Seiki 32 led lcd tv backlight stopped working

[shelby5041]

Me again with another free tv

Tv powers on but backlight wont turn on

I check voltages to the backlight connector....and on standby the voltage is 48v

When tv powered on the volatage goes to 58v.....caps visually looks fine on the power supply board and that voltage reading does seem like it could be fine or not?



Im thinking its another bad led strip issue....very common in led lcd tvs

[shelby5041]

Re: Seiki 32 led lcd tv backlight stopped working

Tv does power on with blue indicator light and all ps voltages seem ok and there is an image on the screen...just no backlight

[shelby5041]

Re: Seiki 32 led lcd tv backlight stopped working

Here is photo of the cheap made all in one board lol

[shelby5041]

Re: Seiki 32 led lcd tv backlight stopped working

Model is SE322FS

[budm]

Re: Seiki 32 led lcd tv backlight stopped working

Picture of the board is low res so I cannot zoom in to look at the board in details especially at the LED connector.
Are there any labels for the connectors' pins?
We also need to see how may wires there are on the LED connector.

[shelby5041]

Re: Seiki 32 led lcd tv backlight stopped working

Hey nevermind....i worked on one of these few years ago and just remembered it was a led strip issue....i looked at an older badcaps thread i made on this tv and 58v going to led connector is a good voltage so im confident to be a bad led strip on this one also....

[shelby5041]

Re: Seiki 32 led lcd tv backlight stopped working

Its a 2 wire to led strips...

But i do believe its a led strip issue since i have 58v going to connector similar to a previous repair i did on same model tv...

[shelby5041]

Re: Seiki 32 led lcd tv backlight stopped working

This pic better?

[shelby5041]

Re: Seiki 32 led lcd tv backlight stopped working

On my last repair i just removed 1 backlight strip and ran tv with 2,strips not 3....and it looked fine....but i dont have the tv anymore..

Would this cause an issue in time since the input voltage to leds are same but im running less leds?

[Th3_uN1Qu3]

Re: Seiki 32 led lcd tv backlight stopped working

LED power supplies are current sources so you will not be running the same voltage with less leds, you will be running less, the power supply will automatically adjust the voltage to keep the same current thru the leds, they should work fine with any number of LEDs up to the maximum they are designed for running, you can remove as many as you like and they will still get driven the same.

[shelby5041]

Re: Seiki 32 led lcd tv backlight stopped working

Not sure i understand....does the voltage stay the same and the amps adjust depending on needs?

[shelby5041]

Re: Seiki 32 led lcd tv backlight stopped working

Oh voltage does adjust.....ok

[shelby5041]

Re: Seiki 32 led lcd tv backlight stopped working

Ok well can u explain this then....when i checked the voltage at led power connector i had the led end disconnected on ps

So i checked the voltage without leds connected and i got about 48v on standby and about 58v when tv powered on

How can this be with leds disconnected...if umsay the voltage adjusts....why would itbadjust to 58v with nothing drawing current?....

[dick_barton]

Re: Seiki 32 led lcd tv backlight stopped working

EB803 looks as though its started to vent?

[Th3_uN1Qu3]

Re: Seiki 32 led lcd tv backlight stopped working

How can this be with leds disconnected...if umsay the voltage adjusts....why would itbadjust to 58v with nothing drawing current?....

It is not made to work with the load disconnected thus all such measurements are irrelevant. Try connecting a few strips and then remove one and see what it does.

[budm]

Re: Seiki 32 led lcd tv backlight stopped working

You have input Voltage feeding the Boost converter circuit, when PS-ON is present, the Boost converter will boost the Voltage up to the needed Voltage for the total Vf of the LED string + the Vdrops on the PWM controlled Power MOSFET on the return side of the LED.
That MOSFET is the device that maintain the constant current thorough the LED string, so when you have LESS LEDs connected then the Volatge drops on the MOSFET will be increased so that the current will be maintain at constant level but that means the MOSFET will run hotter so you must be really careful about bypassing the LEDs. The boosted Voltage at the LED will be a lot higher if there is no LEDs connected, so to prevent the over Voltage condition, the boost converter will go into shutdown.

[shelby5041]

Re: Seiki 32 led lcd tv backlight stopped working

Ok thank you

[shelby5041]

Re: Seiki 32 led lcd tv backlight stopped working

thats not my board photo..it is a stock board from shop jimmy

[Th3_uN1Qu3]

Re: Seiki 32 led lcd tv backlight stopped working

That MOSFET is the device that maintain the constant current thorough the LED string, so when you have LESS LEDs connected then the Volatge drops on the MOSFET will be increased so that the current will be maintain at constant level but that means the MOSFET will run hotter so you must be really careful about bypassing the LEDs.

Is the current-limiting MOSFET a linear circuit? I'm asking because i haven't really done a lot of backlight circuits.

[budm]

Re: Seiki 32 led lcd tv backlight stopped working

The MOSFET is PWM controlled and it is on the LOW side of the circuit instead of on the HIGH side of the circuit.
The Anode of the LED string is connected to the Boosted output pin, the Cathode is connected to the Drain pin of the MOSFET, the Source pin is connected to the current sensing resistor, then the other end of the resistor is connected to GND. The Vdrops on the sensing resistor is used as the feedback to feed the LED backlight driver IC, the output of the IC is then drive the Gate of the MOSFET with PWM signal to main tain constant current through the LED string.