SAMSUNG UE55TU7002KXXH rebooting

[unimatrix93]

Hello everyone, I'm working on a Samsung TV that constantly rebooting.

MB/PSU unit: BN9654185QE

It loads up to the main menu then as soon as the backlight goes brighter it reboots.

First I suspected the LCD panel, so a disconnected the LCD display from the mainboard but did the same thing.

Then I removed the backlight connector which only delayed the bootloop by a minute maybe, I know it because I test it with the remote, it does gives the menu sounds a bit longer.

Also I have to note, that the MB/PSU combo gives out a very noticible chirping noise like it's trying to start up.

So even with the BL and LCD connector removed I can still hear the boot sound every 10-15 sec or so. So it's still rebooting with the disconnected.

I guess it's a mainboard issue, is there anything I should check before replacing the MB?

Thank you in advance.

[unimatrix93]

So I did some measurements on the mainboard and noticed that I have a low resistance to ground (38ohm) on L88.
The test point is marked BV097_PW2.
I measure 0.90V on it during startup.
The Ic next to it is a A0Z2255LQI DC/DC buck regulator and the inductor is connected to the LX pins of it.

The IC has 12V VCC, which is supplied properly.

Is this working properly or something is off?

[lotas]

So I did some measurements on the mainboard and noticed that I have a low resistance to ground (38ohm) on L88.
The test point is marked BV097_PW2.
I measure 0.90V on it during startup.
The Ic next to it is a A0Z2255LQI DC/DC buck regulator and the inductor is connected to the LX pins of it.

The IC has 12V VCC, which is supplied properly.

Is this working properly or something is off?

Everything is correct, low resistance and low voltage is the power supply for the SoC (processor) core.

[unimatrix93]

Everything is correct, low resistance and low voltage is the power supply for the SoC (processor) core.

Ah so this is the power supply of the SoC. Now it makes sense.

Any idea what could cause this issue?
I measuring fluctuating voltages on the backlight.
The board specifies 112V for 55' model like mine, but I measure 123V on it while its working and when it flashes and shuts down, I measure 80V then it quickly climbs up to 150V, then goes back to 123V while working again for a couple of seconds.

[lotas]

With the panel disconnected, you need to put a jumper, check these electrolytes...

[unimatrix93]

With the panel disconnected, you need to put a jumper, check these electrolytes...

I did some measurements based on the photo you made.
I also made the jumper where you showed.
Since my board looks different, I don't know where is the middle capacitor is on my board.

[lotas]

Then you don't need to look for the middle capacitor, since the boards are different, check the ones you have and what is the voltage on the power capacitor (PFC) in standby mode and in operating mode?

[unimatrix93]

Then you don't need to look for the middle capacitor, since the boards are different, check the ones you have and what is the voltage on the power capacitor (PFC) in standby mode and in operating mode?

393V on both standby and operating mode.


CX8015 - 0v
CX8025 -0V
CP8015 275V
CM816 - 10.43V
CM813 - 20V
CM862 - 218V
CM853 - 12V
C9105 - 13V
C9101C - 197V

[lotas]

So your PFC is working, check those capacitors for capacity and esr, with a jumper and the panel disconnected, how does the TV behave?


Have you checked the HDMI, USB, and LAN ports for low or short circuits on the signal lines?

[unimatrix93]

So your PFC is working, check those capacitors for capacity and esr, with a jumper and the panel disconnected, how does the TV behave?

I proceed to check the capacity (I don't have an ESR meter at hand sadly).
With the jumper on and the panel disconnected, the backlight stayed on for a long 2 minutes, at high brigthness. The tv responded to my remote commands, bleeping with menu sounds, then it went into restarting again. The difference is that the backlight had much higher brightness and it the tv were stable for 2 minutes instead of a few seconds.

Also, no short on the signal lines you mentioned.

[lotas]

I think you have over-powered the backlight, check...

[unimatrix93]

I think you have over-powered the backlight, check...

Yes, you are right, I measure 170-200V on the discharge pins while the backlight is trying to stabilise.

Also I found something suspicious:

This small ceramic cap is next to the IC marked DI02072 which is right next to the backlight connector.

[unimatrix93]

I found the schematic of the IC.

Also turned out that the burned component is a resistor. I removed it from the circuit and I measure low resistance (53ohm) to ground on all of the non-GND pins of the IC.
Could this be the issue?

[lotas]

Where is it located, in the backlight chain? You can take a look at a high-quality photo.

[unimatrix93]

Where is it located, in the backlight chain? You can take a look at a high-quality photo.

As far as I can see, it's part of the backlight. I desoldered the IC to measure it's pins outside of the circuit and indeed the GND leg has 56ohm resistance to all other leg on it.

[lotas]

Replace it. It also relates to backlight adjustment... But which resistor burned out, what should its resistance be?

[unimatrix93]

Replace it. It also relates to backlight adjustment... But which resistor burned out, what should its resistance be?

R170, the one missing between the cap and the resistor.

I can measure 140kohm on it, but since it let out magick smoke, the resistance of it might changed.

I couldn't find the exact IC on any supplier sadly, maybe using something else with similar attributes could work?

[lotas]

You can take a high-quality photo of this microcircuit.

[unimatrix93]

You can take a high-quality photo of this microcircuit.

I hope this clear enough.

[lotas]

The marking is not visible in the photo, is it DIO2072?