PCB Technical - PCB circuit board repair experience

Almost all PCB circuit board repairs do not have drawings and materials, so many people are skeptical about circuit board repairs. Although various circuit boards are very different, the same thing is that each circuit board is composed of various integrated blocks, resistors, and capacitors. And other components, so the circuit board damage must be caused by damage to one or some of the components. The idea of circuit board maintenance is established based on the above factors. Circuit board repair is divided into two parts: inspection and repair, of which inspection occupies a very important position. The basic knowledge of repairing each device on the circuit board is tested, until the bad parts are found and replaced, then a circuit board is repaired.

1. Repair experience of PCB circuit board

Circuit board 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.

2. The 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. It is also very difficult to get up. Then the check-in type inspection method from the fault phenomenon to the cause of the fault is an important method of inspection and repair. As long as the circuit board detects the problem, it is easy to repair. The following basic formulas for circuit board repair are summarized for everyone, which is convenient and practical.

An ordinary circuit board, the circuit combination is tens of millions

The appearance of components changes frequently, character recognition is the key

Resistance and capacitance are the most common, damage is also common

The resistance value of the resistance is easy to change, and the leakage of the capacitor is still leaking.

Inductance transformer is a coil, simple test to see the on-off

Diode and triode, measure the positive and negative of the PN junction

MOS tube and thyristor, trigger test is the key

The above are all discrete parts, integrated circuits for many years

The optocoupler isolates the front and rear stages, and the damage is tens of millions

Digital devices are often seen, before 40 and 74 marks

There is also an analog-to-digital converter, which is very laborious to test

Don't forget ROM and CPLD, programming depends on the programmer

CPU, single-chip microcomputer, timing judgment logic instrument

A variety of sensors, the probability of damage ranked first

Let's take a look at the actual case: the fault characteristics and maintenance of the capacitor damage of the industrial control circuit board

Failures 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 as follows: 1. Capacity becomes smaller; 2. Complete loss of capacity; 3. Leakage; 4. Short circuit.

3. Capacitors play different roles in the circuit, and the faults caused by them also 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 less 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. 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 will be 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.

4. The life of the capacitor is directly related to the ambient temperature. The higher the ambient temperature, the shorter the life 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 to it, the greater the possibility of damage.

5. In the PCB circuit board repair, the power supply of an X-ray flaw detector was repaired. The user reported that there was smoke coming out of the power supply. After disassembling the case, it was found that a 1000uF/350V large capacitor had the same oil quality. Things flowed out, and the capacity was only tens of uF when removed. It was found that only this capacitor was closest to the heat sink of the rectifier bridge, and the others far away were intact with normal capacity. In addition, there was a short circuit in the ceramic capacitor, and it was also found that the capacitor was relatively close to the heating component. Therefore, there should be some emphasis in the inspection and search. Some capacitors have serious leakage current, and even burn your hands when touched with your 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, pay attention to the quality of the capacitors, and choose a better brand.