How to judge whether the IC chip is good or bad?

How to accurately judge whether the power supply IC chip in the circuit is good or bad in daily circuit maintenance work is an important work content for repairing TV, audio, and video equipment. The fault still exists, so it is a compulsory course for every maintainer to make a correct judgment on the integrated circuit.

1. Check board method:

1. Observation method: Whether there is burning, blowing, blistering, disconnection on the board surface, and corrosion of the socket.

2. Meter method: Whether the resistance of +5V and GND is too small (below 50 ohms).

3. Power-on inspection: If the board is clearly broken, you can slightly increase the voltage by 0.5-1V, and rub the IC on the board with your hands after turning on the power to make the problematic chip heat up, so as to sense it.

4. Logic pen inspection: Check whether the signals are present or not, and whether they are strong or weak at each terminal of the IC input, output, and control poles that are suspected.

5. Identify the major work areas: most boards have a clear division of labor in the area, such as: control area (CPU), clock area (crystal oscillator) (frequency division), background screen area, action area (characters, aircraft), sound generation and synthesis District etc. This is very important for the in-depth repair of the computer board.

2. Troubleshooting method:

1. For the suspected chip, according to the instructions in the manual, first check whether there is a signal (wave type) at the input and output terminals. If there is no input, then check whether there is any control signal (clock) of the IC. If there is, the IC is broken. The possibility is extremely high, without a control signal, trace to its previous pole until the damaged IC is found.

2. If you find it, don't take it off from the pole temporarily, you can choose the same model. Or the IC with the same program content is carried on the back, and it is turned on to see if it is better, so as to confirm whether the IC is damaged.

3. Use tangent and jumper methods to find short-circuit lines: If you find that some signal lines and ground lines, +5V or other pins that should not be connected to multiple ICs are short-circuited, you can cut off the line and measure again to determine whether it is an IC problem or a board wiring problem. , or borrow signals from other ICs and weld them to ICs with wrong waveforms to see if the phenomenon picture is getting better, and judge whether the IC is good or bad.

4. Comparison method: Find a good computer board with the same content and measure the pin waveform and number of the corresponding IC to confirm whether the IC is damaged.

5. Use the ICTEST software in the microcomputer universal programmer (ALL-03/07) (EXPRO-80/100, etc.) to test the IC.

3. Chip removal method:

1. Foot cutting method: no damage to the board, no recycling.

2. Tin dragging method: Solder both sides of the IC feet with tin, drag it back and forth with a high-temperature soldering iron, and remove the IC at the same time (the board is vulnerable to damage, but the IC can be preserved for testing).

3. Barbecue method: Barbecue on alcohol lamps, gas stoves, and electric stoves, and take out the IC after the tin on the board melts (difficult to grasp).

4. Tin pot method: Make a special tin pot on the electric furnace. After the tin is melted, immerse the IC to be removed from the board into the tin pot. The IC can be removed without damaging the board, but the equipment is not easy to manufacture.

5. Electric heat gun: Use a special electric heat gun to remove the chip, blow the part of the IC pin to be removed, and then the tinned IC can be removed (note that the air gun should be shaken when blowing the board, otherwise the computer board will be blown. But the cost of air gun is high)

As a professional hardware maintenance, board maintenance is one of the most important items. Take a faulty motherboard, how to judge which component is faulty? Main causes of motherboard failure

1. Human failure:

Plugging and unplugging I/O cards with power on, and improper force when installing board cards and plugs cause damage to interfaces, chips, etc.

2. Bad environment:

Static electricity often causes chips on the motherboard (especially CMOS chips) to be broken down. In addition, when the motherboard encounters power supply damage or instantaneous spikes in the grid voltage, it will often damage the chips near the power supply plug of the system board. If the motherboard is covered with dust, it will also cause signal short circuit and so on.

