820-2879 no bkl_pwm

[Gmorb]

Issue started with no image on screen before fixing issue i would get a quick backlight then shut off. After getting image to show on screen i get no backlight.

Measured around the board and noticed u9701 is not getting any voltage on bkl_pwm nothing gets to c9704 or r9704. As far as i can see gpu provides voltage for this?

if the gpu is not sending out the voltage is this only fixable by replacing gpu?

 

[2informaticos]

If no short, or low value at C9704, try first NVRAM reset.
Check R9704 and presence of LCD_BKLT_PWM also.

If nothing changed after reset, remove R9704 and (temporary) run a wire from C9704 to C9723 (pad1).
Boot into OSX, shutdown, remove wire, resolder R9704.
If still no backlight, go back and discard temporary word; no backlight adjust available, of course.

 

[Gmorb]

After nvram reset im getting voltage flowing again but now i have lost image and backlight voltages on lvds connector. Lcd_bklt_pwm is working now. Im back at no image. ?

update:u9000 is not being told to turn on. R9014 getting no voltage.

 

[dukefawks]

LCD not recognized. Check voltages at pin 4 6 and 7 of J9000 and see if they have continuity to the end of a separate LCD cable.

 

[Gmorb]

Pin 4=1.7v
Pin 6=1.8v
Pin 7=1.8v

when i connect a seperate cable i get
Pin 4=0v
Pin 6=3.3v
Pin 7=3.3v

i do get continuity on seperate cable.

 

[dukefawks]

Now look at schematic and pin 4.....

 

[Gmorb]

Ok i see c9010 and l9008 directly tied to it soo something is preventing full voltage from getting there? One of those may be bad?

so i tested a different screen and i get 3.3 on pin 4 with that one. cable on other screen may be bad but it still wouldnt help that i get no voltage on pin 2 and 3 of j9000. What could be preventing u9000 from turning on? Theres no voltage on pin 1 of u9000 or r9014 ?

 

[larossmann]

There's three components to LCD power, in this order.

1) DDC circuit power

2) Image circuit power

3) backlight circuit power

DDC circuit deals with talking to the computer and saying "hey bro, I'm a screen, of X resolution and Y refresh rate."

Image circuit is the part of the screen that makes a picture

backlight circuit is powered at the end ot get a backlight.

the LCD has to talk to the MCP before the MCP sends the signal to pin 1 of U9000 to tell U9000 to send 3.3v. It does this via the DDC circuit.

The DDC circuit has R9008 and R9009 as pullup resistors on the line so that the LCD can talk to the MCP. Are these good? 3.3v on both side with no LCD connected?

 

[Gmorb]

Yes those 2 are 3.3 on both ends without lcd.

 

[dukefawks]

*silver plate* L9008 is fucked. I got fucked by a LCD/LCD cable.

 

[Gmorb]

i changed l9008 and still no luck, i did however test the machine with another board and screen turns on so lcd cable and screen are fine.

 

[dukefawks]

Instead of blindly replacing it why not measure continuity and voltages?

 

[Gmorb]

i do not get any voltage on c9012, l9004, c9015, c9011 and all continuity from those up to the lvds connector are ok

 

[dukefawks]

And where does L9004 get its 3V from? I assume a broken trace, just run a wire.

 

[Gmorb]

l9004 gets its voltage from u9000 which is turned on by r9014/lcd_ig_pwr_en but im getting no voltage telling u9000 to turn on running a wire to force voltage to l9004 still ok?

 

[dukefawks]

Wait wait, I meant L9008 of course. As long as there is no 3V there nothing else with the LCD/BKL will work.