Summary of Common PCB Repair Techniques
Failure Characteristics and Troubleshooting for PCB Capacitor
Failure caused by capacitor damage is most common in electronic equipment, especially damage to electrolytic capacitors is most common.
A capacitor failure manifests itself as: 1. reduction in capacitance 2. complete loss of capacitance 3. leakage 4. short circuit.
Capacitors play different roles in a circuit, and faults they cause also have their own characteristics.
In industrial control boards, digital circuits are vast majority, and capacitors are mainly used for power filtering, and fewer capacitors are used for signal transmission and generation 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 filter is defective, and logic confusion occurs in circuit due to voltage instability. broken or cannot be turned on. If a capacitor is connected between positive and negative poles of digital circuit power supply, fault characteristics are same as above.
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.
I once repaired power supply of an X-ray flaw detector. A user reported that smoke was coming out of power supply. After disassembling case, it turned out that there was a large 1000uF / 350V capacitor with something like oil leaking out. Remove it. The capacity of one is only a few tens of microfarads. It was also found that only this capacitor is closest to rectifier bridge radiator, and rest of capacitors are intact far away and have normal capacity. In addition, a short circuit occurs in ceramic capacitor, and capacitor is also found to be relatively close to heating element. 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 event of major and serious failures during maintenance, with exception of possibility of poor contact, most failures are usually caused by a damaged capacitor. Therefore, when faced with such malfunctions, it is possible to check capacitors. Replacing capacitors is often surprising (of course, pay attention to quality of capacitors, and choose a better brand, such as Ruby, Black King Kong, etc. ).
Damage Resistance Characteristics and Identification
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, metal film resistors, wirewound resistors, and insurance resistors.
The first two types of resistors are most widely used, and their damage characteristics are that damage degree of low resistance value (below 100 ohms) and high resistance value (above 100 kΩ) is relatively high, and intermediate resistance value (for example, from hundreds of ohms to tens of thousands) Europe) is rarely damaged; second is that when low-resistance resistors are damaged, they often burn and turn black, which is easy to detect, and if high-resistance resistors are damaged, there are few traces.
Wirewound resistors are usually used to limit large currents, and their resistance is low. When a cylindrical wire resistor burns out, some will turn black or surface will burst and crack, and some will not leave a trace. Cement resistors are a type of wire-wound resistor that can break when burned out, otherwise there will be no visible traces. When wick burns out, a piece of skin will fly off from some surfaces, and from some there will be no 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 or resistance value becomes larger when resistors are damaged, and high-resistance resistors are easily damaged We can use a multimeter to directly measure resistance at both ends of high-resistance resistor on PCB. If measured resistance is greater than rated resistance, resistor must be damaged (pay attention to stability of equal resistance indication. The conclusion is made after circuit, since it is possible to connect capacitive elements in parallel in circuit , charging and discharging in progress), if measured resistance value is less than nominal resistance value, generally ignore it. This way, every resistor on PCB is re-measured, even if you "wrongly kill" a thousand, you won't miss a single one.
How to tell if an op amp is good or bad
Many electronics repairers have a hard time judging whether an op amp is good or not. A graduate student who specializes in learning frequency conversion control from his mentor is actually same!), II will discuss this with you. here and hope it will be useful for everyone.
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 whether a device is good or bad, you must first determine whether the device is being used as an amplifier or comparator in a circuit.
It can be seen from figure that regardless of type of amplifier, there is a feedback resistor Rf, then we can test this feedback resistor in circuit during maintenance, and use a multimeter to check relationship between output terminal and reverse input terminal. If resistance value between are ridiculously high, such as more than a few megohms, then we can probably be sure that device is being used as a comparator. Between inverting input terminals, if any, it should be used as an amplifier.
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 usually it will not exceed 0.2V. If there is a difference of more than 0.5V, amplifier should fail! (using a FLUKE179 multimeter) < /p>
If device is used as a comparator, non-inverting input terminal and inverting input terminal may be different. If same direction of voltage>reverse voltage, output voltage is close to positive maximum value; same direction voltage < reverse voltage, then output voltage is close to 0V or negative maximum (depending on dual or single power supply). If detected voltage does not follow this rule, device must be faulty! So you do not need to use replacement method, and you do not need to remove chip on PCB to judge whether amplifier is working good or bad.
Tip for Testing SMT Components with a Multimeter
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 sewing needles of smallest size (Deep Industrial Control Maintenance Technology column) and connect them to pen of multimeter, then take a thin copper wire in multi-core cable, and use a thin copper wire to connect test pen and sewing needle. The pins are bonded together and securely soldered. Therefore, there is no risk of a 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 enter directly.about in key parts, and there is no need to worry about scratching these films.
A method for clearing a short circuit in a common power supply on a PCB
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 many components on board, use "The "earth loosening" method can find short circuit point. If there are too many components, whether "loose ground" can find situation depends on luck. Here I recommend a more efficient method whereby you can get twice result with half effort, and you can often find fault quickly.
There must be a power supply with adjustable voltage and current, voltage 0-30V, and current 0-3A. This power supply is not expensive, about 300 yuan. Adjust open circuit voltage to level of device supply voltage, first adjust current to a minimum, and 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 to device with your hand. When you feel that a certain device is clearly warm, this is usually a damaged component. You can remove it 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.
A small eraser solves a big problem
More and more boards are used in industrial control, and many boards use method of inserting gold fingers into slots. Due to harsh environment of industrial site, environment with dusty, humid and corrosive gases is easy to cause bad contact of boards. Malfunction, many friends may have solved problem by replacing board, but purchase cost of board is very significant, especially board of some imported equipment. In fact, you could also use eraser to rub gold finger several times to remove dirt from gold finger, and then try machine again, maybe problem will be solved! The method is simple and practical.
Analysis of good and bad electrical faults
The variety of good and bad electrical faults usually includes following situations in terms of probability:
1. Bad contact
Bad contact between board and slot, disconnection of cable when it breaks inside, poor contact between plug and terminal, poor soldering of components, etc., all fall into this category;
2. Signal broken
For digital circuits, faults only appear when aconditions. It is possible that interference is too high and affects control system to cause errors, or there are changes in parameters of individual components or overall performance of printed circuit board. . Make it so that anti-interference ability tends to a critical point so that a failure occurs;
3. The thermal stability of components is poor
Among a large number of maintenance methods, thermal stability of electrolytic capacitors is first, followed by other capacitors, transistors, diodes, microcircuits, resistors, etc.;
4. There is moisture and dust on circuit board
Moisture and dust will conduct electricity and resist, and resistance value will change during thermal expansion and contraction. This resistance value will have a parallel effect with other components. When this effect is strong, it will change parameters of circuit. , which will crash;
5. Software is also one of considerations
Many parameters in circuit are controlled by software, and margin of some parameters is adjusted too low, which is in critical range. When working state of machine matches reason that software detects a malfunction, an alarm will appear.
How to quickly find information about components
Modern electronic products are diverse, and types of components are becoming more and more. In circuit maintenance, especially in field of industrial printed circuit board maintenance, many components have never been seen or even heard of. The information is complete, but it is necessary to read and analyze information one by one in computer. If there is no quick search method, maintenance efficiency will be greatly reduced. In field of industrial electronics maintenance, efficiency is money.
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