Modern Mobile Phone Repairing: Using a Multimeter in Diode Mode to Test a Mobile PCB

Introduction Modern smartphones, powered by high-performance processors and integrated circuits like UFS, eMMC, and BGA ICs, demand updated diagnostic methods. This guide explores two fundamental techniques every mobile repair technician should master: Cold Testing and Hot Testing. These methods help identify hardware faults in smartphones, whether powered on or off.

Conducting a cold test on a circuit board using a digital multimeter, with careful measurement and analysis.

1. Cold Testing Method (Without Power)

Objective: Detect short circuits, faulty components, or disconnections on the PCB without supplying power.

Tools Required: Digital Multimeter in Diode Mode

Procedure:

Set your multimeter to diode mode.

Place the red probe on the ground (GND) of the mobile PCB.

Use the black probe to test key points across connectors, ICs, and terminals.

Reference Diode Readings: | Component | Expected Reading (Diode Mode)

✅ Battery Connector (+) ⏩ 0.250 – 0.600

✅ Battery Connector (Sense) ⏩ 0.800

✅ Display/Touch Connector (Power) ⏩ 0.200 – 0.400

✅ Display/Touch Connector (Signal) ⏩ 0.500 – 0.800

✅ UFS/eMMC Pads (CMD, CLK, DAT0) ⏩ 0.200 – 0.800

✅ MIC/Audio Section ⏩ 0.400 – 0.900

✅ SIM/SD Card Connector Pins ⏩ 0.400 – 0.800

✅ Ground Pins ⏩ 0.000 with beep

Tips for New Devices:

Always consult updated schematics (ZXW, FlexiSoft, Borneo, Dzkj).

For eMMC/UFS, test the main data lines carefully to ensure no short to ground or VCC.

2. Hot Testing Method (With Power)

Objective: Analyze voltage levels when the phone is powered on to diagnose faulty ICs, power lines, or missing voltages.

Tools Required: DC Power Supply, Multimeter (DC Voltage Mode)

Procedure:

Power the phone via battery or DC supply.

Set the multimeter to DC voltage.

Place the black probe on GND and use the red probe to test voltage at key points.

Reference Voltage Readings: | Component| Expected Voltage (V)

✅ Battery Connector ⏩ 3.7 – 4.4V

✅ Display Power Line (VCC/AVDD) ⏩ 1.8 – 2.8V

✅ CPU Core (VDD_CPU) ⏩ 0.8 – 1.2V

✅ eMMC/UFS Power Lines ⏩ 1.8 – 3.3V

✅ MIC/Audio Supply ⏩ 1.8 – 2.9V

✅ SIM VCC ⏩ 1.8 – 2.9V

✅ Charging Input ⏩ 4.9 – 5.2V

✅ Charging Output ⏩ 3.7 – 4.2V |

Power Consumption Analysis (DC Supply):

👉Reading 👉 Possible Issue

✅ 0.00A ⏩ No power path, dead phone

✅ 0.01 – 0.03A ⏩ Short in PMIC/CPU or minor fault

✅ 0.15 – 0.40A ⏩Boot loop, software or CPU stuck |

Pro Tips for Modern Technicians:

Use freeze spray or thermal camera to detect shorted components.

Employ tools like F64 Ultra, UFI, Easy JTAG for deep-level eMMC/UFS fault analysis.

Replace faulty BGA ICs using a hot air rework station with precise temperature control.

Always work on an ESD-protected workstation.

Conclusion Understanding and mastering cold and hot testing methods is essential for diagnosing today's smartphones effectively. With modern tools and techniques, technicians can repair even the most complex faults at the component level, ensuring fast, professional results.

Stay updated. Stay skilled.

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