How to judge whether an integrated circuit is good or bad and how to disassemble it

The quality of the integrated circuit block can be tested by using a multimeter to measure whether the working voltage, resistance to ground and working current of each pin of the integrated circuit block are normal. The integrated circuit block can also be removed to measure whether the resistance between each pin of the integrated circuit block and the ground is normal. When the integrated circuit block is removed, the resistance to ground of each pin of the external circuit can be measured to see whether it is normal. It should be noted that when replacing the integrated circuit block, attention must be paid to the welding quality and welding time. When replacing the integrated circuit block, it is generally required to replace it with an integrated circuit of the same model and specification. If the original model and specification of the integrated circuit block cannot be found, it can be considered to replace it with an integrated circuit block with similar functions, but it should be noted that when replacing it, the power supply voltage, impedance matching, pin position and peripheral control circuit issues must be clarified.

Method for Recognizing Integrated Circuit Application Circuit Diagrams

1. Integrated circuit application circuit diagram function

Integrated circuit application circuit diagram has the following functions:

① It expresses the external circuit structure and component parameters of each pin of the integrated circuit, thus indicating the complete working condition of a certain integrated circuit

② In some integrated circuit application circuits, the internal circuit block diagram of the integrated circuit is drawn, which is quite convenient for analyzing the integrated circuit application circuit, but this kind of representation method is not common.

③ There are two types of integrated circuit application circuits: typical application circuits and practical circuits. The former can be found in the integrated circuit manual, and the latter appears in practical circuits. These two application circuits are not much different. Based on this feature, the typical application circuit diagram can be used as a reference when there is no actual application circuit diagram. This method is often used in repairs.

④ Generally speaking, the integrated circuit application circuit expresses a complete unit circuit or a circuit system, but in some cases a complete circuit system requires two or more integrated circuits.

2. Characteristics of integrated circuit application circuits Integrated circuit application circuit diagrams have the following characteristics

① Most application circuits do not have internal circuit block diagrams, which is not conducive to diagram reading, especially for beginners to analyze circuit operation. ② For beginners, it is more difficult to analyze the application circuit of integrated circuits than to analyze the circuit of discrete components. This is because they do not understand the internal circuit of integrated circuits. In fact, integrated circuits are more convenient than discrete component circuits for diagram reading and repair. ③ For integrated circuit application circuits, it is more convenient to read diagrams if you have a general understanding of the internal circuit of the integrated circuit and a detailed understanding of the function of each pin. This is because integrated circuits of the same type have regularity. After mastering their commonalities, it is easy to analyze many integrated circuit application circuits with the same function but different models.

3. Methods and precautions for analyzing integrated circuit application circuit diagrams The methods and precautions for analyzing integrated circuits mainly include the following points:

(1) Understanding the function of each pin is the key to understanding the diagram. You can refer to the relevant integrated circuit application manual to understand the function of each pin. After knowing the function of each pin, it is convenient to analyze the working principle of the circuit outside each pin and the function of the components. For example: if you know that pin ① is the input pin, then the capacitor connected in series with pin ① is the input coupling circuit, and the circuit connected to pin ① is the input circuit.

(2) Three ways to understand the function of each pin of an integrated circuit There are three ways to understand the function of each pin of an integrated circuit: one is to consult relevant information; the second is to analyze based on the internal circuit block diagram of the integrated circuit; the third is to analyze based on the external circuit characteristics of each pin in the application circuit of the integrated circuit. The third method requires a good foundation in circuit analysis.

(3) Circuit analysis steps

The steps for analyzing integrated circuit application circuits are as follows:

① DC circuit analysis. This step mainly analyzes the external circuit of the power supply and ground pins. Note: When there are multiple power pins, you need to distinguish the relationship between these power supplies, such as whether it is the power pin of the front stage or the rear stage circuit, or the power pin of the left and right channels; multiple ground pins should also be distinguished in this way. Distinguishing multiple power pins and ground pins is useful for repairs?

②Signal transmission analysis. This step mainly analyzes the external circuit of the signal input pin and output pin. When the integrated circuit has multiple input and output pins, it is necessary to figure out whether it is the output pin of the front stage or the rear stage circuit; for the dual-channel circuit, it is also necessary to distinguish the input and output pins of the left and right channels.

③Analysis of other pins’ external circuits. For example, find out the negative feedback pins, vibration elimination pins, etc. This step of analysis is the most difficult, and beginners need to use pin function data or internal circuit block diagrams.

