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08 Jul

Background:

The MacBook had been damaged by water and had been cleaned by a local computer store in Queensland but still not working. The owner was told the logic board is faulty and the MacBook cannot be repaired. MacBook Model Number A1398 Year 2012 and Logic board Number 820-3332.

Diagnosis:

The logic board is nice and clean, good work! Save our time to clean. We connect it to a power supply and the fans spin but no chime no display. All the power rails are present except the CPU core power.

Analysis:

The CPU power management chip u7400 is responsible for creating CPU core power as shown below:

  • S0 state power rail PP5V_S0_CPUIMVP_VCC provides power to the three modules VCC, VDDA, VDDB of chip u7400.
  • System Management Controller (SMC) sends CPUIMVP_VR_ON to enable u7400.
  • R7402, R7403 set the working mode of u7400 as CPUIMVP_TONA and CPUIMVP_TONB.
  • CPU uses SVID bus CPU_VIDSOUT, CPU_VIDSCLK and CPU_VIDALERT_L to send CPU identification to u7400, and u7400 outputs CPU core power voltage PPVCORE_S0_CPU corresponding to the CPU ID.
  • During POST (Power On Sefl Test), CPU sends a second SVID signal to u7400 to adjust the CPU core power voltage.
  • Once booting into MacOS, CPU keeps communicating with u7400 to modify the core power dynamically corresponding to the load of the application software.

Repair:

  1. Check pin 47 chip enable signal CPUIMVP_VR_ON, ok.
  2. Check pin 46, 29, 19 VCC, VDDA, VDDB power supply to the chip, all ok.
  3. Check pin 1 chip mode setting CPUIMVP_TONB, not ok.
  4. Check resistor R7403, open line.
  5. Replace R7403 with a new 182K resistor, CPU Vcore power is present!
  6. The mac is working again.

Testing:

The mac is tested by Apple Service Diagnostics (ASD) software dedicated to this model.

Note: u7400 is the chip number on Apple logic board electronic circuit diagrams (schematic 820-3332).