Nokia mobile phone signal problem

Speaking of Nokia mobile phone, everyone will admit that its signal is relatively stable, with stroke input, which is very popular with consumers, but now its position is slowly replaced by color screen mobile phone. However, the maintenance of this mobile phone still exists. Maintenance masters often have headaches because of the quality problems of internal equipment. The following little experience about the maintenance of this mobile phone hopes to bring some help to everyone.

First, the signal problem of this mobile phone is the easily damaged components: antenna switch, front-end filter, power amplifier tube, intermediate frequency, audio, 13MHZ, VCO, CPU, software, etc. To give you a better understanding, let's take a few minutes to briefly introduce the component performance:

1, antenna switch (Z670): Its main function is to switch the receiving and transmitting lines of the mobile station (mobile phone) transceiver. Its damage will cause no signal, poor signal and large emission current. There is also a special case where China moves normally and Unicom signals are poor or vice versa.

2. Front-end filter (Z620): Its simple function is to attenuate some unnecessary signals and let useful signals pass. Its damage will lead to no signal or poor signal. The main solution is to connect a fake antenna or short circuit.

3. Amplifier tube (V904\V905): It is mainly used for low-noise amplification of antenna induced signals to provide sufficient amplitude for later detuning. Its magnification is 5~ 10db, and the larger the magnification in the circuit, the better. If it is damaged, it will not work normally, which will cause failures such as no signal or poor signal and unstable signal.

4. Intermediate frequency IC(N605): Its main function is to mix and modulate the received and transmitted frequency signals, and some also divide the frequency of 26MHz into 13MHz for the logic circuit. Damage will cause problems such as failure to start, abnormal start, signal diving, no signal and poor signal. It is the key point of maintenance and inspection of non-signal mobile phones, and it is also one of the parts that are easy to be damaged.

5. Audio IC(N250): Its main function is to demodulate the received baseband signal into a data signal that can be recognized by the machine by the CPU, or to convert the analog signal in the transmission process into a variable transmission baseband signal and transmit it to the intermediate frequency. Its damage will cause two lines of English, no signal, signal diving, receiver noise, receiver mute, no message transmission and other failures.

6. Reference clock oscillation circuit (G502): Its function is to provide synchronous clock for the logic part to work and contact the base station. Its damage and abnormal operation will make the mobile phone unable to turn on, and there will be no network and signal diving.

7. Voltage-controlled oscillator (G800): also called VCO. It is synchronized with the base station by frequency doubling of oscillator circuit. For example, 82 10GSM network works at 3740~3840MHz, while DCS works at 36 10~3760MHz. Its damage will lead to no signal and diving.

8.CPU(D200): It is called microcomputer central processing unit. It is an important part of the logic part, and the whole system depends on it to calculate and analyze data independently and finally output control instructions to make related circuits work. In the mobile phone, its damage will cause no power-on and no signal, and the single function will fail.

9. Software: a machine language program that can be recognized by the machine. However, data loss will also lead to abnormal demodulation, resulting in no signal, no boot, two lines of English and other failures.

Second, the important test points:

A. Amplifier tube (V904\V905): square wave voltage with electrode and collector voltages of 0.8V and 2.0V respectively when receiving. Because it can be judged that there is no signal because the intermediate frequency is damaged or the power IC(N 100) is damaged.

B, R533: the reference voltage 1.20V for audio transmission to the intermediate frequency. The voltage is too low or too high, and the error is 0.2V It may be that the audio is damaged by virtual welding or the power supply of VREF is abnormal, resulting in no signal or signal diving.

C R530: I/Q signal receiving test point. It consists of trapezoidal waveform and AC component. The larger the AC component in the middle, the stronger the signal. If the trapezoidal waveform is abnormal, it may be caused by the power IC, its own damage or the virtual welding between CPU and audio.

D, R543 and R540: test points for emitting I/Q signals. They are two connected resistors. When you make a phone call, you can see an I/Q signal flashing. If there is no I/Q signal, it may be audio virtual welding damage, but it must be done under normal conditions.

E.G502: Generally, the frequency of the output point is 25.999X several MHz measured by a frequency meter. There is also AFC voltage. Search on the Internet is about1.12v ~1.4v.. Too high or too low to check the audio and audio power supply.

F, R205, R206: frequency synthesis clock and frequency synthesis starting voltage. Missing one will also cause no signal.

Three: maintenance examples.

(1) A customer of Nokia 8250 mobile phone sent a message that he could not make a call. After testing, the current oscillates slightly around 250Ma when searching online, and returns to around 100 Ma after the keyboard light is turned off. After that, it oscillated slightly to 150Ma and then returned to 100 Ma.

