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Surprisingly, this is most prone to PCB failure?


1. Capacitor failure

Failure caused by capacitor damage is most common in electronic equipment, especially damage to electrolytic capacitors is most common. Capacitor damage manifests itself in form of: a decrease in capacitance, a complete loss of capacitance, a leak, a short circuit.

Capacitors play different roles in a circuit, and faults they cause also have their own characteristics: in industrial PCBs, digital circuits are vast majority, and capacitors are mainly used for power filtering, as well as signal transmission and transmission. several oscillatory circuits. If electrolytic capacitor used in switching power supply is damaged, switching power supply may not vibrate and have no output voltage.

Or output voltage filtering is bad and circuit is logically confused due to voltage instability. .

This is especially noticeable on computer motherboards. After several years of use, many computers sometimes stop turning on, and sometimes can turn on again. When opening case, one can often see phenomenon of swelling of electrolytic capacitors. If you remove capacitors Measure capacitance and find that it is much lower than actual value.

The service life of capacitor is directly related to ambient temperature: higher ambient temperature, shorter service life of capacitor. This rule applies not only to electrolytic capacitors, but also to other capacitors. Therefore, when looking for faulty capacitors, you should focus on checking capacitors that are close to heat source, such as capacitors near heat sinks and powerful components. The closer you are to them, more likely they are to be damaged. Therefore, more attention should be paid to inspection and search.

Some capacitors leak badly and even burn your hands when you touch them with your fingers. Such capacitors must be replaced. In case of good and serious faults during maintenance, with exception of possibility of poor contact, most faults are usually caused by a damaged capacitor. Therefore, when faced with such a malfunction, you can focus on checking capacitor, and after replacing capacitor it is often a pleasant surprise.

2. Resistance Error

It is common to see many beginners tossing around with resistors when repairing a circuit, disassembling and welding.

Resistance is most numerous component in electrical equipment, but it is not component with highest degree of damage. Open circuit is most common type of resistance failure, resistance value rarely becomes large, and very rarely resistance value becomes small. Common are carbon film resistors, metafilm, wirewound resistors and insurance resistors.

The first two types of resistors are most widely used, and their damage characteristics are that damage rate at low resistance value (below 100 Ω) and high resistance value (above 100 kΩ) is high, and intermediate resistance value (for example, from hundreds of ohms to tens of thousands of euros) is rarely damaged; secondly, if a low-resistance resistor is damaged, it often burns out and turns black, which is easy to find, while few traces remain with a high-resistance one. resistor is damaged.

Wirewound resistors are usually used to limit large currents, and resistance value is small; when cylindrical wire resistors burn out, some of them turn black or surface bursts, cracks, and some leave no marks. ;cement resistors are wirewound resistors One of them can break when burned out, otherwise there will be no visible traces; if a fuse blows, a piece of skin will be torn off from some surfaces, and some will not leave traces, but they will never burn or turn black. According to given characteristics, when checking resistance, you can focus on it and quickly find out damaged resistance.

From above specifications, we can first observe whether there are traces of burnt black on low-resistance resistors on board, and then by fact that most of resistors are open-circuited or resistance value becomes larger when resistors are damaged, and high-resistance resistors are easy to damage characteristics, we can use a multimeter to directly measure resistance at both ends of a high resistance circuit board resistor.

If measured resistance value is greater than nominal resistance value, resistance must be damaged (be careful not to conclude until resistance value becomes stable, because there may be a parallel connection of capacitive components in circuit, and there is a process of charging and discharging), if measured resistance value is less than nominal resistance value, it is usually ignored. This way, every resistor on PCB is re-measured, even if you "wrongly kill" a thousand, you won't miss a single one.

3. Op-amp failure

Many electronics maintainers have a hard time judging whether an op amp is good or not.

An ideal op-amp has "virtual short circuit" and "virtual trip" characteristics. These two characteristics are very useful for analyzing linear op amp circuits. To ensure linear operation, op-amp must operate in a closed loop (negative feedback). If there is no negative feedback, open-loop gain op-amp becomes a comparator. If you want to judge goodwhether it is a device or a bad one, you must first determine if device is being used as an amplifier or comparator in circuit.

