Samsung 40 inch TV -Uncommon Backlight issue.

[alfatv]

Re: Samsung 40 inch TV -Uncommon Backlight issue.

gsmith413, please find MOSFET that drives the LEDs. I think it's the one on heatsink.
Measure voltages at drain, source and gate.
Also measure resistance between source and ground when TV is off.
I think your LEDs are OK.

[gsmith413]

Re: Samsung 40 inch TV -Uncommon Backlight issue.

@alfatv
As per your suggestion ,here are the results,

1. I checked the Gate,Drain,Source
TV on Image 1)
Led driver MOSFET-KF3N50FZ
Gate -0 volts
Drain- 113 volts
Source -0 volts

2.Resistance between source and ground when TV is off.(Image 2)
Ground TV-Black probe
Source MOSFET- Red probe
Multimeter set to 200 ohms- gives 1.5 ohms


I have checked few voltages around and have shown in image attached .
The one diode shown is showing weird voltages as shown in image although when checked diode is okay - checked(TV OFF) its give 473 ohms on multimeter and at opposite terminals 1, also continuity checked-no beep.

Thank you in advance.

[dick_barton]

Re: Samsung 40 inch TV -Uncommon Backlight issue.

There's a couple of 0 ohm resistors around the driver IC / Mosfet that would be worth checking are not open circuit.

[gsmith413]

Re: Samsung 40 inch TV -Uncommon Backlight issue.

@dick_barton:
I checked all of them near LED driver IC & MOSFET.
Image attached.
In continuity mode all 0 ohm resisters beep and at 200 ohms they give similar value 0.3ohms
Thank you .

[dick_barton]

Re: Samsung 40 inch TV -Uncommon Backlight issue.

I have never done this so I do not know for sure if any damage may be done so think carefully before doing it.
If you short the Drain to Source of the mosfet and then switch on the TV, I would expect all the leds to be at full brighthness. I would only do this very quickly to check that the leds come on and then quickly switch off the set.

[gsmith413]

Re: Samsung 40 inch TV -Uncommon Backlight issue.

@dick_barton
Don't know if you read this in my first post .I did something similar but after switching off the TV...

"Help required :Now I think the LED driver IC maybe at fault but again i have no schematics as there is nothing written on the chip ,no matter what magnification level I use, its blank.(Image 3-InkedSamsung PSU3 attached)
I have checked the MOSFET for LED circuit ,that is not shorted but When after switching off the TV ,I put Multimeter red probe to "Drain" and "black probe" to "Gate" terminals the LED strips light up for a short time. That proves its working."

But this check was between Gate and drain and not drain and source...
Does this give you any idea what may be wrong?

Also thank you for advise , i dont want to die from my wife's hands.. :P so let me park your idea for a while and wait for some more suggestions , if not then i can die in peace

[dick_barton]

Re: Samsung 40 inch TV -Uncommon Backlight issue.

If you short the drain and gate then you may well have damagd the mosfet and possibly the driver IC.

When the gate voltage is applied to the Mosfet by the driver IC the mosfet acts like a switch dropping its resistance between the Drain and Source allowing current to flow

The test was to short between Drain and Source to simulate the mosfet being switched on by the driver IC.

There is a possibility that the mosfet could be damaged by shorting the Drain to Gate.

[gsmith413]

Re: Samsung 40 inch TV -Uncommon Backlight issue.

@dick_barton
I understand your logic to perform the test ,as i did read earlier how driver IC & MOSFET work but before attempting to short drain & source Just one question-
I am already getting 169 volts at PIN 9 on LED connector and others pins are zero (led connector disconnected-refer post 19 image) , So if we short drain & source to mimic the switching of MOSFET, then how will it reduce the voltage to 101v required by LED's(as per legend on Board-refer post 16). Am I missing the logic somewhere? Also do you have schematics of this particular board?

[alfatv]

Re: Samsung 40 inch TV -Uncommon Backlight issue.

gsmith413,
So the MOSFET is not driven by IC.
I think this is the chip:
https://www.badcaps.net/forum/showthread.php?t=82041
Also check the diode mentioned in that thread.
Next, please measure the voltages on IC to make sure it's powered.

[dick_barton]

Re: Samsung 40 inch TV -Uncommon Backlight issue.

@dick_barton
I understand your logic to perform the test ,as i did read earlier how driver IC & MOSFET work but before attempting to short drain & source Just one question-
I am already getting 169 volts at PIN 9 on LED connector and others pins are zero (led connector disconnected-refer post 19 image) , So if we short drain & source to mimic the switching of MOSFET, then how will it reduce the voltage to 101v required by LED's(as per legend on Board-refer post 16). Am I missing the logic somewhere? Also do you have schematics of this particular board?

