MONITOR BOARD REPAIR - CHIP IDENTIFICATION

[Hanma]

Hello everybody,

I'm trying to fix an "old" 27' QHD Monitor. The brand is QNIX, a chinese brand so I can't find any schematic.
A couple days ago it shut down and didn't power on anymore, not even standby light.

I opened it and checked the main power rail (will attach the picture) ; 12v gets in two voltage regulators, both outputting 5v measured also from the coils, so the bottom side should be okay.

The problem is on the top side. When I was checking voltages, smoke started coming out from one of two small voltage regulators (attached a macro shot below) near the bios and crystal oscillator. I guess they are the 1.2v and 3.3v power supplies for BIOS and processor.





Could you guys help me identify this chip? I'm not a pro but maybe I can fix it with your help.

Thank you 😁

[Hanma]

Couldn't find any short around the board, except for the chip that started to smoke obviously. Any idea?🤔

[lotas]

CYxx - ETA3409S2F-T

[lotas]

BL8076CB5TR is also suitable.

[Davi.p]

XC9235G36DM ?

[Hanma]

BL8076CB5TR is also suitable.

The pinout looks right. Ordered a couple of these, will update the post once I solder them back on the board.

Thanks 🫱🏻🫲🏼

[Hanma]

XC9235G36DM ?

Can't find much with this code. Sometimes the markings on the chips have nothing to do with them 🤔.

[Hanma]

Ok guys, late update but chips have arrived and I proceeded to replace the blown one.

Unfortunaly I'm back to square one, still no standby led. I measured everything again and on the bottom side of the board the 5v rail is perfectly fine (will show in the attached picture).

The two regulators on top are the one I've been messing with thinking they were the problem, but after replacing the blown one they're both outputting half the voltage they're supposed to.


Correct me if I'm wrong, I think one regulator should feed the Eeprom chip with 3.3, and the other one the MCU with 1,2.

I measured the Regulator 1 at 0,684v and Regulator 2 at 1.3v. What am I missing? Could the MCU be the culprit in the end?


Thanks for helping me 😄

.

[lotas]

What voltage do you have on pin 1 (ENABLE), if it is not there, then there will be nothing at the output. What resistance do you have between pin 3, (on the inductors) relative to the ground. What do you have on pin 5 of one regulator, does it not close with the higher resistor?

[Hanma]

What voltage do you have on pin 1 (ENABLE), if it is not there, then there will be nothing at the output. What resistance do you have between pin 3, (on the inductors) relative to the ground. What do you have on pin 5 of one regulator, does it not close with the higher resistor?

Hi lotas, first of all thank you for taking your time to even take markings on my picture, I appreciate that.

I took the measurements and on Pin 1 there are 4.7v on both regulators.

Then I measured the Output on the inductors relative to ground and Regulator 1 has 18 megaohm, the 2nd 180ohm with the power jack connected.
If i remove the power jack the Regulator 1 measures 180ohm too.

The feedback Pin has 0,4v on Regulator 1 and 0,7v on the second. The pin you circled does not close with the resistor, it's some exces solder I had to put because the pad was gone, so i scraped the track and connected the leg, which actually goes to the back of the board.

The input voltage on both regulator is okay at 4.8v.

[Hanma]

Update, replaced regulator 1 which was definitely gone internally. Now I've got clean output on both regulators, 3.3v and 1.2.

The sad part is that the monitor still doesn't work.

When I press the power button, with a flashlight I can see the logo on the screen, but it doesn't detect the hdmi output (I connected to a pc). I tested one time with the led inverter board connected and also without it, it doesn't change a thing. It seems to "boot" then it goes to standby.

I ran out of ideas 🙄

[lotas]

Each input is controlled by the EDID firmware, the main firmware may fail due to a power failure, or the launch may be blocked due to a faulty backlight (the model is unknown and the switching algorithm is also unknown).

[Hanma]

Each input is controlled by the EDID firmware, the main firmware may fail due to a power failure, or the launch may be blocked due to a faulty backlight (the model is unknown and the switching algorithm is also unknown).

Yeah, I tought that too. Even if it starts, shows the logo and states "no signal", it probably goes to standby because it doesn't detect the backlight.
I checked the led driver board, and both mosfet and fuse are okay. When the mosfet is off, I measure 12v on the fuse that directly connects to the filter cap and led connector.

When I press the power button, the voltage goes up to 70v for a couple seconds, then it drops to 12v again. So there's definitely a problem with the backlight.

How should I troubleshoot from here? Maybe I could open the screen and directly check the led strip, but I don't have a tester for leds.

[Hanma]

Mini update, the theory that the screen doesn't recognize hdmi and goes to standby due to the malfunctioning backlight probably is wrong.



This connector attaches to the backlight driver board, coming from the mainboard. The red cables are for the 12v, the yellow is for Enabling voltage (2.7v) and the orange one for adjusting the brightness.

There is no feedback/Control pin that goes to the main microcontroller. So the screen should be able to recognize and process the image coming from my pc.

Any idea?

[lotas]

So I didn't claim that the problem was due to the backlight, but I assumed (because I didn't have such a motherboard and I don't know what the algorithm is for turning it on), that the PC recognizes the other inputs (DVI and VGA)?

[Hanma]

So I didn't claim that the problem was due to the backlight, but I assumed (because I didn't have such a motherboard and I don't know what the algorithm is for turning it on), that the PC recognizes the other inputs (DVI and VGA)?

Yeah there isn't much on the web about this board, however yesterday I got back to work on the monitor and tried another hdmi and the screen works, except for the backlight 😂.

Backlight is driven by a separate board "LD2732 Rev1.0", on which I found 3 resistors burnt. Then I disassembled the whole screen to test the led strip, and half of the leds were bad.

I'm no expert but the burnt resistors seem part of the current feedback loop between the leds and the driver IC, and that's probably why they're gone kaboom.

Now the problem of the day: I can't identify the value of these resistors due to the burn marks. I will leave a couple pictures to you here:



The IC driver is the BIT3260.

[lotas]

1R30 + 1R30 + 1R00 = 1.3 Om + 1.3 Om + 1.0 Om

[Hanma]

1R30 + 1R30 + 1R00 = 1.3 Om + 1.3 Om + 1.0 Om

A wizard! Thank you again lotas 😎

I probably have these resistors somewhere. I'm also still waiting for the replacement led strip, so will update the post as soon as I have everything.

[lotas]

Ok!