1、 Methods of checking the plate:
1. Observation method: whether there is burning paste, burning, blistering, board surface broken line, socket corrosion.
2. Meter measurement: + 5V, GND resistance is too small (below 50 ohm).
3. Power on inspection: for the damaged board, slightly increase the voltage by 0.5-1v. After starting up, use the IC on the washboard to make the chip in question heat up and sense it.
4. Logic pen check: check whether or not the signal is strong or weak at each end of the IC input, output and control pole.
5. Identify the major work areas: most boards have clear division of labor on the area, such as: control area (CPU), clock area (crystal oscillator) (frequency division), background picture area, action area (people, aircraft), sound generation and synthesis area, etc. This is very important for the in-depth maintenance of the computer board.
2、 Troubleshooting:
1. the chip suspected shall be checked for signal (wave type) at input and output terminals according to the instructions in the manual. If there is any input or output, check whether the IC control signal (clock) is available, if any, the IC may be damaged * extremely, without control signal, and track the previous pole until the damaged IC is found.
2. the same model can be selected for the time being. Or the IC with the same program content is on the top, and the power on observation is improved to confirm whether the IC is damaged.
3. find short route by tangent line and jumper method: if some signal lines and ground wires, + 5V or other IC should not be connected, the short circuit of the foot should not be connected can be cut off and then measured. It is judged whether the IC problem or the board routing problem is the problem, or whether the phenomenon picture is better or not, and the IC is judged whether the picture is better by welding the signal from other IC to the IC with wrong wave type.
4. comparison method: find a good computer board with the same content to check whether the IC is damaged by measuring the pin wave type and its number of corresponding IC.
5. test IC with ictest software in all-03 / 07 (expro-80 / 100, etc.) of microcomputer.
3、 Chip removal method:
1. foot cutting: no damage to the board, no regeneration and utilization.
2. tin dragging method: solder the IC foot on both sides, use high temperature soldering iron to drag back and forth, and then lift out IC (easy to damage the board, but it can preserve the test IC).
3. barbecue method: bake on alcohol lamp, gas stove and electric stove, and then the IC will be produced after tin dissolves on the board (not easy to master).
4. tin pot method: it is used as special tin pot on electric furnace. After tin dissolves, the IC to be unloaded on the board is immersed in tin pot, and IC can be produced without damage to the board, but the equipment is not easy to make.
5. electric hot air gun: use special electric hot air gun to discharge the chip, blow the IC pin part to be unloaded, and the IC after tin melting can be lifted out.
As a professional hardware maintenance, board maintenance is one of the most important projects. Take a faulty motherboard, how to determine which specific components have problems?
The main causes of the main failure of the main board are:
1. human fault: the power plug-in and dial i/o card, and improper force during the installation of board card and plug cause damage to interface, chip, etc.
2. poor environment: static electricity often causes the chip on the main board (especially CMOS chip) to be broken down. In addition, when the main board encounters power supply damage or peak pulse generated by power grid voltage, it will often damage the chip near the power supply plug of the system board. If the motherboard is covered with dust, it will also cause short circuit of signal.
3. device quality problems: damage caused by poor quality of chips and other devices.
Cleaning first of all, it is important to note that dust is one of the biggest enemies of the motherboard. It is better to pay attention to dust prevention. Brush the dust on the main board gently. In addition, some card and chip on the motherboard adopt the pin form, which often contacts badly due to pin oxidation. The surface oxide layer can be removed by eraser and re plugged. Of course, we can use trichloroethane – which has good volatilization performance and is one of the liquids that clean the motherboard.
Also is in sudden power down, should turn off the computer immediately, lest suddenly call to burn the main board and power. technological process. BIOS is set improperly, if the frequency is too high… Jumper can be cleared and reset. If the BIOS is corrupted, such as virus intrusion… The BIOS can be rewritten. Because BIOS can not be measured by instrument, it is in the form of software. In order to eliminate all possible causes of the main board problems, it is better to brush the BIOS on the main board.
