[SOLVED]820-3115-b no backlight

[Gmorb]

macbook boots and i can see an image on screen but not backlight. I get 12.5 volts on the backlight fuse and Q9707 gate and source gets 12.5 volts so based on some louis videos ive seen when both sides have the same voltage the gate wont open. could that be preventing backlight? HAALP.

 

[Gurmon]

Do you get 12.5v on pin21 of j9000 ?

If so you have an issue with dc-dc boost circuit.

Check if PPVOUT_SW_LCDBKLT_FB trace is complete. Check R9731/R9715 voltage divider. If all looks ok, including the LVDS connector get ready to replace u9701.

 

[Gmorb]

i get half a volt on pin 21, and same goes for r9731/r9715 not sure what you mean on trace being complete if you mean physically yes it looks complete but its test point only gives me less than half a volt.

edit:decided to replace q9707 along with the resistors and capacitor around it (r9789,r9782,r9788) that seemed to do the trick! got backlight now.

update:after putting the mac back together the backlight lasted for about 3 minutes before screen started flickering and backlight went out again... what does this mean?

 

[dukefawks]

R0910 is a common corrosion suspect. Also check if SMC_LID is high and see if the battery indicator is not pulling it low.

 

[Gmorb]

replaced r0910 and upon boot the trace leading to test point for f9700 fuse burned! that fuse didnt make it. replaced the fuse and reconnected the trace now i dont get 12.5v on point 2 of the fuse..

is it possible to jump 12v over to the fuse from ppbusg3h using a wire?

 

[smiba]

Bridging a fuse is a really bad idea, please don't connect PPBUS_G3H to PPBUS_S0_LCDBKLT without the fuse in between. (Especially if it keeps breaking!)

The new fuse might have blown already as well. Check the fuse with diode mode, also what is the resistance of PPBUS_S0_LCDBKLT to ground?

 

[Gmorb]

I get 0.450 on pin one of fuse and 0 on pin 2 I also get 60.8 - 61.5 resistance to ground on PPBUS_S0_LCDBKLT_FUSED.

also noticed the pad where fuse sits for PPBUS_S0_LCDBKLT_FUSED i was getting a beep while measuring to ground Imay have screwed up by digging out the trace thinking it was shorting the fuse (oops).

replaced fuse with one that gave me 12.5v on both ends connected the trace back to the ppbus_s0_LCDBKLT_FUSED test point and once again it keeps burning.

 

[dukefawks]

Probably a short in the board now. If it is charred in the fuse area that is usually a write off as there are many other traces nearby and digging out the charred material will destroy traces.

 

[Gmorb]

Well i dug the area out and only see one trace which looks healthy board still turns on as usual with spinning fan i was thinking of just running a wire from the fuse to the transistor on the other end of the board and getting rid of that test point. Sadly last fuse i had got away from me while removing it from my last donor board. Ordered some fuses and will have to give an update later. If you guys think running a wire to get 12v from the fuse to the transistors around q9707 to push12v through the mosfets is a bad idea let know. So far its all i can think of to get this voltage flowing again.

 

[Gmorb]

Just a quick update i was able to get voltage flowing again by running a wire from baklight fuse to r9788 had to cut the burned trace off since it was killing my fuses. Replaced components near q9707 just in case they blew and replaced the r0910 resistor and backlight came back! So far it hasnt failed while wiping and reloading operating system.

 

[Gmorb]

hey guys ressurecting this thread because the issue came back voltage is flowing as before only thing i did notice is that the led returns are outputting les than 1 volt. is this normal? bad LED DRIVER?

 

[dukefawks]

Voltage on backlight output, also BKL_PWM?

 

[Gmorb]

issue solved upon replacing the led driver and r9788, i hope thats the end of this nightmare.