Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

[Topcat]

This was given to me the other day.... All I was told is the power light blinks. It's a 43" Samsung 4K from ~2018. Geez, this one didn't live long...

I plugged it in, and as they said, power/standby LED blinking....but not by the firmware/control. The blinking is caused by a PSU not starting up.... I can hear the PSU 'chirp' in unison with the blinking light. There's no voltages present except the pulsing/intermittent standby. I've checked all the usual suspects, can't seem to find anything bad (caps, diodes, etc). The PSU still does this with everything unplugged.....eliminates LED strips, LVDS, and the panel itself. I tested the LED strips with my tester, they lit up nice & pretty....so its down to this one & only board in the set. This board has already pissed me off.....it's lucky it hasn't been smashed with a hammer. Even after discharging the mains cap, it bit me twice....so it apparently doesn't have discharge circuitry!!

Whatever is wrong with this is apparently a common problem, these boards are nowhere to be found....but I never found any detailed information as to what the actual fault might be.....any of you experts have some input?









Here's the high res of the top & bottom views of the board:

https://www.badcaps.net/forum/attach...1&d=1619719005

https://www.badcaps.net/forum/attach...1&d=1619719005

[Diah]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

its all in one boards.. i think the blinking are error..
take out the heat sink and show here focused photo of this part

BTW: what are yours LED tester voltage reading ?

[Topcat]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

its all in one boards.. i think the blinking are error..
take out the heat sink and show here focused photo of this part

BTW: what are yours LED tester voltage reading ?

It's not an error code, it's a pulsing PSU. There's no voltages present anywhere long enough for it to boot.

LED tester is 140v @ ~100mA, all beads are lighting.

Under the big sink:

https://www.badcaps.net/forum/attach...1&d=1619722041

The switching transistor under the small sink:



The text on the chip:
'magna chip'
6DR360QS
KE3040DGP

[Diah]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

did you try measure the spi flash close to the crystal clock i think if it had 3.3V on pin 1?
of course all cable should be in place

[Topcat]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

did you try measure the spi flash close to the crystal clock i think if it had 3.3V on pin 1?
of course all cable should be in place

It fluctuates with the pulsing; 0 to 3.3, rinse repeat every second. On a digital meter, it's hard to see...but on my analog, the needle dances nicely from zero to 3.3 every second.

[Diah]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

It fluctuates with the pulsing; 0 to 3.3, rinse repeat every second. On a digital meter, it's hard to see...but on my analog, the needle dances nicely from zero to 3.3 every second.

ok. so we do have output volts.

can you check as you have 2 groups of diodes after the transformer ( at cold side ) 2 big diodes and other with 4 diodes.. need to know which one belong to LED feed. so you can terminated the other one which will be main supply feed to the mainboards.. theory the LED sockets will have volts reading or they could lit on in case they are fine.

[budm]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

It fluctuates with the pulsing; 0 to 3.3, rinse repeat every second. On a digital meter, it's hard to see...but on my analog, the needle dances nicely from zero to 3.3 every second.

Is that standby 3.3V?
It looks like the power supply is in shutdown/restarting due to overload, I.E. short circuit.

The board only has one power transformer that is always running, if you looks at the Cathode sies of those two barrel diodes on the cold of the board (with the board has nothing connected to it except the AC cord). Does the Voltage on the Cathode of the diodes fluctuate?

[Topcat]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

All diodes check good.

For grins, I removed the cathode end of the two diodes outlined. This stopped the blinking stby LED. There was also no zero~3.3v fluctuation on the SPI flash (pin1) IC; it was zero volts.

I also tested the LED+ TP, and from chassis ground it had ~360v present with the diodes attached and not attached.

From LED- to LED+, there were zero volts present. This was the same whether the backlight LED strips were connected or disconnected. I could still hear the 'chirp' no matter if the diodes were disconnected or connected, but it was far quieter (barely noticeable).

[Diah]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

LED tester is 140v @ ~100mA, all beads are lighting.

did you take the screen apart to tell all beads lighting? in case yes or no 100mA to 140V not good reading.. tester will never tell you if the LED unit zener shorted even its lighting on.

U series as i know used Concave Lens over LEDs so the distances between LEDs will increase resulting the amount of LED unit at one strip will reduced..