3. Device quality issues:

Damage due to poor quality chips and other components. LED circuit board manufacturers must first remind attention that dust is one of the biggest enemies of the motherboard. It is best to pay attention to dust prevention, and use a brush to gently brush off the dust on the motherboard. In addition, some cards and chips on the motherboard are in the form of pins, which often cause poor contact due to oxidation of the pins. The oxide layer on the surface can be removed with an eraser, and reconnected. Of course, we can use trichloroethane - it has good volatility and is one of the liquids for cleaning the motherboard.

For computer board maintenance, it is necessary to turn off the computer immediately in case of sudden power failure, so as not to burn the main board and power supply due to another sudden call. Due to improper BIOS settings, if overclocking...you can clear the jumper and reset it. If the BIOS is damaged, such as virus intrusion..., the BIOS can be rewritten. Because the BIOS cannot be tested by an instrument, it exists in the form of software. In order to eliminate all possible causes of problems with the motherboard, it is best to refresh the BIOS of the motherboard.

?Plug and swap. There are many reasons for the failure of the host system, for example, the failure of the motherboard itself or the failure of various cards on the I/O bus can cause the system to run abnormally. Using the plug-and-plug maintenance method is a simple method to determine whether the fault is on the motherboard or I/O device. The method is to turn off the power and pull out the plug-in boards one by one. After each board is pulled out, start the machine to observe the running status of the machine. Once a certain board is pulled out, the main board runs normally, and the cause of the fault is the fault of the plug-in board or the corresponding I/O bus slot. and load circuit failure. If the system still does not start normally after pulling out all the plug-in boards, the fault is likely to be on the main board. The exchange method is essentially to exchange the same type of plug-in board, plug-in board with the same bus mode and the same function, or the same type of chip with each other, and judge the fault according to the change of the fault phenomenon. This method is mostly used in easy-plug maintenance environments, such as memory self-test errors, and the same memory chips or memory sticks can be exchanged to determine the cause of the failure. Watching Get a faulty motherboard and scan it with your eyes first to see if there are no traces of burnt out, whether the appearance is damaged, whether the plugs and sockets are skewed, whether the resistance and capacitor pins touch each other, whether the surface is burnt, and whether the chip is damaged. Whether the surface is cracked, whether the copper foil on the motherboard is blown. Also check to see if any foreign objects have fallen between the components on the motherboard. When in doubt, you can use a multimeter to measure it. Touch the surface of some chips, if they are abnormally hot, try another chip.

(1). If the connection is broken, we can use a knife to scrape off the paint on the broken wire, apply wax to the exposed wire, and then use a needle to scratch the wax along the wire, and then drip silver nitrate solution on it. Then use a multimeter to confirm whether the breakpoint is connected. Just one by one, just connect the breakpoints. Be careful to connect them one by one, don't be impatient, as the distance between the traces on the motherboard is very small, if it is not done well, it will be short-circuited.

(2). If it is an electrolytic capacitor, you can find a matching one and replace it. Multimeter and oscilloscope tools Use the multimeter and oscilloscope to measure the power supply of each component on the motherboard. One is to check whether the motherboard supplies power to this part, and the other is whether the voltage of the power supply is normal. Resistance and voltage measurement: power supply faults include +12V, +5V and +3.3V power supplies and Power Good signal faults on the main board; bus faults include faults of the bus itself and faults caused by bus control rights; component faults include resistors, capacitors, integrated circuit chips and Failure of other components. To prevent accidents, measure the resistance value between the power supply +5V and ground (GND) on the motherboard before powering on. The easiest way is to measure the resistance between the chip's power supply pin and ground. When the power plug is not plugged in, the resistance should generally be 300Ω, and the minimum should not be lower than 100Ω. Measure the reverse resistance value again, there is a slight difference, but the difference cannot be too large. If the forward and reverse resistance is very small or close to conduction, it means that there is a short circuit, and the cause of the short circuit should be checked.

