In this article, we gonna share an A1466 With Memory Issue Repair case for your reference.
- Model: A1466 (820-00165)
- Year: 2015
- Defect Phenomenon: the boot current reach around 0.4A then stop
Troubleshooting process
First, disassemble the MacBook and check the cosmetic carefully. After checking we can find, besides one water damage indicator sticker turned into red, the rest are OK. There is no corrosion or even any water trace on the motherboard. So we can not find the clues for the memory issue, and also can confirm this Macbook must be cleaned or repaired by another repair technician before.
As we can see from the schematic, the water damage indicator sticker is near the connector J8300.
2. Since we cannot find defects from the cosmetic, then we can only measure the signals to diagnose.
Step 1. Check the Platform Reset signal status to confirm it is the hardware start or the soft start issue. The voltage on the PLTRST is 3.3V which is normal. Besides that, we can also measure the waveform from the #2 pin of the BIOS chip, which means the CPU should have read the BIOS.
Step2. Normally, the SM bus of the memory slot should be measured at this time. But the motherboard is onboard memory, there is no SM bus to read the memory EEPROM chip, also no memory reset signal gets measured. So we can just jump over the memory, and measure the AUX bus of the screen.
01. Self-checking graphics card then generate the power supply (PP3V3_S0SW_LCD) for the screen
02. Screen connection detects signal DP_INT_HPD will be generated. Normally it should be high-level voltage
03. Read the screen EEPROM information by the AUX bus
The measuring result: there is no waveform for the AUX. The voltage for the screen PP3V3_S0SW_LCD is low level. So we should track the source of PP3V3_S0SW_LCD.
After tracking, we find the power comes from U8300. However, the voltage supply for U8300 is normal. The enable signal EDP_PANEL_PWR is low level, which is come from the CPU. We suspect that the self-test has not reached this step yet, and it should still be stuck on the memory side, and then go back to check the conditions of the memory.
Step 3. Refer to the memory basic working sequence as below
Power — ODT — SMBUS — CKE — DRAMRST# — WE#
Normally we can start to measure from the SMBUS and WE#, but as mentioned before this motherboard with memory onboard, there is no SMBUS. So we can measure the CKE signal.
The signal should be high-level, but the measured result is low-level. The basic working conditions for the memory are not get satisfied. Measure the power supply voltage and VTT are OK. And measure the rest 3 CKE are all low-level voltage, the CKE signals are all pulled-up by PP0V6_S0_DDRVTT（VTT）through the resistors.
Measure the voltage on the other side of the resistor, it’s still low-level which is abnormal. And the voltage value on the side of the memory power supply chip is 0.6V, normal!
So we should jump a wire to get the rail connected.
After jumping the wire and testing again, the screen can be lightened and Macbook can be turned on normally. The issue gets fixed by jumping wire.
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