[budm]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

All diodes check good.

For grins, I removed the cathode end of the two diodes outlined. This stopped the blinking stby LED. There was also no zero~3.3v fluctuation on the SPI flash (pin1) IC; it was zero volts.

I also tested the LED+ TP, and from chassis ground it had ~360v present with the diodes attached and not attached.

From LED- to LED+, there were zero volts present. This was the same whether the backlight LED strips were connected or disconnected. I could still hear the 'chirp' no matter if the diodes were disconnected or connected, but it was far quieter (barely noticeable).

Those two diodes are the main DC output power supply.
The other 4 smaller diodes are for the backlights power supply.

[Topcat]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

Is that standby 3.3V?
It looks like the power supply is in shutdown/restarting due to overload, I.E. short circuit.

The board only has one power transformer that is always running, if you looks at the Cathode sies of those two barrel diodes on the cold of the board (with the board has nothing connected to it except the AC cord). Does the Voltage on the Cathode of the diodes fluctuate?

If you're referring to the ones outlined above (DM851C and DM852C), the cathode has a steady +14v, chassis ground as reference; nothing connected except the AC cord. +3.3v still fluctuates from 0 to 3.3; chassis ground as reference.

Yes Diah, all beads lit....I did check.

[Diah]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

test each LED of its zener diode if in tact. with meter diode mode

[budm]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

If you're referring to the ones outlined above (DM851C and DM852C), the cathode has a steady +14v, chassis ground as reference; nothing connected except the AC cord. +3.3v still fluctuates from 0 to 3.3; chassis ground as reference.

Yes Diah, all beads lit....I did check.

That power supply is not going into cycle then, 14V is an always on output of this power supply, it is then fed to switched MOSFETs to provide various switched power supply and regulators.

[Topcat]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

That power supply is not going into cycle then, 14V is an always on output of this power supply, it is then fed to switched MOSFETs to provide various switched power supply and regulators.

...but with the fluctuating stby 3.3v, logical deductions would tell me it's not bad strips, as it does this with or without them connected.....inspite of the steady +14v?

It does this with nothing but line connected....Even if the backlights are dead shorted (They're not), standby should not be fluctuating like this.

[budm]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

...but with the fluctuating stby 3.3v, logical deductions would tell me it's not bad strips, as it does this with or without them connected.....inspite of the steady +14v?

It does this with nothing but line connected.

That is correct, otherwise the LED power supply Voltage would be fluctuating too since it comes from the same power supply transformer, the main key right now is that the 14V and the high Voltage for the LED are stable with no LED connected but the 3.3V is fluctuating.

Can you check the resistance between circuit ground and the 3.3V test point?
Can you zoom in to that red dot of the diode that you marked?
Also that white connector next to that diode, what is connected to that plug?

[Diah]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

as all of you know every transformer had layout out of coils in side. some had at secondary coils as feed back to primary ( called in circuit protection ) unfortunately no one looking after the transformer coils direction which its key to know how it work when there are only ONE transformer between HOT & COLD and how to build the protection circuit.

[R_J]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

It could be a bad electrolytic in the primary side for the standby vcc, this vcc voltage usually supplies the pfc ic and the main power supply ic (if used)
The two small elec. caps across from your white clips are what I would check/replace or it could be the one near the bottom of the board (CM813), next to the right of the main filter

[Topcat]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

Can you check the resistance between circuit ground and the 3.3V test point?

It's reading ~37k

Can you zoom in to that red dot of the diode that you marked?

Also that white connector next to that diode, what is connected to that plug?

That connects to the backlight LED's.

It could be a bad electrolytic in the primary side for the standby vcc, this vcc voltage usually supplies the pfc ic and the main power supply ic (if used)
The two small elec. caps across from your white clips are what I would check/replace or it could be the one near the bottom of the board (CM813), next to the right of the main filter

All of those capacitors were spot-on.

[budm]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

OK, that is Boost converter section (you can see the black inductor and the SMD MOSFET).
Where is the 3.3V test point?

[Topcat]

Re: Samsung UN43NU6900FXZA 43" 4K - Power Supply Issue

OK, that is Boost converter section (you can see the black inductor and the SMD MOSFET).
Where is the 3.3V test point?

Pin 1 of IC503 next to the crystal, as per Diah's advice.