There are several reasons for this phenomenon:

1) There are broken chips on the system board. Generally speaking, such faults are difficult to eliminate. For example, the +5V of the TTL chip (LS series) is connected together, the solder on the +5V pin can be sucked off, it is suspended, and the faulty chip can be found by measuring one by one. If the secant method is adopted, it will inevitably affect the life of the motherboard.

(2) There are damaged resistors and capacitors on the board.

(3) There are conductive debris on the board. After eliminating the short-circuit fault, plug in all I/O cards and measure whether +5V, +12V and ground are short-circuited. Especially whether +12V collides with surrounding signals. When you have a good motherboard of the same type on hand, you can also use the method of measuring the resistance value to test the suspicious points on the board. Through comparison, you can quickly find out where the chip is faulty. When none of the above steps work, you can plug in the power supply and power on for measurement. Generally measure +5V and +12V of the power supply. When it is found that a certain voltage value deviates too far from the standard, the voltage can be measured by separating or cutting some leads or unplugging some chips. When a certain lead wire is cut off or a certain chip is unplugged, if the voltage becomes normal, then the components led out by this lead wire or the unplugged chip is the fault. Diagnosis of programs and diagnostic cards Through random diagnostic programs, special maintenance diagnostic cards and according to various technical parameters (such as interface addresses), self-compiled special diagnostic programs to assist hardware maintenance can achieve twice the result with half the effort. The principle of the program test method is to use software to send data and commands, and to identify the fault location by reading the line status and the status of a certain chip (such as a register). This method is often used to check various interface circuit faults and various circuits with address parameters. But the premise of the application of this method is that the CPU and the base bus are running normally, the relevant diagnostic software can be run, and the diagnostic card installed on the I/O bus slot can be run. The diagnostic program written should be strict, comprehensive and targeted*, which can make regular signals appear in some key parts, can conduct repeated tests on occasional faults, and can display and record error conditions.

How to judge whether the power IC chip in the integrated circuit is normal?

1. First of all, you must master the purpose of the IC in the circuit, the internal structure principle, the main electrical characteristics, etc., and analyze the internal electrical schematic diagram if necessary. In addition to these, if there are DC voltages, waveforms, and forward and reverse DC resistance values of each pin to ground, then more favorable conditions are provided for the judgment before inspection.

2. Then judge its location according to the fault phenomenon, and then find the faulty component according to the location. Sometimes multiple judgment methods are needed to prove whether the device is indeed damaged.

3. Generally, there are two ways to check and judge the IC in the circuit: one is off-line judgment, that is, the judgment that the IC in the circuit is not welded into the printed circuit board. This method is difficult to determine the quality of the IC in the circuit without special instruments and equipment. In general, the DC resistance method can be used to measure the forward and reverse resistance values of each pin corresponding to the grounding pin. And compare with the intact integrated circuit, also can adopt the replacement method to insert suspicious integrated circuit on the position of normal equipment same model integrated circuit to determine its quality. Of course, there are conditions to use the integrated circuit tester to carry out quantitative inspection of the main parameters, so that the use is more guaranteed.

4. There is also online inspection and judgment, that is, the method of judging that the integrated circuit is connected to the printed circuit board. Online judgment is the most practical method for overhauling integrated circuits in TV, audio, and video equipment. The following situations are described:

DC working voltage measurement method: mainly to measure the DC working voltage value of each pin to ground; and then compare it with the nominal value to judge whether the integrated circuit is good or bad. Using the voltage measurement method to judge whether the integrated circuit is good or bad is one of the most commonly used methods in maintenance, but attention should be paid to distinguishing non-faulty voltage errors. When measuring the DC working voltage of each pin of the integrated circuit, if the voltage of individual pins does not match the voltage value marked in the schematic diagram or maintenance technical data, do not rush to conclude that the integrated circuit is damaged, and the following factors should be ruled out first Make sure again.

