the screen will just be at the max brightness or what?
just curious