How To Use A Multimeter To Check The Circuits Of Android Phone Motherboard


For some difficult phone malfunction caused motherboard faulty, we must use a multimeter to find out the faulty parts and replace them. Therefore, a multimeter is essential for a phone repairer. In other words, you must learn how to use a multimeter.

There are numerous parts on the motherboard. Which one should we test? How to find out their logic connection? We need the motherboard inode bitmap and circuit diagram. Generally speaking, the procedure goes like this:

  • First, check the circuit diagram, search and find the relative parts, and then measure the circuits to analyze the phone faulty.
  • Then use a multimeter to find out faulty circuits such as open circuits, short circuits, and power supply.
  • Lastly, fix or replace the faulty circuits.

Here we will share how to use the multimeter, and motherboard inode bitmap and circuit diagram.

The Basic Knowledge Of A Multimeter
  • Capacitors: Normally, we use a multimeter to measure the capacitors and resistors. 11 o ‘clock position is for the capacitor measurement, but it is seldom used for phone fix. The most commonplace is to measure the resistors.
  • Resistors: The Ohm range is for measuring the resistance of the resistors. Just put the two meters pens on both ends. Then use the black pen to measure the resistors, the red one pointing to the ground. When the resistance is less than 100 Ohm, there will be a buzz. This method can detect short circuit resistors.

Please note: the internal resistance of several groups of SoC power supply is less than 100 ohms, and it will sound when measuring with the buzzer, but not a short circuit to ground.

  • Diode Readings: Switch to 20mA position to measure the diode readings of the circuits. When there is no value or the value is infinite, then it is an open circuit. If the value is 0 or less than 100 compared with the standard value, it is a short circuit.

  • DC Voltage: Switch to 2V position to measure the power supply voltage. Use the red pen to measure the voltage, the black one pointing to the ground.

A multimeter has many functions, however, this is enough for phone repair.

Motherboard Inode Bitmap

Motherboard inode bitmap is a detailed diagram form of the position and position number of the motherboard parts. We use an inode bitmap to locate the parts and polarity, then check the circuits on the circuit diagram according to their codes. It is very useful and easy to learn.

Motherboard Circuit Diagram

The motherboard circuit diagram is also the schematic diagram. The first page usually has a table of contents. According to the catalog page number, we can locate the circuits quickly. Nevertheless, in practice, we prefer to search the circuits by inputting the keywords or codes.

We will introduce them on a motherboard circuit diagram

Power Supply Parts

This page(as shown below) is the power supply parts of SoC. There are 6 channels of VBUCK and 10 channels of VOUT with different voltages. The parts need testing are the capacitors on the circuits.

Check out this page:

  • VDD_CPU_B is the big core power supply
  • VDD_CPU_L is the small core power supply
  • VDD_GPU is GPU power supply

Please note: The impedance of these three power supplies is less than 100 ohms, so unless the value is 0, it cannot be a short circuit.

The Main Clock Data Control Parts

Here are the main clock data control parts. Since all functions need the clock, data, and control, so they are all controlled by SoC.

SOC GPIO Bus Control Parts

This is the SOC GPIO bus control parts. The sensor parts of phones are located on the I2C bus, controlled by SOC. If the sensor has been faulty, it is mostly caused by I2C line, so we need to measure the capacitors and resistors on it.

Please note: GPIO bus is for some high-speed data transmission, CAM, USB, GPS, audio, etc. It is in the same line with the PCB internal parts and cannot be measured directly.

SoC RF Control Layer

SoC RF control layer if for data exchange with BB, and then control it.

Please note: It is also connected to PCB internal parts and cannot be measured directly.

So much for the motherboard circuit diagram. It is quite complicated, and you need to practice a lot to master it.

We have given a demonstration on this video, please check it here: