Understanding how this broken component caused these symptoms in a MacBook Pro

Thread Starter

Flippers McCoy

Joined Mar 10, 2018
TL;DR I was working on a broken MacBook motherboard, and couldn't figure it out. I left it plugged in for ages, when I came back the broken component was burned up, I replaced it, it works fine. I'm now trying to figure out how I could have learned of the bad component without it being physically damaged.

So I had a 13" MacBook Pro Retina Early-2015, motherboard model 820-4924-A
When I plugged in the adapter with the battery plugged in, it would turn from green to orange like normal, but when I hit the power button it would show no signs of life, if I shorted the power on pins (R5116) it would still do nothing. An SMC reset by shorting R5101 would reset the SMC, but it didn't solve the issue. I tested PPBUS_G3H and only read 1V, when I was supposed to be reading 8.6V, and PP3V42_G3H was fine with 3.42V.
After discovering this information, I had to leave the office and didn't come in the next day. When I came in next, I discovered that I had accidentally left the adapter plugged into the bare board on my desk, so I disconnected it. I calculated it was left there for ~38 hours. Upon further inspection, I noticed that a component had been reduced to ashes during that time! This was C7140. I removed it, cleaned the board, and nothing else seemed to have been damaged. I replaced it with a part from a donor board, and the computer worked fine!
So here I am, trying to figure out how I could have deduced this if the component hadn't caught fire. It is obviously directly related to the nonfunctioning PPBUS_G3H rail, but I am unable to see how I could have figured it out without that visible damage.

I have attached a zip archive with the schematics and boardview for this model

Any advice or tips would be greatly appreciated! :)