7 tips on LED display circuit board repair technology

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Fault characteristics and repair of damaged circuit board capacitors

Failures caused by capacitor damage are the highest in electronic equipment, among which damage to electrolytic capacitors is the most common.

The symptoms of capacitor damage are: 1. Capacity becomes smaller; 2. Complete loss of capacity; 3. Leakage; 4. Short circuit.

Characteristics and identification of resistor damage

It is often seen that many beginners are tossing around with resistors when repairing circuit boards, disassembling and soldering them. In fact, they can repair a lot more. As long as you understand the damage characteristics of resistors, you don't have to spend a lot of time.

Resistors are the most numerous components in electrical equipment, but they are not the components with the highest damage rate. The most common type of resistor damage is an open circuit. It is less common for the resistance value to increase, and it is very rare for the resistance value to decrease. Common ones include carbon film resistors, metal film resistors, wirewound resistors and fuse resistors.

We can first observe whether there are any traces of burnt black on the low-resistance resistors on the circuit board. Then according to the characteristics that most open circuits or the resistance increases when the resistors are damaged, and high-resistance resistors are easily damaged, we can use a multimeter to check the circuit First directly measure the resistance at both ends of the high-resistance resistor on the board. If the measured resistance is greater than the nominal resistance, the resistor must be damaged (note that the resistance value display is stable before making a conclusion, because there may be Parallel capacitive components have a charging and discharging process). If the measured resistance is smaller than the nominal resistance, generally ignore it. In this way, every resistor on the circuit board is measured once. Even if a thousand are "killed by mistake", one will not be missed.

How to judge the quality of operational amplifiers

The amplifier has the characteristics of "virtual short" and "virtual open". These two characteristics are very useful for analyzing operational amplifier circuits used in linear applications. In order to ensure linear operation, the op amp must work in a closed loop (negative feedback). If there is no negative feedback, the op amp under open-loop amplification becomes a comparator. If you want to judge the quality of a device, you should first distinguish whether the device is used as an amplifier or a comparator in the circuit.

According to the principle of virtual short circuit of the amplifier, that is to say, if the operational amplifier works normally, the voltage of its non-inverting input terminal and the inverting input terminal must be equal. Even if there is a difference, it will be mv level. Of course, in some high input impedance circuits, a multimeter The internal resistance will have some influence on the voltage test, but generally it will not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier must be broken!

If the device is used as a comparator, the non-inverting input terminal and the inverting input terminal are allowed to be different. If the same direction voltage > the reverse voltage, the output voltage is close to the positive maximum value; if the same direction voltage < the reverse voltage, the output voltage is close to 0V or the negative maximum value (depending on the dual power supply or single power supply). If the detected voltage does not comply with this rule, the device will definitely be damaged! This way you don't have to use substitution, and you don't have to remove the chip on the circuit board to determine whether the op amp is good or bad.

Tips for testing SMT components with a multimeter

Some chip components are very small, and it is very inconvenient to use ordinary multimeter test leads to test and repair them. First, it is easy to cause short circuits. Second, it is inconvenient for circuit boards coated with insulating coating to touch the metal parts of component pins. Here I will tell you a simple method, which will bring a lot of convenience to the detection.

Take two of the smallest sewing needles (In-depth Industrial Control Maintenance Technology Column) and place them close to the multimeter pens. Then take a thin copper wire from a multi-strand cable and tie the test pens and sewing needles with the thin copper wire. At the same time, solder it firmly. In this way, when using a test pen with a small needle tip to test those SMT components, there is no risk of short circuit, and the needle tip can pierce the insulating coating and penetrate the key parts, so you no longer have to bother to scrape those membranes.

Repair method of circuit board public power short circuit fault

During circuit board maintenance, if you encounter a public power supply short-circuit fault, you will often have a big problem, because many devices share the same power supply, and every device using this power supply is suspected of being short-circuited. If there are not many components on the board, use "hoeing the ground" The short circuit point can be found after all. If there are too many components, it depends on luck whether the problem can be found by "hoeing the ground". Here we recommend a more effective method. Using this method, you can get twice the result with half the effort, and you can often find the fault point quickly.

You need a power supply with adjustable voltage and current, voltage 0-30V, current 0-3A. This power supply is not expensive, about 300 yuan. Adjust the open circuit voltage to the device power supply voltage level. First, adjust the current to the minimum. Then add this voltage to the power supply voltage point of the circuit, such as the 5V and 0V terminals of the 74 series chip. Depending on the degree of the short circuit, slowly increase the current. Touch the device with your hands. When you feel that a certain device is obviously hot, this is often a damaged component. You can remove it for further measurement and confirmation. Of course, the voltage must not exceed the working voltage of the device during operation, and it cannot be connected reversely, otherwise other good devices will be burned out.

A small piece of rubber can solve big problems

More and more boards are used in industrial control, and many boards use gold fingers to insert into the slots. Due to the harsh environment of the industrial site, dusty, humid, and corrosive gas environments can easily cause poor contact failures in the boards. Many boards My friend may have solved the problem by replacing the board, but the cost of purchasing the board is very considerable, especially for some imported equipment. In fact, you might as well use an eraser to rub the gold finger repeatedly, clean the dirt on the gold finger, and then try the machine again. The problem may be solved! The method is simple and practical.

Analysis of good and bad electrical faults

Various electrical faults that come and go may include the following situations in terms of probability: 1. Poor contact Poor contact between the board and the slot, disconnection when the cable is broken internally, poor contact between the wire plug and terminal block, and weak soldering of components all fall into this category; 2. Signal interference For digital circuits, faults will occur under specific circumstances. It may be that the interference is too great and affects the control system, causing it to go wrong. There may also be changes in individual component parameters or overall performance parameters of the circuit board, making the anti-interference Capabilities tend to reach critical points and thus malfunctions occur; 3. The thermal stability of components is not good Judging from a large number of maintenance practices, the thermal stability of electrolytic capacitors is first of all, followed by other capacitors, transistors, diodes, ICs, resistors, etc.; 4. There is moisture, dust, etc. on the circuit board. Moisture and dust will conduct electricity and have a resistance effect, and the resistance value will change during the process of thermal expansion and contraction. This resistance value will have a parallel effect with other components. When this effect is relatively strong, it will change the circuit parameters and cause malfunctions. occur; 5. Software is also a consideration Many parameters in the circuit are adjusted using software. The margin of some parameters is adjusted too low and is in a critical range. When the machine operating conditions meet the reasons for the fault determined by the software, an alarm will appear.