How to find a short in a Macbook logic board by using your brain.

Posted Sunday, August 17, 2014

Here I demonstrate one basic, low tech method to find a short to ground in a Macbook Pro logic board. I am commonly asked where I went to school to learn this, told I am a genius, etc, and I really want to demystify that. I am not a genius. I am a regular dumbass, just like you, and I often use low tech, low brainpower methods to do my troubleshooting. This is a standard troubleshooting technique we use for our own Macbook logic board repair service.

PPBUS_G3H is the 12.6v the system runs off of. The charger puts in 18 to 19v, and ISL6259 controls two transistors that turn this into 12.6v.

A little backstory so you have a clue wtf is going on to make the 12.6v for PPBUS_G3H, when you look at U7000, Q7035, Q7030.

a) A transistor is nothing but a TRANSFORMING RESISTOR. See here.

b) Now that you know that a transistor is just a resistor with a variable resistance, pretend these two transistors are just resistors. They are arranged to act as a voltage divider. See here for information on what a voltage divider does.

So these two transistors act as a voltage divider to turn the 18-19v from the charger into 12.6v for the laptop. The transistor’s resistance is regulated by ISL6259.

Now that that is out of the way, the first transistor turns on, quickly notices that it is shorting to ground, and the system turns it the fuck off. When measuring PPBUS_G3H to ground, I get 2 ohms. this means there is a short to ground on the PPBUS_G3H 12.6v line on the board, which is why it is not turning on.

Some component on the board is shorting PPBUS_G3H to ground. But WHICH??? And how do we find out which?

Here’s a basic way to do it. We know that PPBUS_G3H is shorting to ground. This short circuit means that anything we put on PPBUS_G3H shows up on ground. Therefore, if we inject 12v into PPBUS_G3H from another power supply, the 12v will show up on ground. Let’s go ahead and do that. We do that, and 12v is showing up on ground of the board from PPBUS_G3H which is shorted to ground.

What if we tried to power a 12v circuit, and attached the positive rail of this 12v circuit to ground on this board? That would mean 12v would have to be moving through PPBUS_G3H to ground on the logic board THROUGH WHATEVER COMPONENT IS CAUSING THE SHORT. If we put a demanding load on there, such as, 10+ LED lighting strips, that component will get VERY HOT.

Then we feel around the board for the hot component. Once we feel the boiling component, it’s time to remove it and check if the short is still there.

After removing the GPU on this particular board, the short did indeed go away.