1. Automatic Power-On

2. Automatic Power-Off (Automatic Shutdown)

• Shuts down automatically when vibrating

This is mainly due to poor contact between the battery and the battery contacts.

• Shuts down when pressing any key

The phone does not shut down if the keyboard is not pressed, but it shuts down automatically when any key is pressed. This is mainly due to poor soldering of the CPU and memory. Re-soldering the CPU and memory usually solves the problem.

• Shuts down when pressing the transmit button

The phone shuts down automatically when the transmit button is pressed. This is mainly due to a fault in the power amplifier section (or power control), such as a damaged or shorted power amplifier. If the power amplifier is normal, it may be due to the power IC's poor load-carrying capacity, which can be resolved by replacing it.

3. Does Not Power On

• No current response when pressing the power button after charging

This is mostly due to a broken or poor contact in the battery spring or power line. If the voltage has reached the power block, it indicates that the power block is poorly soldered or damaged.

• Current around 10mA when pressing the power button

This is mostly due to the 13M not working.

• Current around 50mA when pressing the power button

This is mostly a software or logic circuit issue. Rewriting the software and re-soldering the logic circuit can solve the problem.

• High current (around 500mA) when charging

This is mostly due to a short circuit in the power amplifier or CPU. You can remove the inductor in the power supply circuit for testing.

• High current when pressing the power button

This is mostly due to a damaged power block.

Method for determining high current: Lower the supply voltage to keep the current around 200mA (to prevent further damage). After charging for a while, touch the components on the circuit board to find the one that heats up, which is the damaged component.

4. Cannot Connect to Network

There are two situations: having a signal but not connecting to the network, and no signal and not connecting to the network. For Ericsson and Samsung series phones, as long as the receiving channel is good, the signal strength will be displayed, regardless of the transmit circuit. Other series phones only display signal strength after connecting to the network. For other series phones, when diagnosing the fault range, insert the SIM card, navigate the menu, and manually search for the network. If the network is found, it proves that the receiving channel is good, and the issue is with the transmit channel. If the network cannot be found using the menu method, it indicates a fault in the receiving channel, which should be repaired first.

Common faults in the receiving channel include 13M frequency deviation (measurable with an oscilloscope and frequency counter), local oscillator stop (determinable by checking for phase-locked level), and damage to the high amplifier or filter (testable with a dummy antenna).

Except for Motorola phones, the presence of normal RXI/Q can be measured to determine whether the issue is with the RF circuit or the logic circuit.

5. Weak Signal Transmission, Signal Drop

• Weak signal transmission: When the phone is in standby mode, it does not show weak signal, but as soon as a call is made or after making a few calls, it immediately shows weak signal and low battery warning. This phenomenon is primarily caused by dirt or poor contact between the battery and the contact interface; secondly, it may be due to poor contact between the battery contact and the phone circuit board; and thirdly, it could be due to damage to the power amplifier itself.
• Signal drop: When the phone is in standby mode, the signal is normal, but as soon as the phone transmits, the signal drops immediately. This phenomenon is caused by a faulty or damaged power amplifier.

6. Power Leakage

Power leakage in phones is a relatively difficult fault to repair. First, determine if the power supply section or the power switch tube is burned out causing a short circuit. Secondly, check if the power amplifier is damaged. Additionally, if the leakage current is not too high, apply power to the phone for 1-2 minutes and use the back of your hand to feel which component is overheating; this component is likely faulty and should be replaced. If the above methods do not resolve the fault, then check if there are any short circuits in the resistors, capacitors, or printed lines in the circuit.

7. Unstable Signal (Signal Drop)

This is caused by poor soldering of components in the receiving channel (phones that have been dropped are prone to this fault). Mainly re-solder the receiving filter, surface acoustic wave filter, intermediate frequency filter, and receiving IC components, which can mostly restore normal function.

8. Software Faults

• Summarizing, the main phenomena of mobile phone software faults are:

• The user locks the phone themselves, but all the original factory passwords have been changed, making the factory unlock password useless. Rewriting the code chip data can resolve this.
• The phone can make calls, but settings are not remembered, the screen is black, the backlight does not turn off, and the battery shows normal weak signal warnings. These faults can be caused by software issues if the related hardware circuits are normal, and rewriting the code chip data is necessary.

9. Diagnosing Faults Based on Phone Current

• The phone does not turn on, and pressing the power button does not move the ammeter needle, which is mostly due to a broken battery spring or power supply line.
• The phone does not turn on, and pressing the power button causes the ammeter needle to move slightly or not at all. This phenomenon is mainly due to a broken power-on signal or a non-working power IC.
• The phone does not turn on, and pressing the power button causes the ammeter to return to around 10mA, which is mostly due to the 13M not working.
• Pressing the power button shows a leakage current of about 20-30mA, indicating a short circuit or damage in the power supply section.
• The phone does not turn on, and pressing the power button shows a current much smaller than the normal value of around 200mA, but releasing the power button causes the ammeter needle to return to zero. This phenomenon indicates that the power supply section is basically normal, but the clock circuit or CPU is not working properly.
• The phone does not turn on, and pressing the power button shows a current of around 200mA, which then quickly returns to 0mA. This is a typical software fault caused by corrupted code chip data.
• Pressing the power button shows a large leakage current, indicating a short circuit in the power supply section or damage to components in the power amplifier section.
• If there is a leakage after installing the battery, first remove the power IC. If the leakage stops, it indicates that the fault is caused by the power IC. If the leakage persists, it suggests that the fault is due to a damaged component directly powered by the battery's positive terminal or a short circuit in the power supply line itself (this fault often occurs due to water ingress). Check the circuit or components based on the principle of the battery supplying power to the phone (power filter capacitor, power protection diode may sometimes short circuit).