④ After you have a certain ability to read diagrams, you should learn to summarize the rules of the pin external circuits of various functional integrated circuits and master these rules, which is useful for improving the speed of reading diagrams. For example, the rule of the input pin external circuit is: connected to the output end of the previous circuit through a coupling capacitor or a coupling circuit; the rule of the output pin external circuit is: connected to the input end of the next circuit through a coupling circuit

⑤ When analyzing the signal amplification and processing process of the internal circuit of the integrated circuit, it is best to refer to the internal circuit block diagram of the integrated circuit. When analyzing the internal circuit block diagram, you can use the arrows in the signal transmission line to know which circuits the signal has been amplified or processed, and which pin the signal is finally output from.

⑥ Understanding some key test points of integrated circuits and the rules of DC voltage on pins is very useful for circuit maintenance. The DC voltage at the output of the OTL circuit is equal to half of the DC working voltage of the integrated circuit; the DC voltage at the output of the OCL circuit is equal to 0V; the DC voltages at the two output ends of the BTL circuit are equal, equal to half of the DC working voltage when powered by a single power supply, and equal to 0V when powered by dual power supplies. When a resistor is connected between two pins of an integrated circuit, the resistor will affect the DC voltage on the two pins; when a coil is connected between the two pins, the DC voltages of the two pins are equal. If they are not equal, the coil must be open; when a capacitor or an RC series circuit is connected between the two pins, the DC voltages of the two pins must be unequal. If they are equal, it means that the capacitor has broken down.

⑦ Generally speaking, do not analyze the working principle of the internal circuit of an integrated circuit, as it is quite complicated.

Common integrated circuit pin identification:

Different integrated circuit pins have different identification marks and different identification methods. Mastering these marks and identification methods is extremely important for use, purchase, maintenance and testing.

⒈Notch There is a semicircular or square notch at one end of the IC.

⒉Pit, color spot or metal piece There is a pit, color spot or metal piece in one corner of the IC.

⒊ Bevel and cut corner There is a bevel cut corner on one corner of the IC or the heat sink.

⒋ No identification mark There is no identification mark on the entire IC. Generally, you can face the IC model face to face, look at the model, from the lower left to the right counterclockwise, they are 1, 2, 3...

⒌ ICs with reverse "R" mark Some IC models have "R" at the end, such as HAXXXXA, HAXXXXAR. The electrical performance of the above two ICs is the same. It's just that the pins are opposite to each other. As shown in Figure ⑤.

⒍Metal round shell IC: The pins of this type of IC have different arrangement orders from different manufacturers. You should consult relevant information before use.

⒎Three-terminal voltage regulator IC: Generally, there is no identification mark, and various ICs have different pins. A simple way to judge the quality of commonly used integrated circuits: At first glance: the packaging is exquisite, the model is clearly marked, the handwriting is clear, the trademark and the factory number are complete, and the printing quality is good (some are painted, laser etched, etc.). Such manufacturers have strict quality control during the production and processing. Second inspection: The pins are smooth and shiny, there is no corrosion, plug-in and pull-out marks, the production date is short, and the store is operated by a regular store. Third test: For commonly used digital integrated circuits, in order to protect the input terminal and factory production needs, each input terminal is connected to a diode (reverse connection) to VDD GND. Use the multimeter's diode test gear to measure the diode effect. If the static resistance value between VDD GND is above 20K and less than 1K, it must be bad.

Commonly used analog and linear integrated circuits usually need to be inserted into the application circuit before they can be judged. For safety reasons, it is recommended to first solder the integrated circuit socket with the same pin position to make sure that there are no errors in the peripheral circuits before inserting the integrated circuit. If it is not good, you can ask the merchant to replace it.

IC Replacement Tips

1. Direct Substitution

Direct replacement means replacing the original IC with another IC without any modification, and the replacement does not affect the main performance and indicators of the machine. The replacement principle is: the functions, performance indicators, packaging form, pin usage, pin number and interval of the replacement IC are the same. The same function of the IC not only means the same function; it should also be noted that the logic polarity is the same, that is, the output and input level polarity, voltage, and current amplitude must be the same. For example: the ICs in the image, TA7607 and TA7611, the former is a reverse high-amp AGC, and the latter is a forward high-amp AGC, so they cannot be directly replaced. In addition, there are ICs that output AFT voltages of different polarities and synchronous pulses of different polarities that cannot be directly replaced. Even if they are products of the same company or manufacturer, they should be distinguished. Performance indicators refer to the main electrical parameters (or main characteristic curves), maximum power dissipation, maximum operating voltage, frequency range, and various signal input and output impedances of the IC, which should be similar to the original IC. The heat sink should be enlarged for low-power substitutes.

