一、 The capacitance of the LED display circuit board is damaged
The failure caused by capacitor damage is the highest in electronic equipment, especially the damage of electrolytic capacitor. Capacitor damage is manifested as: 1. reduced capacity; 2. Complete loss of capacity; 3. Leakage; 4. Short circuit.
二、 Resistance damage
It is common to see many beginners fidgeting with resistors while repairing circuit boards, either dismantling or soldering. In fact, with more repairs, as long as you understand the damage characteristics of resistors, you don't need to worry too much. Resistance is the most numerous component in electrical equipment, but it is not the component with the highest damage rate. Resistance damage is most common in open circuits, with increasing resistance values being rare and decreasing resistance values being rare. Common types include carbon film resistors, metal film resistors, wire wound resistors, and fuse resistors. We can first observe if there are any signs of burning black on the low resistance resistance on the circuit board. Based on the characteristics of most open circuits or increased resistance when the resistance is damaged, as well as the tendency of high resistance to be easily damaged, we can use a multimeter to directly measure the resistance values at both ends of the high resistance resistance on the circuit board. If the measured resistance value is greater than the nominal resistance value, If the resistance is definitely damaged (it should be noted to wait until the resistance value shows stability before making a conclusion, as there may be a charging and discharging process in parallel with capacitor components in the circuit), if the measured resistance value is smaller than the nominal resistance value, it is generally ignored. In this way, every resistor on the circuit board is measured once, and even if you accidentally kill a thousand, you won't miss a single resistor.
三、 A Method for Judging the Quality of Operational Amplifiers
Amplifiers have the characteristics of "virtual short" and "virtual break", which are very useful for analyzing linear operational amplifier circuits. To ensure linear application, the operational amplifier must operate in a closed loop (negative feedback). If there is no negative feedback, the operational amplifier 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 amplifier virtual short, that is to say, if the operational amplifier works properly, the voltage at the same input and reverse input terminals must be equal, even if there is a difference, it is still at the level of mv. Of course, in some high input impedance circuits, the internal resistance of the multimeter may have a slight impact on voltage testing, but it generally does not exceed 0.2V. If there is a difference of 0.5V or more, the amplifier will undoubtedly fail! If the device is used as a comparator, it is allowed to have unequal input terminals in the same direction and reverse directions. If the same voltage is greater than the reverse voltage, the output voltage is close to the maximum positive value; If the same voltage<reverse voltage, the output voltage is close to the maximum value of 0V or negative (depending on dual or single power supply). If the voltage detected does not comply with this rule, the device will undoubtedly fail! In this way, you don't need to use substitution method or remove the chips on the circuit board to determine the quality of the operational amplifier.
四、A Tip for testing SMT components with a multimeter
Some SMD components are very small, making it inconvenient to use ordinary multimeter probes for testing and maintenance. Firstly, they can easily cause short circuits, and secondly, it is inconvenient for circuit boards coated with insulation to come into contact with the metal parts of the component pins. Here is a simple method that will bring a lot of convenience to testing. Take two of the smallest sewing needles, (Deep Industrial Control Maintenance Technology Column) and place them tightly against the multimeter pen. Then, take a thin copper wire from a multi-strand cable, tie the pen and sewing needle together with the fine copper wire, and solder them firmly. This way, when measuring SMT components with a stylus with a small needle tip, there is no longer a risk of short circuits, and the needle tip can puncture the insulation coating and directly hit key parts, eliminating the need to bother scraping the film.
五、Maintenance Method for Short Circuit Fault of Circuit Board Common Power Supply
In circuit board maintenance, if there is a short circuit to a common power supply, it is often the most common fault, because many devices share the same power supply, and every device using this power supply is suspected of a short circuit. If there are not many components on the board, using the "hoe the earth" method can ultimately find the short circuit point. If there are too many components, whether the "hoe the earth" can hoe the condition depends on luck. Here is a recommended method that works well. By using this method, you can get twice the result with half the effort and often quickly find the fault point. There should be a power supply with adjustable voltage and current, with a voltage of 0-30V and a current of 0-3A. This power supply is not expensive and costs around 300 yuan. Adjust the open circuit voltage to the level of the device's power supply voltage. First, adjust the current to the minimum. Apply this voltage to the power supply voltage points of the circuit, such as the 5V and 0V terminals of the 74 series chip. Depending on the degree of the short circuit, gradually increase the current. Touch the device with your hand. When a certain device heats significantly, it is often a damaged component. You can remove it for further measurement and confirmation. Of course, the voltage during operation must not exceed the working voltage of the device, and it must not be reversed, otherwise it will burn out other good devices.
六、A small rubber to solve big problems
The number of boards used in industrial control is increasing, and many boards use the method of inserting gold fingers into slots. Due to the harsh industrial environment, which is dusty, humid, and corrosive, it is easy for boards to have poor contact faults. Many friends may have solved the problem by replacing boards, but the cost of purchasing boards is very considerable, especially for some imported equipment boards. In fact, everyone can use an eraser to repeatedly wipe the dirt on the golden finger a few times, clean it up, and then try the machine again. Perhaps the problem will be solved! The method is simple and practical.
七、Analysis of electrical faults with good and bad timing
In terms of probability, various electrical faults with good and bad times may include the following situations:
1. Poor contact between the board and slot, failure to connect when the cable is broken internally, poor contact between the wire plug and terminal, and faulty soldering of components all belong to this category;
2. For digital circuits, malfunctions only occur under specific conditions due to signal interference. It is possible that excessive interference has indeed affected the control system and caused it to make mistakes, and there are also changes in individual component parameters or overall performance parameters of the circuit board, leading to a critical point in anti-interference ability and resulting in malfunctions;
3. Poor thermal stability of components From a large number of maintenance practices, the thermal stability of the first electrolytic capacitor is poor, followed by other capacitors, triodes, diodes, ICs, resistors, etc;
4. There is moisture, dust accumulation, etc. on the circuit board. Moisture and dust will conduct electricity with resistance effect, and the resistance value will change during thermal expansion. This resistance value has a parallel effect with other components. If this effect is strong, circuit parameters will be changed, causing faults;
5. Software is also one of the factors to consider. Many parameters in the circuit are adjusted using software, and the margin of some parameters is set too low, which is within the critical range. When the machine's operating conditions meet the reason for the software to determine the fault, an alarm will appear.
Post time: Jun-21-2023