There are many reasons for the fault of the plug-in and exchange host system, such as the failure of the main board itself or the various card failures on the i/o bus can cause the system to operate abnormally. The plug-in maintenance method is a simple method to determine the fault in the main board or i/o equipment. This method is to shut down and pull out the plug-in board one by one, and start up every board to observe the operation status of the machine. Once the main board is running normally after pulling out a certain block, the fault is caused by the fault of the plug-in board or the corresponding i/o bus slot and load circuit fault.
If the system starts normally after pulling out all the plug-in boards, the fault is likely to be on the main board. In essence, the exchange method is to exchange the plug-in boards with the same type, bus mode and same function or chips of the same type with each other, and judge the fault location according to the change of the fault phenomenon. This method is mostly used in the maintenance environment which is easy to plug in, such as memory self check error. The same memory chip or memory bar can be exchanged to determine the cause of the failure. To see if you get a defective motherboard, first scan with your eyes to see if there is no burn marks, whether the appearance is damaged, see if the plugs and sockets are slanted, whether the resistance and capacitance pins touch each other, whether the surface is burnt, whether the chip surface is cracked, and whether the copper foil on the main board is burnt. Also check if there are foreign objects falling between the components of the main board. If you have any questions, you can use universal meter to measure it. Touch the surface of some chips. If it is abnormal hot, try a chip.
(1) . if the connection is broken, we can use a knife to wipe the paint at the broken wire, apply wax to the exposed wire, and then we can use the needle to scratch the wax along the line, and then drop silver nitrate solution on it. Then you need to use a universal table to confirm whether the breakpoint is connected. Just one by one, you can connect the breakpoints. Pay attention to one company, never worry, like some places on the motherboard between the distance between the lines is very small, if not good will be short-circuit.
(2) . if it is electrolytic capacitance, you can find the matching replacement. Universal meter and oscilloscope tool are used to measure the power supply of main board components with universal meter and wave device. One is to check whether the main board is supplying power to this part, and then whether the voltage of power supply is normal. Resistance and voltage measurement: power failure includes + 12V, + 5V and + 3.3V power supply and power good signal failure on the main board; Bus fault includes bus itself fault and bus control right fault; The fault of the components includes the resistance, capacitance, IC chip and other component faults. To prevent accidents, measure the resistance between power supply + 5V and ground (GND) on the main board before power up. The easiest way to do this is to measure the resistance between the power pin and the ground of the chip. When the power plug is not inserted, the resistance shall be 300 Ω generally and the minimum shall not be less than 100 Ω. Measure the reverse resistance value again, which is slightly different, but it cannot be too different. If the positive and negative resistance is very small or close to the conduction, it indicates that there is a short circuit, and the reason for the short circuit should be checked.
The reasons for this kind of phenomenon are as follows:
(1) There are broken down chips on the system board. Generally speaking, such faults are difficult to be eliminated. For example, the + 5V of TTL chip (LS series) is connected together, and solder on the + 5V pin can be sucked off to make it suspended, and the fault sheet can be found out by measuring one by one. If the method of cutting is adopted, it will affect the life of the motherboard.
(2) There is a damaged resistance capacitance on the board.
(3) There are conductive debris on the board.
After troubleshooting the short circuit, plug in all i/o cards and measure whether + 5V and + 12V are short to the ground. Especially, whether + 12V collides with surrounding signals. When there is a good motherboard of the same model, the suspect points on the board can be measured by measuring the resistance value. By comparing, the chip failure can be found quickly. When none of the above steps are effective, the power supply can be plugged in to power up measurement. Generally, measure the + 5V and + 12V of the power supply. When it is found that a voltage value is too far away from the standard, it can be measured by separating or cutting some leads or pulling off some chips. When cutting off a lead or pulling off a chip, if the voltage becomes normal, the components or chips drawn out of the lead are the fault.
