A2779 no backlight, incomplete boot

[Mike H]

I have an A2779 820-02841-05 that came in with no power, not charging, some corrosion on bottom side of board.

Unable to find a schematic so I’m using 820-02098 which appears to be almost identical.

Relieved a short on PP3V8_AON_SPMU_ISNS by removing C771C. Obvious liquid damage in this area. Now it charges and I get an Apple, black screen, Apple repeating most of the time but otherwise just a white circle instead and never a backlight. Boot is incomplete although it occasionally plays the startup sound but just stays on the Apple/circle.



PPBUS_AON 12V

PP3V8_AON 3.8V

PP3V3_AON 5V

PP5V_AON_P3V8_AON 3.8V

PP1V8_AON 1.8V



LUXE_ISL_R_N dead short to G

LUXE_ISL_R_P G dead short to G



LUXE_UG1_R 0.05V .441 diode, 4.2 MΩ



How should I proceed?

 

[2informaticos]

"PP3V3_AON 5V"
Are you sure about this?

"PP5V_AON_P3V8_AON 3.8V"
I don't know how to interpret this; can't find such power rail on the schematic.

"LUXE_ISL_R_N dead short to G
LUXE_ISL_R_P G dead short to G"
Normal readings, check schematic...

 

[Mike H]

I was looking for PP5V_AON and this is what came up in Flexboardview. Perhaps erroneous or I am interpreting it incorrectly.

 

[2informaticos]

Yeah, you've wrote it incorrectly; that's the reason I couldn't find it.
And should have 5V there, not 3.8V.

Check UP701/30 output voltages.

BTW, use schematic, not boardview, when mention signals.

 

[Mike H]

For UP701 I have

3.8V on PP3V8_AON and PPMDB_AWAKESW_TCON
1.8V for a few seconds and then 0 on EDP_PANEL_PWR_EN

For UP730

3.8V on PP3V8_AON
1.8V then 0V on EDP_PANEL_PWR_EN
.6V then 0V on FB_BLKT_2D
3.2V then 0V on PPBCON_AWAKESW_BKLTCONN

 

[2informaticos]

Did you check with LCD connected?