Circuit board detection is the process of finding, determining and correcting each electronic component fault on the circuit board.
In fact, the whole testing process is a thinking process and a testing process that provides logical reasoning clues. Therefore, testing engineers must gradually accumulate experience in the process of circuit board maintenance, testing and overhaul, constantly improve the level.
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 checking each component in the PCB circuit board one by one, it is also very difficult to implement.
Then the maintenance method from the fault phenomenon to the fault cause is an important maintenance method.
As long as the circuit board detects the problem, maintenance is easy.
The following is a summary of the basic formula for circuit board maintenance, which is convenient and practical.
A common circuit board, the circuit is combined into tens of millions, the component appearance is often changed, the character recognition is the most common key resistor capacitor, the damage is also very common, the resistance value is easy to change, the capacitor leakage and leakage, inductive transformer is a coil, simple test to see the on-off, diode and triode, test the PN junction and anti-MOS tube and SCR, trigger test is the key above are discrete parts, integrated circuit for many years, the optocoupler isolates the front and back stages, and the damage is tens of millions. Digital devices are often seen. There is an analog-to-digital converter in front of the 40 and 74 standards. It is very laborious to test. Don't forget ROM and CPLD, programming depends on programmer CPU, single chip microcomputer, timing judgment logic instrument, various sensors, and the damage probability ranks first. Let's look at the actual case: the fault characteristics of capacitor damage in industrial control circuit board and the fault caused by maintenance capacitor damage are the highest in electronic equipment, especially the damage of electrolytic capacitor is the most common. The capacitance damage is as follows: 1. Smaller capacity; 2. Completely lose capacity; 3. Leakage; 4. Short Circuit.
Capacitors play different roles in circuits, and the faults caused by them also have their own characteristics.
In industrial control circuit boards, digital circuits account for the vast majority. Capacitors are mostly used for power filtering, while capacitors used for signal coupling and oscillation circuits are less.
If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not vibrate and have no voltage output;
Or the output voltage is not well filtered, and the logic of the circuit is chaotic due to unstable voltage, which shows that the machine works well or badly or cannot be turned on. If the capacitor is between the positive and negative poles of the power supply of the digital circuit, fault performance ibid.
This is especially obvious on the computer motherboard. Many computers have been in use for several years, sometimes they can't turn on the machine, sometimes they can turn on the machine, open the chassis, the phenomenon of electrolytic capacitor bulging can often be seen. If the capacitor is removed to measure the capacity, it is found to be much lower than the actual value.
The lifetime of the 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 faulty capacitors, we should focus on checking capacitors that are closer to the heat source, such as capacitors next to the heat sink and high-power components. The closer they are to them, the greater the possibility of damage.
I once repaired the power supply of an X-ray flaw detector. The user reported that smoke came out of the power supply. After disassembling the chassis, I found a 1000 uF/350 V large capacitor with oil-like stuff flowing out, the removed capacity is only a few dozen uF, and it is also found that only this capacitor is closest to the heat sink of the rectifier Bridge, and the other ones that are far away are intact and the capacity is normal.
In addition, there is a short circuit of the ceramic capacitor, and it is also found that the capacitor is closer to the heating component. Therefore, attention should be paid to maintenance and search.
Some capacitors have serious leakage, and even hot hands when touched with fingers. This kind of capacitor must be replaced.
In the repair of good and bad faults, the possibility of poor contact is eliminated, and most of them are caused by capacitor damage.
Therefore, when encountering such faults, the capacitor can be checked emphatically, and it is often surprising to replace the capacitor (
Of course, we should also pay attention to the quality of capacitors and choose better brands, such as Ruby and Black King Kong).