820-00165 No Backlight Missing BKL_PWM

[it-solve]

Hi Guys,

Dealing with 820-00165 which has no Backlight. Replaced LP8550 already but still no backlight. When it turns on, backlight flashes for a second and I can measure BKL_PWM going up for a sec but drops after that.

BKL_EN = 2.8V
BKL_ISET = 0V
PPVOUT_SW_LCDBKLT_FB = Jumps to 25V when backlight flashes then drops to 8.5V
LCDBKLT_BOOST = 8.5V
Return Resistors measures OK

There is no liquid around SMC
EDP_BKLT_EN = 3.3V
EDP_PANEL_PWR = 3.3V
EDP_BKLT_PWM = 0V

Not sure what other measurements I can post.

Cheers

 

[2informaticos]

First check if you have steady image.
Next, try known good LCD assembly.

Read the sticky post about backlight issues; do NOT forget to check XW7720...

 

[it-solve]

Thanks for your reply. Yes went through the tread and based on that was measuring these voltages.
BKL_EN Voltage Divider has 2.7V
A5 ball seem to be connected properly to the feedback line
Tried with a spare LCD which works with another 00165 still only matrix, no backlight

What is XW7720 ? I saw it on the schematic but don't have anything placed on the board between L7701 and D7701. I tough it is the trace itself or a test point.

Any suggestions?

 

[2informaticos]

"What is XW7720 ? "
Search function will bring you out some pictures of it.

Supposing XW7720 is good, remove R7704 and run a wire from C7704 to R7715.
Then do few times PRAM reset.
Remove the wire, put back R7704 and test the machine.

 

[it-solve]

Thanks for the reply. Apparently XW7720 is a trace and not a component. I can measure it A5 all the way to the D7701. So trace should be okay.

Did the wire-jump and it turned on with backlight. Did 3 PRAM resets and removed the jump-wire. Still comes with no backlight.

I wonder if it is an issue with SMC. Perhaps permanent jumpwire is the only solution here, considering if customer is happy with no backlight adjustment?

 

[2informaticos]

PWM adjust signal comes from PCH, not SMC...

Did you resolder R7704?
You can join its pads directly, no reasistor really needed.
Very strange to have just 0V there; be sure R7704 is not open.