Backlight problem - Asus M3200WU, board Gapal LA-L291p

[Honziq]

Hello,

I have this All-in-one PC, after SSD change there is no picture. I was found out it is a backlight problem, but LEDs are OK. There is 4 led strings each working on 42,4V, but from board is comming only 5V.

I was repaired many backlights before but this design i will not understand.

For me it looks like it is waiting for some signal from screen to start up the backlight or something like this. Nothing seems to be damaged or shorted here.

Can someone explain me please what to do, what to check?

Thanks.

[Honziq]

+Have those switches something to do with backlight?

[Honziq]

This capacitors jumps to 79V shortly after power on, then it sloooowly discharges to 19V rail voltage and then its still 19V.
Im sure it is for backlight, but I can't find a way how it is connected to BL conn. BL conn has 6pins center two is + and 2 + 2 on sides is - to separate strings with common +. But it goes to another layer of board and I can't measure where it leads.

[Honziq]

Bl conn

[R_J]

Check if the pins from the backlight connector, connect to the two switches, they do seem to be in the backlight circuit, The voltage going to 79v (boosted voltage) and then dropping to 19v is likely due to no load (open backlights)

[Honziq]

Check if the pins from the backlight connector, connect to the two switches, they do seem to be in the backlight circuit, The voltage going to 79v (boosted voltage) and then dropping to 19v is likely due to no load (open backlights)

Thank for your advice.

That switches leads to this circuit.
Maybe someone messed with that when changing SSD.
This computer is designed in way you have to remove motherboard to gain access to SSD.

This four mosfets will be each for one LED string I think.
Leds in LCD are not open every string lights up with psu at 44V. But I think it's maybe open somewhere in the mobo.
I already tried all combinations of switch configuration but none of them worked.

EDIT: No that mosfets are for 3,3V and 5V rail, there are not backlight related.

[rogfanther]

Since the board needs to be removed to install the SSD, review the cables and connectors. It is possible some was connected in the wrong place when reassembling the machine.

[Honziq]

Since the board needs to be removed to install the SSD, review the cables and connectors. It is possible some was connected in the wrong place when reassembling the machine.

Thanks for reply, but there is no possible way to connect it wrong, it can be connected only to webcam port, but this is prevented by cable length. Other connectors are different. Its hard for me to find fault on this board, because there are no schematic anywhere and this design is complicated for me.
Everything from BL connector leads inside board, cant even find where it leads.

Maybe I can try connect that 100V capacitor + directly to + on BL connector, but I dont think it will be a good idea.

Or use step-up module from 5V or 19V rail, but it is not clean repair and without original brightness control.