PCB News - Indirect Substitution of IC Substitution Techniques in Circuit Board Repair

Indirect Substitution of IC Substitution Techniques in Circuit Board Repair

Indirect substitution refers to the IC board that cannot be directly substituted slightly modify the peripheral circuit, change the original pin arrangement or increase or decrease individual components, etc., to make it a method of replacing the 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 arranged 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 the same, and they can be connected according to the pin function. The dual-row IC AN7114 and AN7115 are basically the same as the LA4100 and LA4102 packages, and the pins and heat sinks are exactly 180° apart. The aforementioned A620 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 A620. 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 a difference between positive and negative comparisons, and they can be replaced as long as an inverter is connected to the output end.

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 lead pins in the internal equivalent circuit and application circuit are not marked. When there are empty lead pins, the industrial circuit board repair price should not be grounded without authorization. These lead pins are alternate or spare pins, sometimes 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.

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 the external components of the two ICs that are substituted for each other. Pay attention to the actual operation:

1. The numbering sequence of the integrated circuit pins should not be wrongly connected;

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

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

4. After the replacement, 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, and decoupling and adjustment must be carried out at this time. If the gain is different from the original, the resistance of the feedback resistor can be adjusted;

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

6. Make full use of the pin holes and leads on the original circuit board when making changes, and the external leads must 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. Before power on, connect a DC current meter in series in the Vcc loop of the power supply, reduce the voltage resistance value from large to small, and observe whether the change in the total current of the integrated circuit is normal.

Classification of circuit boards
If you want to do a good job of repairing the circuit board, you must understand the circuit board.

According to the number of layers, circuit boards are divided into three major categories: single-sided, double-sided, and multi-layer circuit boards.

On a basic PCB, the parts are concentrated on one side, and the wires are concentrated on the other side. Because the wires only appear on one side, this kind of PCB is called a single-sided circuit board. Single-sided panels are usually simple to manufacture and low in cost, but the disadvantage is that they cannot be applied to products that are too complex.

Double-sided boards are an extension of single-sided boards. When single-layer wiring cannot meet the needs of electronic products, double-sided boards should be used. There are copper-clad wires on both sides, and the lines between the two layers can be connected through vias to form the required network connections.

Multi-layer board refers to a printed board with more than three conductive pattern layers and insulating material between them laminated at intervals, and the conductive patterns between them are interconnected as required. Multilayer circuit boards are the product of the development of electronic information technology in the direction of high speed, multi-function, large capacity, small volume, thinner and lighter weight.

Circuit boards are divided into flexible boards(FPC), rigid boards (PCB), and rigid-flex boards (FPCB) according to their characteristics.

In circuit board maintenance, if you encounter a short circuit of the public power supply, the fault is often serious, because many devices share the same power supply, and every device using this power supply is suspected of short circuit. If there are not many components on the board, use "hoe the earth" After all, you can find the short-circuit point. If there are too many components, it will depend on luck whether the "hoe the earth" can hoe the situation. In this more effective method, using this method will get twice the result with half the effort, and the point of failure can often be found quickly.

It is necessary to have a power supply with adjustable voltage and current,circuit board repair price, voltage 0-30V, current 0-3A, this power supply is not expensive, Wuhu circuit board repair, about 300 yuan. Adjust the open circuit voltage to the device power supply voltage level, first adjust the current to a smaller value, and repair the circuit board. Add this voltage to the power supply voltage point of the circuit such as the 5V and 0V terminals of the 74 series chip. When the current increases, touch the device with your hand. When you touch a device that heats up significantly, this is often a damaged component, which can be removed for further measurement and confirmation. Of course, the voltage must not exceed the working voltage of the device during operation, and the connection cannot be reversed, otherwise other good devices will be burned out.