1. Replacement of ICs of the same model. Replacement of ICs of the same model is generally reliable. When installing the integrated circuit, be careful not to make a mistake in the direction, otherwise the integrated circuit may be burned when powered on. Some single-row in-line amplifier ICs have the same model, function, and characteristics, but the direction of the pin arrangement sequence is different. For example, the dual-channel amplifier IC LA4507 has "positive" and "negative" pins, and the starting pin marking (color dot or pit) has different directions; ICs without suffixes and ICs with suffix "R" such as M5115P and M5115RP.

2. Replacement of different IC models

⑴ Replacement of ICs with the same prefix letters but different numbers. As long as the pin functions are exactly the same and the internal circuits and electrical parameters are slightly different, they can be directly replaced. For example, the LA1363 and LA1365 ICs are placed in the accompaniment. The latter has a voltage regulator diode added to the ⑤th pin of the IC, and the rest are exactly the same.

⑵ Replacement of ICs with different prefix letters but the same numbers. Generally, the prefix letters indicate the manufacturer and the type of circuit. If the numbers after the prefix letters are the same, most of them can be directly replaced. However, there are a few that have the same numbers but completely different functions. For example, HA1364 is a sound IC, while uPC1364 is a color decoding IC; 4558, the 8-pin is an operational amplifier NJM4558, and the 14-pin is a CD4558 digital circuit; therefore, the two cannot be replaced at all.

⑶ Replacement of ICs with different prefix letters and numbers. Some manufacturers import unpackaged IC chips and then process them into products named after their own factories. Others improve products in order to improve certain parameter indicators. These products are often named with different models or distinguished by model suffixes. For example, AN380 and uPC1380 can be directly replaced; AN5620, TEA5620, DG5620, etc. can be directly replaced.

2. Indirect Substitution

Indirect replacement refers to a method of making an IC that cannot be directly replaced by slightly modifying the peripheral circuit, changing the original pin arrangement or adding or removing individual components, etc. to make it a replaceable IC. Replacement principle: The replacement IC can have different pin functions and appearances from the original IC, but the functions must be the same and the characteristics must be similar; the replacement should not affect the performance of the original IC.

1. Replacement of ICs with different packages. For IC chips of the same type but with different package shapes, when replacing, just reshape the pins of the new device according to the shape and arrangement of the pins of the original device. For example, AFT circuit CA3064 and CA3064E, the former is a round package with radial pins; the latter is a dual in-line plastic package. The internal characteristics of the two are exactly the same, and they can be connected according to the pin function. The dual-row IC AN7114, AN7115 and LA4100, LA4102 have basically the same package form, and the pins and heat sinks are exactly 180° apart. The AN5620 dual in-line 16-pin package with heat sink and the TEA5620 dual in-line 18-pin package mentioned above, pins 9 and 10 are located on the right side of the integrated circuit, which is equivalent to the heat sink of AN5620. The other pins of the two are arranged the same. Connect pins 9 and 10 and ground them for use.

2. Replacement of ICs with the same circuit functions but different functions of individual pins can be carried out according to the specific parameters and instructions of each model IC. For example, the AGC and video signal outputs in TV sets have positive and negative polarities, and they can be replaced by adding an inverter to the output end.

3. Replacement of ICs of the same type but with different pin functions. This replacement requires changes to the peripheral circuit and pin arrangement, and therefore requires certain theoretical knowledge, complete information, and rich practical experience and skills.

4. Some empty pins should not be grounded without authorization. Some lead pins in the internal equivalent circuit and application circuit are not marked. When you encounter empty lead pins, you should not ground them without authorization. These lead pins are replacement or spare pins, and are sometimes used as internal connections.

5. Replace IC with discrete components Sometimes you can use discrete components to replace the damaged part of the IC to restore its function. Before replacing, you should understand the internal function principle of the IC, the normal voltage of each lead pin, the waveform diagram, and the working principle of the circuit composed of peripheral components. At the same time, you should also consider:

⑴ Whether the signal can be taken out from the IC and connected to the input end of the peripheral circuit:

⑵ Can the signal processed by the peripheral circuit be connected to the next level inside the integrated circuit for further processing (the signal matching during connection should not affect its main parameters and performance). If the IF amplifier IC is damaged, from the typical application circuit and internal circuit, it is composed of the accompanying audio IF amplifier, frequency discrimination and audio amplifier stage. The signal injection method can be used to find the damaged part. If the audio amplifier part is damaged, it can be replaced with discrete components.

