Réparation du problème de démarrage du MacBook Pro A1278

Received a Macbook Pro A1278 cannot boot, connect the power cable with MacBook the green light can be turned on normally, but when pressing powder button no response. The client said the issue is caused by water damage and he has disassembled the Macbook and cleaned the motherboard by himself. 

Disassemble the Macbook and check the motherboard, it’s has been cleaned thoroughly and looks nice. Check under the microscope again the motherboard also looks OK, didn’t find obvious corrosive components.

Then we check whether there is short on motherboard by multimeter, normaly we start from the inductors.

And after measuring, it’s found the L7260 get shorted, the rest inductors are OK. Refer to the schematic, we can see the L7260 is relative with the PP5V_S3_REG power generation, check the cosmetic around this components also cannot find the corrosive components and chips.

So we can only use the DC power supply to apply voltage and find out the shorted components.

Tips, even through the voltage is PP5V_S3_REG, but we cannot apply 5V voltage directly, as the instantaneous large current might cause serious damage to the motherboard!

(5A 30V DC Power Supply – Suitable for Laptop Repair)

It’s advised to apply voltage from 0Volt to 5volts gradually. And when we apply voltage to 2V (current around 2A) a capacitor get burned with smoke. 

Check the schematic, C6601 is a filter type capacitor only. So we can just remove it directly.

Assemble the MacBook to test again, the issue remains still cannot be booted.

Then continue to check the public voltage, it’s normal. But when we measure the voltage PP3V3_S5_REG on L7220, there is no voltage.

Based on the boot sequence

First, when EC is ready to work, it will receive the SMC_BC_ACOK signal(adapter detect signal), then read the adapter information. As the green light is on, it means the adapter can be detected normally.

Then, EC will send out voltage enable signal PP3V3S5_EN_L to start the PP3V3_S5_REG voltage output. Measure the voltage on the PP3V3S5_EN_L signal, it’s in low-level status which is normal. So PP3V3_S5_REG cannot be generated might be caused by relative chip damage.

Check the cosmetic of components related to PP3V3_S5_REG, with no corrosive or obvious damage.

Then replace Q7220 with a new one directly. But the issue still did not get resolved.

Then based on repair experience, go to check the standby chip U7200

• the Pin 16 with normal power supply
• the Pin 8/17 no linear voltage
• the Pin 13 linear voltage enable signal no voltage

Pin 13 linear enable signal P5V3V3_REG_EN will be generated through R2723. Measure both sides of R2723, there is no voltage. Power off to measure the diode values are OK.

Track the signal P5V3V3_REG_EN is from pin 3 of U7941. Check the U7941, chip appearance looks fine. Measure the voltage on U7941

• Pin 2 with high voltage level which is OK
• Pin 1 voltage supply is also OK

So the chip U7941 might be damaged. Get it replaced with a new one, then the MacBook can be booted normally.

Summary: The issue finnaly gets fixed by removing the shorted filter type capacitor C6601 and replacing chip U7941.