PCB Tech - Do you know PCBA circuit board integrated circuit replacement skills?

Do you know PCBA circuit board integrated circuit replacement skills?
1. Direct substitution

Direct substitution refers to directly replacing the original IC with other ICs without any modification, and the main performance and indicators of the machine will not be affected after the substitution.

The replacement principle is: the function, performance index, package form, pin usage, pin number and interval of the replacement IC are the same. The same function of the IC not only refers to 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: IC is placed in the image, TA7607 and TA7611, the former is reverse high-amp AGC, the latter is forward high-amp AGC, so it cannot be replaced directly. In addition, there are also ICs that output different polarity AFT voltages and output synchronization pulses of different polarity. ICs cannot be directly replaced. Even products of the same 270_f8 company or manufacturer should be distinguished. Performance indicators refer to the IC's main electrical parameters (or main characteristic curve), maximum power dissipation, maximum operating voltage, frequency range, and various signal input and output impedance parameters that are similar to those of the original IC. Substitutes with low power should increase the heat sink.

1. Substitution of the same type of IC

The replacement of the same type of IC is generally reliable. When installing the integrated circuit, be careful not to make a mistake in the direction, otherwise, the integrated circuit is likely to be burned when the power is turned on. Some single in-line power amplifier ICs have the same model, function, and characteristic, but the direction of the pin arrangement order is different. For example, the dual-channel power amplifier IC LA4507, its pins are divided into "positive" and "reverse", and the initial pin markings (color dots or pits) are in different directions; there is no suffix or IC with the suffix "R", etc., For example, M5115P and M5115RP.

2. Substitution of different types of IC

⑴ Substitution of ICs with the same prefix letter and different numbers. As long as the functions of the pins are exactly the same, the internal circuits and electrical parameters of this substitution are slightly different, and they can also be directly substituted for each other. For example: IC LA1363 and LA1365 are put in the sound, the latter adds a Zener diode inside IC pin 5. compared with the former, and the others are exactly the same.

⑵ Substitution of ICs with different prefix letters and the same number. In general, the prefix letter indicates the manufacturer and the category of the circuit. The numbers after the prefix letter are the same, and most of them can be replaced directly. But there are also a few, although the numbers are the same, but the functions are completely different. For example, HA1364 is a sound IC, while uPC1364 is a color decoding IC; 4558, 8-pin is an operational amplifier NJM4558, 14-pin is a CD4558 digital circuit; so the two cannot be replaced at all.

⑶ Type prefix letters and numbers are different for IC substitution. Some manufacturers import unpackaged IC chips and then process them into products named after the factory. Another example is to improve products in order to improve certain parameters. 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 substitution refers to a method in which an IC that cannot be directly replaced is a method of slightly modifying the peripheral circuit, changing the original pin arrangement or adding or removing individual components, etc., to make it a replaceable IC.

Substitution principle: The IC used in the replacement can have different functions and different appearances from the original IC, but the functions should be the same and the characteristics should be similar; the performance of the original machine should not be affected after the replacement.

1. Substitution of different packaged ICs

For IC chips of the same type but with different package shapes, only the pins of the new device need to be reshaped according to the shape and arrangement of the pins of the original device. For example, the AFT circuit CA3064 and CA3064E, the former is a circular 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. Dual-row IC AN7114, AN7115 and LA4100, LA4102 are basically the same in package form, and the pins and heat sinks are exactly 180° apart. The aforementioned AN5620 dual in-line 16-pin package with heat sink and TEA5620 dual in-line 18-pin package. 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 in the same way. 9 and 10 feet can be used when connected to ground.

2. Substitution of ICs with the same circuit function but different individual pin functions

The replacement can be carried out according to the specific parameters and instructions of each type of IC. For example, the AGC and video signal output in the TV have the difference between positive and negative polarity, as long as the inverter is connected to the output terminal, it can be replaced.

3. Substitution of ICs of the same type but different pin functions

This kind of substitution needs to change the peripheral circuit and pin arrangement, which requires certain theoretical knowledge, complete information, and rich practical experience and skills.

4. Some empty feet should not be grounded without authorization

Some of the lead pins in the internal equivalent circuit and application circuit are not marked. When there are empty lead pins, they should not be grounded without authorization. These lead pins are alternate or spare pins, and sometimes they are also used as internal connections.

5. Replace IC with discrete components

Sometimes discrete components can be used to replace the damaged part of the IC to restore its function. Before replacement, you should understand the internal function principle of the IC, the normal voltage of each pin, the waveform diagram and the working principle of the circuit composed of the peripheral components. Also consider:

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

(2) Whether the signal processed by the peripheral circuit can be connected to the next stage inside the integrated circuit for reprocessing (the signal matching during connection should not affect its main parameters and performance). If the intermediate amplifier IC is damaged, from the typical application circuit and internal circuit, it is composed of audio intermediate amplifier, frequency discrimination and audio amplifier stages. Signal injection method can be used to find the damaged part. If the audio amplifier part is damaged, discrete components can be used instead.

6. Combination substitution

Combination substitution is to reassemble the undamaged circuit parts of multiple ICs of the same model into a complete IC to replace the poorly functioning IC. It is very applicable when the original IC is not available. However, it is required that the intact circuit inside the IC used must have interface pins.

Note: The key to indirect substitution is to find out the basic electrical parameters, internal equivalent circuit, the function of each pin, and the connection relationship between the IC and external components of the two ICs that are substituted for each other. Pay attention to the actual operation:

⑴The numbering sequence of integrated circuit pins should not be connected wrongly;

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

⑶ The power supply voltage should be consistent with the replaced IC. If the power supply voltage in the original circuit is high, try to step down; if the voltage is low, it depends on whether the replacement IC can work.

⑷After the substitution, the quiescent working current of the IC should be measured. If the current is much larger than the normal value, it means that the circuit may be self-excited. At this time, decoupling and adjustment must be carried out. If the gain is different from the original, the resistance of the feedback resistor can be adjusted;

⑸ After the replacement, the input and output impedance of the IC must match the original circuit; check its drive capability.

⑹ Make full use of the pin holes and leads on the original circuit board when making changes. The external leads are required to be neat and avoid front and back crossing, so as to check and prevent circuit self-excitation, especially to prevent high-frequency self-excitation;

(7) It is best to connect a DC current meter in series in the Vcc loop of the power supply before power-on. The resistance of the step-down resistor is from large to small, and observe whether the change of the total current of the integrated circuit PCB is normal.