6. Combination replacement Combination replacement is a method of combining the undamaged circuit parts of multiple ICs of the same model into a complete IC to replace the IC with poor function. It is very suitable when the original IC is not available. However, it is required that the intact circuit inside the IC must have interface pins. The key to indirect replacement is to find out the basic electrical parameters of the two ICs to be replaced, the internal equivalent circuit, the function of each pin, and the connection relationship between the IC and external components.

Please pay attention to the following points during actual operation:

⑴ The numbering sequence of the integrated circuit pins should not be wrong

⑵ In order to adapt to the characteristics of the replaced IC, the components of the peripheral circuits connected to it must be changed accordingly;

⑶ The power supply voltage must match the replaced IC. If the power supply voltage in the original circuit is high, try to reduce it. If the voltage is low, see if the replaced IC can work.

⑷After replacement, measure the static working current of IC. If the current is much larger than the normal value, it means that the circuit may be self-excited. In this case, decoupling and adjustment are required. If the gain is different from the original, adjust the feedback resistor value.

⑸ The input and output impedance of the replaced IC must match that of the original circuit; check its driving capability.

(6) When making changes, make full use of the pin holes and leads on the original circuit board. The external leads must be neat and avoid crossing each other in order to check and prevent circuit self-excitation, especially high-frequency self-excitation.

(7) Before power is turned on, it is best to connect a DC ammeter in series in the power supply Vcc loop, and observe whether the change of the total current of the integrated circuit is normal by decreasing the resistance of the voltage-dropping resistor from large to small.

How to disassemble an integrated circuit

When repairing a circuit, it is often necessary to remove the integrated circuit from the printed circuit board. Since the integrated circuit has many pins and they are densely packed, it is difficult to remove it, and sometimes it will damage the integrated circuit and the circuit board. Here are some effective integrated circuit removal methods for your reference.

Desoldering machine desoldering method:

Using a solder sucker to disassemble the integrated block is a common professional method. The tool used is an ordinary dual-purpose electric soldering iron with a power of more than 35W. When disassembling the integrated block, just place the heated dual-purpose electric soldering iron head on the pin of the integrated block to be disassembled, and wait for the solder at the solder point to melt and be sucked into the fine tin. After the solder of all the pins is sucked, the integrated block can be removed. Medical hollow needle disassembly method: Take a few medical hollow needles of size 8 to 12. When using, it is best that the inner diameter of the needle just covers the pin of the integrated block. When disassembling, use a soldering iron to melt the solder of the pin, cover the pin with the needle in time, then remove the soldering iron and rotate the needle, and pull out the needle after the solder solidifies. In this way, the pin is completely separated from the printed circuit board. After all the pins are done in this way, the integrated block can be easily removed.

Disassembly method using soldering iron brush:

This method is simple and easy to use. All you need is a soldering iron and a small brush. When disassembling the integrated block, heat the soldering iron first. When the soldering iron reaches the melting temperature and melts the solder on the pins, use the brush to sweep away the melted solder. This will separate the pins of the integrated block from the printed circuit board. This method can be done by pin or row. Finally, use pointed tweezers or a small "S" screwdriver to pry off the integrated block.

Add solder melting disassembly method:

This method is a hassle-free method. Just add some solder to the pins of the integrated block to be disassembled, so that the solder joints of each column of pins are connected, which is conducive to heat transfer and easy to disassemble. When disassembling, use a soldering iron to heat each column of pins and pry them with sharp tweezers or a small "S" screwdriver. Heat the two columns of pins alternately until they are removed. Generally, each column of pins can be removed after heating twice.

Multi-strand copper wire detinning method:

It is to use multi-strand copper core plastic wire, remove the plastic outer skin, and use multi-strand copper core wire (short wire ends can be used). Before use, apply rosin alcohol solution to the multi-strand copper core wire, and after the electric soldering iron is heated, put the multi-strand copper core wire on the manifold pin to heat it, so that the solder on the pin will be absorbed by the copper wire, and the part that absorbs the solder can be cut off. Repeat several times to absorb all the solder on the pin. If conditions permit, you can also use the braided wire in the shielded wire. As long as the solder is absorbed, use tweezers or a small "one" screwdriver to gently pry it, and the manifold can be removed.

Frequency Converter Maintenance Training

Integrated circuit testing method (using only a multimeter as a testing tool)

Although there are ways to replace integrated circuits, disassembly is troublesome after all. Therefore, before disassembly, you should accurately determine whether the integrated circuit is really damaged and the degree of damage to avoid blind disassembly. This article introduces the methods and precautions for off-circuit and on-circuit detection of integrated circuits using only a multimeter as a detection tool. The four methods of on-circuit detection described in the article (measurement of DC resistance, voltage, AC voltage and total current) are practical and commonly used detection methods in amateur maintenance. Here, we also hope that everyone can provide other practical (integrated circuit and component) identification and detection experience.

