Has your MacBook A1989 (motherboard number 820-00850) suddenly stopped turning on, the screen gone black, and the power button stopped working? A malfunctioning motherboard is frequently the reason of this "no trigger" and incapacity to pull up to 20V. Phonefix explains the necessary tools and techniques in this post, which takes you step-by-step through the full repair process, from first diagnostic to repair.
Important Note: Motherboard-level repairs necessitate professional repair tools, precise operating methods, and specific electronics understanding. The device is very likely to sustain irreversible harm if amateurs attempt this operation. The purpose of this article is to disseminate information and offer technical references. It is recommended that novices leave repairs to qualified experts.
Step 1: Symptom and Initial Diagnosis
· Symptom:
The MacBook will not boot up at all. There's no sound when pressing the power button, the fans aren't spinning, and the screen isn't lighting up. Using a Current Voltage Tester that supports power display, the adapter fails to properly handshake and deliver 20V (PD protocol failure), and may remain stuck at 5V or even output nothing.
· Initial Diagnosis:
This indicates a major issue with the motherboard's power management sequence. The fault may involve the USB-C power chip, the motherboard's power supply circuitry, or even the T2 security chip and its firmware responsible for boot management.
Step 2: Deep Disassembly and Safety Preparation
Before accessing the motherboard, be fully prepared.
1. Cut Off the Power: Completely shut down the computer and disconnect all peripherals.
2. Precision Disassembly: Use a Pentalobe screwdriver to carefully remove the bottom case. Be sure to note the length and placement of all screws and keep a record.
3. Disconnect the Battery: This is a crucial step! Before any motherboard-level work, disconnect the battery connector to prevent accidental short circuits and further damage. Use non-metallic tools such as plastic spudgers.
4. Remove the Motherboard: Completely remove the motherboard from the case and place it on an anti-static mat. Repair personnel must wear an anti-static wrist strap to prevent static electricity from damaging delicate chips.
Step 3: Motherboard-Level Electrical Inspection and Diagnosis
Now, we begin measuring the motherboard after it's been removed from the motherboard. This stage requires specialized measurement tools.
Diagnostic Process:
1. Check the CD3215 chip: A1989 motherboards typically have multiple CD3215 (or similar) chips, responsible for USB-C power management and PD protocol negotiation. Refer to the forum thread on key signal timing to measure the basic operating conditions of these chips.
· Use a digital multimeter to measure the chip's PP3V3_UPC_X_LDO (3.3V linear regulated output) to ensure it's functioning properly. This is the "lifeblood" of the chip.
· If this voltage is missing, there's a problem with the CD3215 chip itself or its power supply circuitry.
2. Trace the timing of key signals: If the CD3215 is providing power but still can't request 20V, the problem may be more profound. At this point, you need to troubleshoot according to the timing of key signals (this is a logically rigorous check chain):
· Check the T2 chip power supply: Use a multimeter to measure whether the multiple power supplies surrounding the T2 chip (such as PP1V8_S2, PP0V9_S2, etc.) are all functioning properly.
· Check the T2 chip clock: As mentioned in the reference example, the T2 chip requires a 24MHz clock to operate. Use a digital oscilloscope to verify whether the crystal oscillator is oscillating properly. Without a clock, the chip is like a "heart stopped."
· Check the communication between the PMU and T2: Use an oscilloscope to measure whether the wake-up signal (such as PMU_TO_T2_WAKE) sent by the PMU (power management unit) to T2 and the T2's response signal have a momentary pulse. A normal momentary chip select signal indicates that the communication link is attempting to start.
Diagnostic Conclusion:
If the above measurements reveal:
· The CD3215 power supply is functioning properly.
· T2's 24MHz clock is oscillating properly.
· The critical signal between the PMU and T2 is momentary (chip select is normal).
As mentioned in the reference case, the most likely cause is that the firmware of the T2 security chip is corrupted or missing. This is a software-level issue, not a physical hardware failure.
Step 4: Restoring the Firmware in DFU Mode (Core Repair Step)
If the issue is confirmed to be a T2 firmware issue, there's no need to replace the chip; the firmware can be re-flashed using DFU (Device Firmware Update) mode. This method requires a working Mac and official tools.
Steps:
1. Connecting the Motherboard: Connect the faulty A1989 motherboard to a working Mac via a fast charging cable. At this point, there's no need to install any components like the battery, fan, or keyboard; only the motherboard itself needs to be connected.
2. Entering DFU Mode:
· Open Apple Configurator 2 on a working Mac.
· For the faulty motherboard: Locate the DFU pin connector on the motherboard (usually labeled "DFU" or "FORCE DFU"). Using a pair of tweezers, short-circuit the two pins for about 2-3 seconds.
· If successful, the Apple Configurator 2 window will show a connected device in DFU or Recovery Mode (usually indicated by a yellow or gray icon).
3. Perform a recovery:
· In Apple Configurator 2, right-click the detected DFU device icon.
· Select "Advanced" -> "Recovery." The software will automatically download the latest firmware and flash it to the T2 chip on the faulty motherboard.
· The entire process may take 15-30 minutes. During this time, neither the working Mac nor the motherboard should be powered off or disconnected.
4. Completion: After the recovery is successful, the software will indicate completion. You can now disconnect the USB-C cable.
Step 5: Verification and Assembly:
1. Initial Verification: Connect the motherboard to a USB Power Adapter (without installing it in the case) to see if it can normally draw 20V and check for current fluctuations during startup (use an adjustable power supply to observe current fluctuations).
2. Basic Assembly Test: Connect the battery, heat sink, screen, and other necessary components, but do not install the bottom case yet. Try to power on the computer. If all goes well, you should see the long-awaited startup screen or charging icon.
3. Complete Assembly: After confirming proper functionality, disconnect the power cord, reassemble all components, and secure the screws.
It made sense to fix the MacBook A1989's inability to turn on and its lack of 20V electricity. From straightforward voltmeter readings to intricate signal timing analysis, we finally located a T2 firmware problem, which was fixed using a DFU restore—a quick and non-destructive technique. Keep checking back to China Phonefix for continued advice on electronic repair as well as expert repair tools. We hope that this comprehensive tutorial will give you clear repair techniques.