The program and diagnostic card diagnosis can achieve the double effect of half the effort by randomly diagnosing program, special maintenance diagnostic card and self-made special diagnostic program according to various technical parameters (such as interface address). The principle of program test is to send data and commands by software, and recognize the fault location by reading the circuit state and the state of a chip (such as register).
This method is often used to check the fault of various interface circuits and circuits with address parameters. But the premise of this method is that CPU and base bus are running normally, can run relevant diagnostic software, and can run diagnostic cards installed on i/o bus slot. The diagnostic procedure should be strict and comprehensive, and it can make some key parts appear regular signals, and can repeatedly test the occasional faults and display the record error.
4、 The method of judging IC IC
1. Not on the road detection
This method is carried out when IC is not welded into the circuit. Generally, multimeter can be used to measure the positive and negative resistance values between the pins corresponding to the grounding pins. The second is to compare with the intact IC. In-circuit detection is a method to detect the DC resistance of each pin in the circuit (IC in the current circuit), the AC / DC voltage and the total working current through multimeter. This method overcomes the limitation of replaceable IC and the trouble of removing IC in the substitution test method. It is the most common and practical method to detect IC.
2. DC working voltage measurement
This is a kind of measurement of DC supply voltage and working voltage of peripheral components with Multimeter DC voltage block when it is powered on; The DC voltage of each pin of IC is detected and compared with the normal value, and then the fault range is compressed to produce the damaged components.
The following eight methods should be noted in measurement:
(1) . the multimeter shall have enough internal resistance, less than 10 times greater than the resistance of the circuit under test, so as to avoid large measurement error.
(2) . usually turn the potentiometer to the middle position. If it is a TV set, the signal source shall adopt the standard color bar signal generator.
(3) , the probe or probe shall take anti slip measures. IC is easily damaged due to any short circuit in any moment. The following methods can be taken to prevent the probe from sliding: take a bicycle and use the valve core to cover the probe tip, and grow the probe tip about 0.5mm. This can not only make the probe tip contact with the tested point well, but also effectively prevent slipping, even if it touches the adjacent point, it will not be short circuited.
(4) When the voltage of a pin is not in conformity with the normal value, the analysis shall be made according to whether the voltage of the pin has an important influence on the normal operation of the IC and the corresponding changes of other pin voltage, so as to judge the IC quality.
(5) The IC pin voltage will be affected by peripheral components. When the leakage, short circuit, open circuit or variable value occurs in peripheral components, or the peripheral circuit is connected with a potentiometer with variable resistance value, the position of the potentiometer sliding arm is different, which will change the voltage of the pin.
(6) If the voltage of each pin of IC is normal, it is generally considered that IC is normal; If the voltage of the IC part of the pin is abnormal, it should start from the maximum deviation from the normal value, check whether the peripheral components are faulty, if there is no fault, the IC will be likely to be damaged.
(7) For dynamic receiving devices, such as TV sets, the voltage of each pin of IC is different when there is signal or not. If it is found that the pin voltage should not change but change greatly, the IC damage can be determined if the change is not changed with the signal size and the different position of the adjustable element.
(8) For devices with various working modes, such as video recorder, the voltage of IC pins is different under different working modes.
3. AC working voltage measurement method
In order to master the change of IC AC signal, the AC working voltage of IC can be measured by using a multimeter with DB Jack. When testing, the multimeter is placed in AC voltage, and the positive probe is inserted into DB socket; For multimeter without DB jack, a 0.1-0.5 meter should be connected in series with the positive probe μ F. direct capacitance. This method is suitable for IC with low frequency, such as video amplification level and field scanning circuit of TV. Because of the different natural frequency and different waveform of these circuits, the measured data are approximate values and can only be used for reference.
4. total current measurement method
This method is a method to judge the quality of IC by detecting the total current of the IC power supply. Because the most of IC is directly coupled, the saturation and cutoff of the later stage will be caused when the IC is damaged (such as a PN junction breakdown or open circuit), which will change the total current. So the IC can be judged by measuring the total current. The voltage drop of resistance in power supply path can also be measured and the total current value can be calculated by Ohm law.