1. Off-circuit detection This method is performed when the IC is not soldered into the circuit. Generally, a multimeter can be used to measure the forward and reverse resistance values ​​between each pin corresponding to the ground pin, and compared with the intact IC.

2. In-circuit detection This is a detection method that uses a multimeter to detect the in-circuit DC resistance of each IC pin (IC in the circuit), AC and DC voltage to ground, and total working current. This method overcomes the limitation of the substitution test method that requires a replaceable IC and the trouble of disassembling the IC, and is the most commonly used and practical method for detecting ICs.

1. In-circuit DC resistance detection method This is a method that uses the ohm range of a multimeter to directly measure the forward and reverse DC resistance values ​​of each IC pin and peripheral components on the circuit board, and compares them with normal data to discover and determine the fault. Pay attention to the following three points when measuring:

(1) Disconnect the power supply before measuring to avoid damage to the meter and components during testing.

(2) The internal voltage of the resistance block of the multimeter should not be greater than 6V, and the best range is R×100 or R×1k. (3) When measuring IC pin parameters, pay attention to the measurement conditions, such as the model of the measured device, the position of the slider of the potentiometer related to the IC, etc., and also consider the quality of the peripheral circuit components.

2. DC working voltage measurement method This is a method of measuring the DC voltage and the working voltage of peripheral components using the DC voltage block of a multimeter under normal conditions; detecting the DC voltage value of each IC pin to ground and comparing it with the normal value, thereby narrowing the fault range and finding the damaged component. Pay attention to the following eight points when measuring:

(1) The multimeter must have a sufficiently large internal resistance, at least 10 times greater than the resistance of the circuit being measured, to avoid large measurement errors.

(2) Normally, turn each potentiometer to the middle position. If it is a TV, the signal source should use a standard color bar signal generator.

(3) Anti-slip measures should be taken for the test leads or probes. Any instantaneous short circuit can easily damage the IC. The following method can be used to prevent the test leads from slipping: Take a section of bicycle valve core and put it on the tip of the test lead, and extend it about 0.5mm beyond the tip of the test lead. This can not only make the tip of the test lead contact well with the tested point, but also effectively prevent slipping, and will not cause a short circuit even if it touches an adjacent point.

(4) When the measured voltage of a certain pin is inconsistent with the normal value, the quality of the IC should be judged based on whether the pin voltage has a significant impact on the normal operation of the IC and the corresponding changes in other pin voltages.

(5) IC pin voltage will be affected by peripheral components. When peripheral components leak, short-circuit, open-circuit or change value, or the peripheral circuit is connected to a variable resistance potentiometer, the position of the potentiometer sliding arm will change, which will cause the pin voltage to change.

(6) If the voltage of the IC's pins is normal, the IC is generally considered to be normal. If the voltage of some of the IC's pins is abnormal, you should start from the point where the voltage deviates most from the normal value and check whether there are any faults in the peripheral components. If there are no faults, the IC is likely to be damaged.

(7) For dynamic receiving devices, such as televisions, the voltages of the IC pins are different when there is a signal or not. If the voltage of a pin that should not change changes greatly, and the voltage of a pin that should change with the signal size and the position of the adjustable component does not change, it can be determined that the IC is damaged.

(8) For devices with multiple working modes, such as video recorders, the voltages of each IC pin are different in different working modes. 3. AC working voltage measurement method In order to grasp the changes in the IC AC signal, a multimeter with a dB jack can be used to approximately measure the AC working voltage of the IC. When testing, the multimeter is set to the AC voltage block and the positive probe is inserted into the dB jack; for multimeters without a dB jack, a 0.1-0.5μF DC blocking capacitor needs to be connected in series with the positive probe. This method is suitable for ICs with relatively low operating frequencies, such as the video amplifier stage and field scanning circuit of a television. Since these circuits have different inherent frequencies and waveforms, the measured data are approximate 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 IC power supply line. Since most of the internal IC is directly coupled, when IC is damaged (such as a PN junction breakdown or open circuit), it will cause the saturation and cutoff of the subsequent stage, causing the total current to change. Therefore, the quality of IC can be judged by measuring the total current. It is also possible to measure the voltage drop of the resistor in the power supply path and calculate the total current value using Ohm's law. The above detection methods have their own advantages and disadvantages. In practical applications, it is best to combine various methods and use them flexibly.