[SOLVED]820-4924 Has CPU Vcore but doesn't chime or boot.

[bjf]

Not sure where to go with this. USB mouse lights up. PRAM and SMC reset do nothing. No chime. No video/backlight. Vcore at 1.85V and 7Ω to ground. Any idea what might be happening? The board was very clean with only some minor corrosion around the SSD. I attached some pre-ultrasonic pics.

 

[dukefawks]

PRAM reset done with external USB keyboard? Internal keyboard will not work when no trackpad or possible other issues.

 

[bjf]

No change. I've had a trackpad hooked up and I could see that SMC reset was changing the magsafe light. I tried it again with an external keyboard and nothing happened.

 

[bjf]

Ok, just tried the PRAM reset with the internal trackpad and keyboard unplugged and it worked - board is booting. Those are my known-good test parts but it appears something about them being plugged in wouldn't let the board reset. No backlight still but other than that it seems good.
• I replaced the LCD connector. No change.
• The keyboard backlight is working.
• LCDBKLT_EN_L is at 12.4V
• 0.3V on pin 1 of LCD connector.
• Voltage present on pins 28-30.
• F7700 is good. PPBUS voltage present.
• SMC_LID is at 3.4V.
• Data pins 3 and 4 have continuity to R7752 and R7753 and all the way through to pin 15 and 16 on U7700. Resistors are good.
• 3.3V on pin 8.
• Known-good 2015 screen. Image present.

 

[dukefawks]

SMC_LID high? You really sure you have a good 2015 LCD? Checked continuity to the end of a separate LCD cable to confirm connector is properly soldered?

 

[bjf]

Yeah, I just used that same screen a few minutes ago for another 2015 13" Retina with a backlight issue. That was just an LCD connector replacement. The screen worked normally with that board after the replacement. It's my test screen that I always use.

Yes, SMC_LID is high at 3.4V.

To be sure, I went through and tested all the pins through a cable. All good.

 

[dukefawks]

Diode mode on pin 3 and 4? Not shorted together? Inspect U7700 area.

 

[bjf]

Pin 3: .314
Pin 4: .313

Resistance between them is normal, around 3.1K

U7700 was not hit by the spill and the whole area looks fine. I measured the pins in diode mode and they all match up with a good board. Resistors are all good. Continuity in the lines is good.

 

[dukefawks]

EDP_BKLT_PSR_EN present?

 

[bjf]

It's at .35V

 

[dukefawks]

That's no good! EDP_BKLT_EN present then?

 

[bjf]

You got me there, thanks. Blown trace near a pad at the top of the board in the EDP_BKLT_PSR_EN line. Working.