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820-4924 Powers Up But No Chime or Display

C

CRC2018

Member
Ok this one has gotten me completely stumped. Initially the macbook came in with water damage and the keyboard wasnt working. Opened it up and found Pin 4 of J4813 was burned away also some corrosion around Q7520. Replaced J4813 and cleaned up the area around Q7520 - turned on got to the boot selection screen tested the keyboard to find some keys are still not functional. Tried it once more with a new keyboard and looks like working fully with the new keyboard. Replaced the keyboard in the case plug everything back in - turns on but no display or chime but CPU is getting warm, do some more troubleshooting find out it turns on fine and goes to the flashing folder screen without the touchpad connected. Quick inspection of the touchpad later see some corrosion on there and decide best option is to replace the touchpad and cable. Fitted the new touchpad testing once more before plugging it in that the machine still gives the flashing folder screen which it did, plug the new touchpad in and same problem as before no display, no chime. Think i'll just unplug the touchpad and hopefully get back to where I was before but no even with no other parts connected other than Screen and Magsafe port I get no display or chime. Tried external display also no display, also USB mouse isnt lighting up either. Tried PRAM reset with external keyboard but nothing.

Checked some voltages:
I have CPU VCore and cpu is warming up
SMC_LID is High
SMC_RESET_L is High
PPBUS is 12V
PP5V_S0 is 5V
PP3V3_S0 is 3.3V


What I have done to try to rectify the issue so far:
Removed Q7520 and replaced - same
Inspected board for other signs of corrosion finding a tiny amount around C7372 which I have removed, also a tiny amount around pins 1 and 2 of u7700 which i have cleaned up
Removed the glue around the SMC and reflowed it with flux

As it stands now i've reached a point where I could use some pointers in the right direction as to what to check/try next to get this board functioning properly again. Thanks in advance for any help you can provide!
 
C

CRC2018

Member
Checked for corrosion around U1900 - none visible. Replaced U1900 anyway from a donor board but same - still got cpu vcore but no chime/display. As for activity on USB - nothing at all with USB mouse or keyboard - if I plug a USB flash drive with a light on it though it does light up for a few seconds.
 
J

JohnB8812

New member
For sure listen to Duke on this because I literally just had one like this where it worked once and then had no image, chime etc despite VCore being present. Replaced U1950 and it worked, so it can definitely screw things up even if it looks ok.
 
C

CRC2018

Member
Checked U1950 - looked clean but replaced anyway - unfortunately it hasnt made any difference. SMC has been reflowed also with no effect.
 
D

dukefawks

Administrator
At this point it could be anything ranging from corrosion under a chip to a small component knocked off while doing the work.

Seeing that there was corrosion on C7372 is it possible there is more corrosion in the U7200 area?
Check carefully for any damaged or knocked off components.
 
C

CRC2018

Member
Given U7200 another look - can't see any other signs of corrosion around it. One interesting observation I have made is that the PP3V42_G3H rail appears to have quite a low resistance to ground 45ohms or so. I'm not sure if that is normal on this 2015 board as I do not have another one to compare it to.
 
D

dukefawks

Administrator
That is way to low. Is that with everything disconnected from the board? 45 Ohm is too high for heating anything up so it will be a process of elimination.
 
C

CRC2018

Member
Yes that was with everything disconnected from the board. Decided to take a risk and pull the SMC off the board - looked fairly clean underneath if i'm being honest but testing PP3V42_G3H for shorts afterwards was showing 2.2k ohms and not the 45ohms it was before. It's pointing me to think that either there was crap under the SMC which caused the problem and pulling it off has cleaned it or the SMC itself is bad.

Update: Reballed and replaced SMC - partial short on PP3V42_G3H is back this time at 48ohms so that leads me to conclude this SMC is the issue.
 
Last edited:
J

jadao

Member
You could confirm for short directly on smc while removed by testing on gnd and pp3v42_g3h balls. So you don't need to reball and solder it back to finally confirm the short.
 
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