Troubleshooting: the power supply of the power amplifier tube V904 is normal, and the voltage of R533 1.2 1v measured at 1.20V is normal, but when R530 receives I/Q, it is found that the I/Q trapezoidal waveform becomes smooth, which is suspected to be caused by the abnormal power supply of the power IC. However, all the voltages measured by RF are normal, and it is invalid to replace the IF audio IC.

(2) A Nokia 82 10 mobile phone can be used in the urban area according to the customer's description, but there is no signal in the suburbs. After the test, it is found that the signal stick will jump two or three squares at the antenna where the signal is blocked by hand, but it has no effect on the call.

Troubleshooting: Because there is a signal, you can call to explain that the fault of this mobile phone is not very serious. The failure causes are: a. The front end of the receiver is damaged or its performance is poor, including (filter, antenna switch, amplifier tube, resistance-capacitance element of this loop, etc.). ) b, caused by low transmission power. When making a phone call, observe that the current is about 450MA, indicating that the power is still enough, so the fault of replacing the antenna switch still exists. Finally, short-circuit the front-end filter (Z670) by hand to cover the signal at the antenna. Return it to the user and never come back for warranty.

(3) When a Nokia 8250 is turned on, China Unicom works normally, while after China Mobile is turned on for a period of time, the signal difference sometimes leads to signal diving.

Troubleshooting: Because Unicom is normal, most components are free, and most failures occur in VCO. But after the replacement, it is still the same, and the intermediate frequency replacement has not improved. Finally, rewrite the software for troubleshooting. but

(4) Nokia 82 10 has no signal. After replacing IF, China moves normally, while Unicom is unstable due to poor signal.

Troubleshooting: first deal with the invalid software, replace the VCO, and then move the big pieces. Then think carefully whether the signal difference is caused by the damage of the front-end components, and finally change the antenna switch and find that the fault has improved. After waiting for a long time, the signal was not found to be unstable, indicating the troubleshooting.

(5) When a Nokia 8250 is turned on, the signal is full, but the signal dives late (diving: it means that there is a signal to no signal), but it will automatically find the network later. Repeat this.

Troubleshooting: I/Q signal R530 is normal, but the voltage of R533 is1.23v. It is suspected that the audio part is faulty. Repair welding audio is invalid. After being removed, I found that the feet in the stereo were dirty, so I had to troubleshoot them repeatedly.

(6) Nokia 82 10 started with a signal, but later it became no signal, and the network could not be found.

Troubleshooting: I/Q signal is sparse, and there are few AC components in the middle of trapezoidal waveform. At first, it was suspected that it was caused by intermediate frequency virtual welding or damage, and it was invalid after replacement. After writing software to change the audio frequency, the oscilloscope is used to measure the RF power supply again, and it is found that the voltage provided by VCXO to the main clock is unclear. After removing C 156, the fault was eliminated.

(7) Nokia 8250 has a signal and can't make a call.

Troubleshooting: The measured I/Q signal is normal, and the transmission loop is good when the maintenance engineering software is tested, but the induced luminous doll can't sense light when it approaches. It shows that there is no transmitted signal, and it is suspected that IF or VCO is damaged, and it will not work after replacement. Even if the fake antenna is connected to the current collector of V80 1, you can't make a phone call. Normally, measure the control signal TXC sent from audio to IF (this value is generally around 0.50V), then look at D5 of N605 diagram and mark the transmission power level of TX-PWR. Psychological doubt, because there are no peripheral components to measure, we can only remove the electric barrier of the multimeter to measure the resistance to ground, and find that the resistance is infinite. Digging D5# with a needle, I found it dropped. After the flight, the fault is repaired.

(8) After the Nokia 8250 battery is fully charged, one or two phone calls will prompt that the battery is low. Please turn it off automatically after charging.

Troubleshooting: The peak current reaches 700MA when using the maintenance power supply to make a phone call. According to the maintenance experience, it is generally a problem that the power amplifier N702 is damaged or the power control circuit is faulty. Because of the high cost of power amplifier, the power control detection resistor R754 is short-circuited from the power supply to the ground, and the current returns to about 450MA, and the current is normal. The returned users can use it normally.

(9) A low voltage alarm appears when the Nokia 8250 calls.

Troubleshooting: When overhauling, worry about the power amplifier. Remove the power amplifier and connect the dummy antenna. At the input end of the power amplifier, dial 1 12 to measure that the power control end of the power amplifier has a jump of about 2.75V before it is normal. Then, the square wave normal voltage V80 1C = 2V, and B =0.8V of the front-end amplifier tube is measured by a spectrum analyzer. The variable values of C86 1 and C862 detected by subsequent components will not lead to the amplification of V80 1.

Nokia service hotline: 4008 800 123