The open circuit voltage is always higher because there is no current flowing and so no backlight light. When the current flows through the leds , through the mosfet into the source resistor a voltage is developed across the resistor that feeds back to the led driver IC to maintain the current through the leds. If the current rises then the voltage across the resistor increases or if it falls the voltage across it decreases.

The diode alfatv suggested to be checked is D9151C

I don't have a schematic and only have your pictures to go on.

[gsmith413]

Re: Samsung 40 inch TV -Uncommon Backlight issue.

hi @alfatv & @dick_barton
I checked the IC , it is only receiving the 1.75 volts from from PWM_BLU(4th pin of CNM803) first resistor before the IC and the first pin of IC is only 0.44 ohms, but all other pins show zero volts .(image attached)
Also ,after trying so many angles & lighting, I finally read the IC numbers partially..

SM125*
DA1712
******
Did some research on internet & found the IC is SM1251

I did check you link too ,but then the number I found is different on this chip.
I also check the diode

I have also checked the diode marked D9151C-It is okay and showing 0.465 ohms one side only .(range should be 0.4 to 0.7 right) -images attached

I have not been able to find the correct data sheet of IC SM1251, can any of you help?

Also Should I replace the IC?

Thank you for your support.

[alfatv]

Re: Samsung 40 inch TV -Uncommon Backlight issue.

Supply voltage is missing. Either it's not supplied or IC is shorting it.
Measure DC resistance between each pin and ground.
Can you remove IC and measure voltages at the pins?
Or try to find the path of supply voltage and see where it's lost.

[jesurajan]

Re: Samsung 40 inch TV -Uncommon Backlight issue.

Hi Respected forum members,
I have trying to resolve a Samsung TV MODEL-UA40K5000ARLXL multiple issue since last 1.5 months, but now i am stuck having tried almost everything as per my limited knowledge, and hence I seek you support.


Issue : The TV turned off automatically one day.
No Backlight ,No image on screen(flashlight test done),no standby
Samsung repair technician could not find the fault and asked to replace the PSU board, which my ego did not agree with
The Board is L40MSFN-FHS BN44-00851B.
I have searched the whole internet and haven't been able to find the circuit diagram from last 1.5 months.

Diagnostic Steps taken by me :

I opened the TV after watching several YouTube videos for common faults with LCD/LED Tv's and how to fix them.
1.I checked voltages in the Hot section till high voltage capacitors and the voltages are between 325 to 345volts DC after bridge rectifier.
Fuse was okay too.
However the secondary side (Cold side) one schottky Diode was found shorted .( I thought I found the common fault and could fix easily)
While proceeding with multimeter checks, I might have shorted the board somewhere , then the fuse went out.
2.I misread the fuse number and replaced with 10 amps normal fuse as opposed to 3.15 amps ceramic one (its not easy to find components in India ,even with full specs) ,next when plugged in AC mains , the thermistor blew up.
I was scared but not done yet.
3.I increased my knowledge about the primary circuit and learned function of each and every component .
Finally I found the MOSFET to be shorted on primary side .
I searched for proper components in local market as per original component data sheets ,then replaced equivalent values Fuse ,MOSFET & Thermistor on primary side & schottky diode on secondary side.(image 1 -InkedSamsung PSU1 attached)

4.Once Again the voltages were reaching the capacitors but not beyond .
I suspected the PWN IC to be bad which switches the MOSFET on/off to start the main transformer ,After searching more on internet ,I found a post which explained how to check PWM IC 3S111 and found the PWM IC was the issue and replaced with original(Image 2-InkedSamsung PSU2) and yes I was able to bring the TV back to life .

Partial problem solved :The Standby came on , sound came on , image came on BUT NO backlight.

5.I once again searched internet and then opened by full TV to check the LED strips to my surprise each LED checked individually was lighting up, since i didn't have bench power supply to check full strips .(Dis-Assembling & Assembling TV took 6 pain full hours)

Plugged again in AC main the TV still runs with image & sound but no backlight.

Help required :Now I think the LED driver IC maybe at fault but again i have no schematics as there is nothing written on the chip ,no matter what magnification level I use, its blank.(Image 3-InkedSamsung PSU3 attached)
I have checked the MOSFET for LED circuit ,that is not shorted but When after switching off the TV ,I put Multimeter red probe to "Drain" and "black probe" to "Gate" terminals the LED strips light up for a short time. That proves its working.

Additionally I have also checked that it's not the main board at fault ,but PSW itself(Check method -Switch on TV with everything plugged in ,then disconnect the main board cable, if led lights up the issue with main board , if not PSU fault ).
Fault code :Now after switching on tv, Standby LED blinks 3 times,Sound okay ,image okay

Any help next steps will be highly appreciated.
Let me know if you need specific voltage checks at connectors , I can capture an send.

Thank you in advance

I found a nearst ic no. LC5910s from LG power Pcb for this backlight controller ic SM1251