[SOLVED]820-2879 no backlight

[Bohmmen]

There are signs of liquid damage in the area between the lvds connector and the fan.
12.6 v at backlight fuse
I get image on screen

 

[dukefawks]

BKL_EN? BKL_PWM? Voltage on pin 2 D9701?

 

[Bohmmen]

BKL_EN 0 volt
BKL_PWM 3.3 volt
Pin 2 D9701 0 volt

 

[larossmann]

Even with BKL_EN missing there should be 12.6v going through the circuit.

What voltage is on pin 4 of Q9706? Pin 3 of Q9706?

How do R9731 and R9715 look?

 

[dukefawks]

R0910 is also a common suspect. Did you measure on both sides of the fuse?

 

[Bohmmen]

larossmann I don't have Q9706 on this board. But I suppose it is the same as Q9806.
Pin 3 12.22 volt
Pin 4 12.58 volt

I belive that means that it don't let power thru

Q9807
pin 5 3.3 volt
pin 2 0 volt


R9715 , R9731 looks good

 

[Bohmmen]

R0910 is good.
Yes, I meassured on both sides of fuse.

 

[dukefawks]

This is weird cause that would mean PLT_RST is low. Check it somewhere else like R2891 or 92. If also low check resistance to ground on there.

 

[Bohmmen]

R2891, R2892 looks good. 5.8 k resistance to groud and 0 volt on both

 

[Bohmmen]

is U1400 bad or could it be something before it?

 

[dukefawks]

You can remove R2891/92/93/94/95 and see if PCIE_RESET_L comes high again. Could be crap under firewire chip if it is still low after removing all that. Last thing to pull would be firewire chip. If it is still low after that the MCP will be dead and that would be the end.

 

[Bohmmen]

i removed the five resistors but still get 0 volt
I accidently touched both pads where R2893 was located with my probe that caused backlight to light up!

Is U4100 firewire chip?

 

[dukefawks]

U4100 is FireWire can be removed without issue, well firewire won't work but who cares. Also remove U4290 while you are doing that. If there is still no PLT_RST by then the MCP89 will probably be fried.
My bet is that MCP is bad as with a little "pull up" from the multimeter it works.

 

[Bohmmen]

No luck.

Thanks for helping me with this one

 

[Bohmmen]

I found a note that i wrote a couple of months ago about this board and i just found it. Dont know if it matters but i meassured L2590 it should be 3.3 volt but it is 2.5 volt. is this power to the MCP?

 

[dukefawks]

2.5V is fine, depends if U2592 is fitted or not. This is a dead MCP not pulling up PLT_RST.
You could try to add a 100K pull up resistor to 3V3_S0 to PLT_RST, but no idea if stuff wil properly initialize like that.

 

[Bohmmen]

Ok thanks.
Mark as solved.