According to virtual short-circuit principle of amplifier, that is, if op-amp is working normally, voltages at same input terminal and at reverse input terminal must be equal, even if there is a difference, it is at mv level. Of course , in some circuits with high input impedance, internal resistance of multimeter will have little effect on voltage test, but it will usually not exceed 0.2V. If there is a difference of more than 0.5V, amplifier must be damaged.

If device is used as a comparator, non-inverting input and inverting input may not be equal. If same direction voltage>reverse voltage, output voltage is close to positive maximum value; if same direction voltage < reverse voltage, output voltage is close to 0V or negative maximum value (depending on dual power supply or single power supply). If detected voltage does not comply with this rule, device is subject to failure! So you don't have to use replacement method, and you don't have to remove IC on PCB to determine if op amp is good or not.

4. SMT component failure

Some of chip's components are very small and very inconvenient to test and repair with normal multimeter pens. Firstly, it is easy to cause a short circuit, and secondly, it is inconvenient to touch metal part of component's contacts. on a board covered with an insulating coating. Here to tell you a simple method, bring a lot of convenience to discover.

Take two smallest size sewing needles and bring them to handle of multimeter, then take thin copper wire from stranded cable, tie test pen and sewing needle with thin copper wire, and then use solder firmly. Therefore, there is no risk of short circuit when using a test pen with a small needle point to measure these SMT components, and needle point can pierce insulation coating and go directly to key parts, and there is no need to worry about scratching these films.

5. Short circuit in mains

When servicing circuit boards, if you encounter a short circuit in a public power supply, it is often a big headache because many devices use same power supply, and every device using this power supply is suspected of having a short. closure.

If there are not so many components on board, then place of short circuit can still be found by "digging ground"; if there are too many components, then whether it will be possible to "dig through ground" to find situation depends on luck. Here I recommend a more efficient method by whichwow, you can get twice result with half effort, and you can often find problem quickly.

There should be a power supply with adjustable voltage and current, voltage 0-30V, and current 0-3A. Such a power supply is not expensive, about 300 yuan. Adjust open circuit voltage to level of device supply voltage, first adjust current to a minimum, add this voltage to circuit supply voltage point, such as 5V and 0V terminals of 74 series chip, and gradually increase current according to degree of short circuit.

Touch device with your hands. When you feel that a certain device is clearly heating up, it is usually a damaged component. You can take it off for further measurement and confirmation. Of course, voltage must not exceed operating voltage of device during operation, and it must not be reversed, otherwise it will burn out other good devices.

6. Board failure

In industrial control, more and more boards are used, and many boards use method of inserting gold fingers into slots. Due to harsh environment of industrial site, dusty, humid and corrosive gas environments may cause poor contact on boards, many friends can solve problem by replacing boards, but purchase cost of boards is very significant, especially for some boards of imported equipment.

Actually, you can use an eraser to rub gold finger a few times to get dirt off gold finger and then try machine again. The problem can be solved. This method is simple and practical.

7. Electrical failure

The variety of good and bad electrical faults usually includes following situations in terms of probability:

  • Bad contact: poor contact between board and slot, disconnection of cable when it breaks inside, poor contact between plug and terminal, poor soldering of components, etc. class;

  • Signal interference: For digital circuits, faults will only appear under certain conditions. It is possible that interference is too large and affects control system to cause errors, and there are parameters of individual components of circuit board or general performance parameters have changed so that noise immunity approaches a critical point and a failure occurs;

  • The thermal stability of components is poor: In terms of a large number of maintenance methods, thermal stability of electrolytic capacitors is first, followed by other capacitors, triodes. , diodes, ICs, resistors, etc.;

  • Moisture, dust, etc. on PCB: Moisture and dust will conduct electricity and resist, and resistance will change during thermal expansion and contraction. The resistance value will have a parallel effect with other components.ents. When this effect is strong, it will change parameters of circuit and cause a malfunction;

  • Software is also one of factors to consider: Many circuit parameters are software configurable, and some parameters are marginally adjusted too low to be in critical range. When operating conditions of machine match software. When cause of failure is determined, an alarm will appear.