1) Whether the provided nominal voltage is reliable, because the values marked on some manuals, schematic diagrams and other materials are quite different from the actual voltage, sometimes even wrong. At this time, you should find more relevant information for comparison, and if necessary, analyze the internal schematic diagram and peripheral circuits and then perform theoretical calculations or estimates to prove whether the voltage is wrong.

2) It is necessary to distinguish the nature of the nominal voltage provided, and which working state the voltage belongs to. Because the individual pins of the integrated block change significantly with different injected signals, so you can change the position of the band or the recording and playback switch at this time, and then observe whether the voltage is normal. If the latter is normal, it means that the nominal voltage is a certain working voltage, and this working voltage refers to a certain condition, that is, the measured voltage is different when the working state of the test is different.

3) Pay attention to the change of the pin voltage due to the variable components of the peripheral circuit. When the measured voltage does not match the nominal voltage, it may be because individual pins or peripheral circuits related to the pins are connected to a variable resistance potentiometer or switch (such as volume potentiometer, brightness, contrast, video, fast forward, fast rewind, record playback switch, audio amplitude modulation switch, etc.). The positions of these potentiometers and switches are different, and the pin voltages will be significantly different. Therefore, when a certain pin voltage does not match, the position change of the pin or the potentiometer and switch associated with the pin should be considered. You can turn or pull the head to see if the pin voltage is near the nominal value.

4) To prevent errors caused by measurement. Due to the different internal resistance of the multimeter head or different DC voltage ranges, errors will be caused. In general, the marked DC voltage is tested with the internal resistance of the test instrument greater than 20KΩ/V. When a multimeter with an internal resistance of less than 20KΩ/V is tested, the measured result will be lower than the original marked voltage. In addition, it should also be noted that the voltage measured on different voltage files will be different, especially when using a large range file, the influence of reading deviation is more significant.

5) When the measured voltage of a pin does not match the normal value, it should be analyzed according to whether the pin voltage has an important impact on the normal operation of the IC and the corresponding changes in the voltage of other pins, so as to judge whether the IC is good or bad.

6) If the voltage of each pin of the IC is normal, it is generally considered that the IC is normal; if the voltage of some pins of the IC is abnormal, you should start from the place where the deviation from the normal value is the largest, and check whether the peripheral components are faulty. If there is no fault, the IC is likely to be damaged. damage.

7) For dynamic receiving devices, such as TV sets, the voltage of each pin of the IC is different when there is a signal or not. If it is found that the pin voltage should not change but changes greatly, and that which should change with the signal size and the different positions of the adjustable components does not change, it can be determined that the IC is damaged.

8) For devices with multiple working modes, such as video recorders, the voltage of each pin of the IC is also different under different working modes.

The above points are that when the IC in the circuit is not faulty, the measured result is different from the nominal value for some reason, so in general, a test should be specified when performing the DC voltage or DC resistance test of the integrated block. Conditions, especially when it is to be recorded as actual measured experience data, more attention should be paid to this point. Usually, each potentiometer is turned to the mechanical middle position, and the signal source adopts a standard signal under a certain field strength. Of course, if the position of each function switch can be recorded again, it will be more representative. If the measured individual pin voltage still does not meet the nominal value after the above factors are excluded, further analysis of the reason is required, but there are no more than two possibilities. One is caused by the failure of the integrated circuit itself; the other is caused by the peripheral circuit of the integrated block. Distinguishing these two fault sources is also the key to repairing integrated circuit home appliances.

AC working voltage measurement method: In order to grasp the change of IC AC signal, a multimeter with a dB jack can be used to approximate the IC's AC working voltage. When testing, the multimeter is placed in the AC voltage block, and the positive test lead is inserted into the dB jack; for a multimeter without a dB jack, a 0.1-0.5uF DC blocking capacitor needs to be connected in series with the positive test lead. This method is suitable for ICs with relatively low operating frequency, such as video amplifier stages of TV sets, field scanning circuits, etc. Since the natural frequencies and waveforms of these circuits are different, the measured data are approximate values, or as presence or absence.