PCB now is everywhere almost all electronic products show in our life need [PCB Board](jlcpcb.com/m), while nothing will last in good condition forever, when the PCB board get problems, we have to repair it. Today we will discuss **[how to repair the PCB Board](jlcpcb.com/m)**. Almost all [PCB repair](jlcpcb.com/m) without drawing materials, so a lot of people are skeptical of circuit board maintenance, although all kinds of circuit boards differ in thousands of ways, the same is each circuit board is composed of integrated blocks, resistor, capacitor, and other components, so the damage of circuit board must be the result of a damaged one or several components, circuit board maintenance is established based on the above factors. Circuit board maintenance is divided into two parts: testing and maintenance, in which testing occupies a very important position. Test the repair fundamentals of each component on the circuit board until the defective component is found and replaced. Then a circuit board is repaired. [Printed Circuit board ](jlcpcb.com/m )inspection is the process of finding, determining, and correcting the failure of each electronic component on the circuit board. In fact, the entire inspection process is a thinking process and a test process that provides logical reasoning clues. Therefore, the inspection engineer must gradually accumulate experience and continuously improve the level in the process of circuit board maintenance, testing, and repair. General electronic equipment is composed of thousands of components. During maintenance and overhaul, it will be very time-consuming to find problems by directly testing and inspecting every component in the PCB circuit board. Very difficult. Then the check-in type inspection method from the fault phenomenon to the cause of the fault is a vital inspection method. As long as the circuit board detects the problem, it is easy to repair. **There is a Formula for reference.** 　　1 The appearance of components changes frequently, and character recognition is the key 　　2 Resistance and capacitance are the most common, and the damage is also common 　　3 The resistance value of the resistance is easy to change, and the capacitance leakage or leakage 　　4 Inductance transformer is a coil, simple test to see the on-off 　　5 Diodes and transistors, measure the positive and negative of the PN junction 　　6 MOS tube and thyristor, trigger test is the key 　　7 All of the above are discrete parts, integrated circuits for many years 　　8 Optocoupler isolates the front and rear stages, the damage is tens of millions 　　9 Digital devices are often seen, before 40 and 74 　　10 There is also an analog-to-digital converter, which is very laborious to test 　　11 Don't forget ROM and CPLD, programming depends on the programmer 　　12 CPU, single-chip microcomputer, timing judgment logic instrument 　　13 Various sensors, the probability of damage ranks first Let's take a look at the actual case: the fault characteristics and repair of industrial control circuit board capacitor damage Faults caused by capacitor damage are the highest in electronic equipment, and damage to electrolytic capacitors is the most common. The performance of capacitor damage is 1. The capacity becomes smaller; 2. The capacity is completely lost; 3. Leakage; 4. Short circuit. Capacitors play different roles in the circuit, and the faults they cause have their own characteristics. In industrial control circuit boards, digital circuits account for the vast majority, and capacitors are mostly used for power supply filtering, and fewer capacitors are used for signal coupling and oscillation circuits. If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not vibrate, and there will be no voltage output; or the output voltage is not well filtered, and the circuit is logically chaotic due to voltage instability, which shows that the machine is working well or broken No matter the machine if the capacitor is connected between the positive and negative poles of the power supply of the digital circuit, the failure behavior is the same as above. This is especially obvious on computer motherboards. Many computers sometimes fail to turn on after a few years, and sometimes they can be turned on. Open the case, you can often see the phenomenon of electrolytic capacitors bulging if you remove the capacitors to measure the capacity, Found to be much lower than the actual value. The lifetime of a capacitor is directly related to the ambient temperature. The higher the ambient temperature, the shorter the lifetime of the capacitor. This rule applies not only to electrolytic capacitors but also to other capacitors. Therefore, when looking for a faulty capacitor, you should focus on checking the capacitors that are close to the heat source, such as the capacitors next to the heat sink and high-power components. The closer you are, the greater the possibility of damage. I have repaired the power supply of an X-ray flaw detector. The user reported that smoke was coming out of the power supply. After disassembling the case, I found that a 1000uF/350V large capacitor had something like oil flowing out. Removed a certain amount of capacity. It is only tens of uF, and it is found that only this capacitor is the closest to the heat sink of the rectifier bridge, and the others far away are intact with normal capacity. In addition, the ceramic capacitors were short-circuited, and the capacitors were also found to be relatively close to the heating components. Therefore, there should be some emphasis when checking and repairing. Some capacitors have serious leakage currents and even burn hands when touched with fingers. This type of capacitor must be replaced. In the case of ups and downs during maintenance, except for the possibility of poor contact, most of the failures are generally caused by capacitor damage. Therefore, when encountering such failures, you can focus on checking the capacitors. After replacing the capacitors, it is often surprising (of course, you should also pay attention to the quality of the capacitors, and choose a better brand, such as Ruby, Black Diamond, etc.).