Macbook logic board repair case - power cycling due to PP5V_S4 short
August 31, 2019
One of the things I've taken pride in for many years is doing Macbook logic board repair at our store in Manhattan, rather than sending it out to another company like was done eras ago. You can see a video of that below.
Here, we had a machine that was missing PPBUS_G3H, and PP3V42_G3H. PP3V42_G3H must show up before other rails, because
a) PP3V42_G3H powers the onewire circuitry that allows the charger to speak to the machine.,
b) PP3V42_G3H powers the SMC and the ISL6259 which begins the startup sequence as well as turning on main power rails.
The ISL6259 was short circuiting PP3V42_G3H to ground. As a result, we had no green light on the charger. After replacing it, we had PPBUS_G3H, but now we had power-cycling. It was going from 22-38 milliamps. This is why having a way to read how much power the board is taking from the charger is an important thing - whether it's a USB-C ampmeter for the newer models, or attaching a magsafe cord to a power supply for the older models.
We checked down the list of power rails and found out that PP5V_S4 was short circuited to ground, due to a failed transistor. After replacing it - BAM! Working Macbook!