Welcome back, everyone. In the last chapter, we established a foundational understanding of how to interpret the initial readings from a DC power supply to get a general idea of the fault in a dead mobile phone. We learned to identify auto amperage (primary short), no trigger faults, high booting (secondary short), stuck readings (eMMC/UFS issues), fluctuations (RAM issues), and the very common low booting fault.

However, a good technician doesn't just stop at suspicion. Diagnosis is about gathering evidence. This chapter will teach you how to move from a potential cause to a confirmed fault. We will explore how to verify the issues we discussed and introduce a few more complex scenarios.

Diving Deeper: Confirming a "Stuck" Reading (eMMC/UFS Fault)

In the last chapter, we saw that a reading stuck between 50mA to 150mA (e.g., 0.125A) strongly points to a problem with the memory, specifically the eMMC or UFS chip. This happens because the Power Management IC (PMIC) provides initial power, the CPU wakes up and tries to communicate with the memory to load the bootloader, but it fails. The boot sequence halts, and the current consumption "sticks" at that point.

But how can we be 100% sure before removing the chip?

Step 1: The USB Port Test – Your Best Friend

This is one of the most powerful and simple verification steps. After pressing the power button and seeing the stuck reading on the DC supply, connect the phone to a computer's USB port. Now, open Device Manager on your PC.

Qualcomm CPUs

If you see a device listed as QHSUSB_BULK 9008 or something similar, it's a massive clue. This is Qualcomm's Emergency Download (EDL) mode. It means the CPU is alive and is trying to boot, but it cannot find a valid bootloader on the eMMC. It has essentially given up and entered a forced programming mode. This is a very strong confirmation of an eMMC communication failure or a corrupted/dead eMMC.

MediaTek (MTK) CPUs

For MTK devices, you might see MediaTek USB Port or the device may connect and disconnect repeatedly. This also indicates the CPU is attempting to initialize but is failing to communicate properly with the memory chip.

If the phone enters these modes, your focus should be 99% on the eMMC/UFS, its connections, and its required voltages. You can avoid wasting time on the CPU or PMIC.

Step 2: Voltage Verification

Before you decide to reball or replace the memory chip, do a quick voltage check. The eMMC/UFS chip requires specific voltages to operate, typically supplied by the PMIC.

• VCC: Main power supply for the chip's internal logic, often 2.8V or 3.0V.
• VCCQ: Power for the I/O lines communicating with the CPU, typically 1.8V.

If either of these voltages is missing, the chip can't function. The fault might not be the eMMC itself, but the LDO (Low-Dropout Regulator) within the PMIC that supplies this voltage.

The Art of Pinpointing a Short Circuit

We learned that "Auto Amperage" (current draw without pressing power) indicates a short on a primary line (V_BAT or V_PH). "High Booting" (e.g., immediately jumping to 400mA) suggests a short on a secondary line (a buck or LDO output).

The DC supply tells you that there is a short, but it doesn't tell you where. Here’s how you find the exact culprit component.

Technique 1: The Rosin Method

This is a classic, low-cost, and effective technique.

1. Isolate the Area: Based on the symptom, identify the line that is likely shorted (e.g., the V_PH line for auto amperage).
2. Apply Rosin: Gently heat a piece of rosin so its smoke coats the suspected area of the PCB in a thin white layer.
3. Inject Voltage: Set your DC power supply to a low voltage, like 1V, and a limited current, like 1A. Connect the negative probe to the board's ground and briefly touch the positive probe to the shorted line.
4. Observe: The shorted component will heat up instantly, melting the rosin on top of it and creating a visible dark spot. That's your faulty component!

Technique 2: Using a Thermal Camera

This is the modern, professional version of the rosin method. A thermal camera allows you to see heat signatures on the board in real-time. When you inject voltage into a shorted line, the faulty component will light up like a bulb on the thermal camera's screen, instantly revealing its location without any mess.

The "Flicker and Die" Scenario (Advanced Low Booting)

Sometimes, you'll encounter a scenario that is a variation of "low booting." You press the power button, the DC supply reading jumps briefly to maybe 20mA - 80mA and then immediately drops back to zero, even if you keep holding the power button.

This often points to a breakdown in the crucial "handshake" between the PMIC and the CPU. Here's what's happening:

1. Trigger: You press the power button.
2. PMIC Wakes Up: The PMIC generates the initial, essential voltages (buck outputs).
3. PMIC Sends "Wake Up" Signal: The PMIC sends a signal called PON_RST_N (Power On Reset) to the CPU.
4. CPU Responds: A healthy CPU starts internal processes and sends back PS_HOLD (Power Sequence Hold) to keep the power rails active.

The "Flicker and Die" fault happens when Step 4 fails. The PMIC sends the "wake up" signal, but the CPU never sends the PS_HOLD signal back. The PMIC shuts everything down to prevent damage.

Causes of Flicker and Die Fault

• A dead CPU.
• Broken or "dry" solder joints under the CPU.
• A missing clock signal required for the CPU to start.
• A short on a critical LDO voltage required by the CPU just as it starts.

Diagnosing this requires checking the PS_HOLD line with an oscilloscope, but the symptom itself is a very strong indicator that your problem lies with the CPU's ability to initialize, making it the primary suspect.

Conclusion of Chapter 2

Today, we've moved beyond initial guesses and into the realm of verification.

• We learned to use the USB port and Device Manager to confirm eMMC/UFS faults.
• We explored practical methods like rosin and voltage injection to find the exact location of a short circuit.
• We broke down the PS_HOLD handshake to understand the "flicker and die" symptom, pointing us directly toward CPU initialization failure.

Remember, the DC power supply is your guide. It tells you the story of the phone's boot process. Your job is to listen to that story and then use these other techniques to find out exactly where the story went wrong.

In our next chapter, we will discuss how different CPU brands (Qualcomm, MediaTek, Exynos) can show slightly different DC readings for the same fault and how to analyze the boot sequence even further. Thank you.