820-01598 No backlight

[prodigy83]

Board has liquid damage, I have replaced the following from a donor board: Q8401, Q8400
New chips piurchased and replaced already: U8510, R8402, C8411, R8433.
I have 0v on LCDBKLT_FET_DRV. I cannot seem to track down why there is no backlight. I have ordered some extra LP8548B1SQ chips.

 

[2informaticos]

Check diode mode to ground at both pins of D8410; w/ and w/o LCD connected.
Do you get image for sure?

 

[prodigy83]

Yes, I see the apple logo come up and it wants to connect to wifi.
Pin 1 is .838, pin 2 is 1.174.

 

[2informaticos]

Did you test with known good LCD assembly?

Post Q8400 voltages.
Do you get BKLT_EN_R?

Be sure all R8450-53 are good, as their traces too.
Check voltage and diode mode at SCL/SDA lines.

 

[prodigy83]

Did you test with known good LCD assembly?

Yes, assembly screen is good.
BKLT_EN_R I get nothing.
R8450-53 all look good.
BKLT_SCL is at 5.1v, SDA is also the same voltage.
Q8400 pin 1= .045, pin 3=12.27, pin 4= 12.58

My reference only: Ticket # REF1125

 

[2informaticos]

R8442 is good?
Missing EDP_BKLT_EN too?

U8400 gets correct VDDA/D?
Check all surrounding components and traces.

 

[prodigy83]

PPBUS_HS_CPU only has .800. I noticed U7270 had corrosion around it so I replaced 3 caps: C7273, C7273, CD7A2, and reflowed U7270. I noticed the processor started getting hot so I hooked it up to the external display and I am getting picture now. The spinning globe was cutting in and out at first, may be a bad USB-C connection but I am happy for getting video now.

R8442 is good?
Missing EDP_BKLT_EN too?

No voltage for R8442.
Yes EDP_BKLT_EN is missing.
U8400 VDDA/D both get 5v.
R8433 Pin 2 still has no voltage.

 

[2informaticos]

Forget R8433; backlight is not even enabled yet.

Do you get 5V at pin 45 of J8500?

 

[prodigy83]

Do you get 5V at pin 45 of J8500?

Yes I do, hooked it up to the built in screen and I also have picture there, but no backlight.

 

[2informaticos]

If there is image, LCD is correctly detected.
So CPU should enable backlight.
Unless there is some problem on backlight circuytry and U8400 sned back to CPU an error through I2C bus.
You've mentioned that changed many componentes there.
May have corroded trace, bad solder, knocked off component.

Just in case, try SMC reset.

 

[prodigy83]

I do have 3.2v on U8510 pin 2. I had worked on this some. Going to try and reseat this.

 

[2informaticos]

That pin controls power to LCD matrix; previous stept, before enabling backlight.
Backlight enable signal comes directly from CPU.

Did you try SMC reset?

 

[prodigy83]

I will try that now but after further visual inspection BKLT_SD test point the traces on both sides seem to be corroded so bad that they are missing.

 

[prodigy83]

Thats what it was! Ran the jumper from one side of the trace to the other. Thank you so much!!