Summary of circuit board failures and several tricks for repair technology
1.Fault characteristics and repair of capacitor damage in industrial control circuit boards
Faults caused by capacitor damage are the highest in electronic equipment, especially electrolytic capacitor damage is the most common.
Capacitor damage is manifested as:
Capacitance decreases;
Complete loss of capacity;
Leakage;
Short circuit.
Capacitors play different roles in circuits, and the faults they cause also have their own characteristics. In industrial control circuit boards, digital circuits account for the vast majority, and capacitors are mostly used for power filtering, and 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 oscillate and there will be no voltage output; or the output voltage filtering is not good, and the circuit has logical confusion due to unstable voltage, which is manifested as the machine working sometimes well and sometimes badly or cannot be turned on. If the capacitor is connected between the positive and negative poles of the power supply of the digital circuit, the fault manifestation is the same as above.
This is particularly evident on computer motherboards.
Many computers can sometimes not be turned on after a few years of use, but can be turned on again. When the chassis is opened, electrolytic capacitors are often seen to be bulging. If the capacitors are removed and the capacity is measured, it is found to be much lower than the actual value.
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 faulty capacitors, the capacitors that are close to the heat source should be checked, such as capacitors next to the heat sink and high-power components. The closer 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, it was found that a large 1000uF/350V capacitor had something like oil flowing out. After disassembling it, the capacity was only a few dozen uF. It was also found that only this capacitor was closest to the heat sink of the rectifier bridge, and the others that were far away were intact and had normal capacity. In addition, there are cases where ceramic capacitors are short-circuited, and it is also found that the capacitors are close to the heating components. Therefore, there should be a focus when inspecting and searching.
Some capacitors have serious leakage, and they are even hot when touched with fingers. Such capacitors must be replaced.
When repairing a fault that is sometimes good and sometimes bad, in addition to excluding the possibility of poor contact, most of the faults are generally caused by capacitor damage. Therefore, when encountering such a fault, you can focus on checking the capacitor. After replacing the capacitor, it is often a pleasant surprise (of course, you should also pay attention to the quality of the capacitor and choose a better brand, such as Ruby, Black Diamond, etc.).
2.Characteristics and judgment of resistor damage
It is often seen that many beginners are tossing around with resistors when repairing circuits, disassembling and welding. In fact, if you repair more, you don’t have to go to great lengths as long as you understand the characteristics of resistor damage.
Resistors are the most numerous components in electrical equipment, but they are not the components with the highest damage rate. The most common type of resistor damage is open circuit, the resistance value increases less common, and the resistance value decreases very rare. Common types include carbon film resistors, metal film resistors, wirewound resistors and fuse resistors.
The first two types of resistors are the most widely used.
The characteristics of their damage are that low resistance (below 100Ω) and high resistance (above 100kΩ) have a higher damage rate, and intermediate resistance (such as hundreds of ohms to tens of kiloohms) are rarely damaged; second, low resistance resistors are often burnt and blackened when damaged, which is easy to find, while high resistance resistors rarely leave traces when damaged.
Wirewound resistors are generally used as large current limiters and have low resistance values.
When cylindrical wirewound resistors are burned, some will turn black or have skin peeling and cracks on the surface, while others will have no traces. Cement resistors are a type of wirewound resistor. When burned, they may break, otherwise there will be no visible traces. When the fuse resistor is burned, some will have a piece of skin blown off on the surface, while others will have no traces, but they will never be burnt and blackened. According to the above characteristics, you can focus on checking the resistors and quickly find the damaged resistors.
According to the characteristics listed above, we can first observe whether there are traces of burnt black on the low-resistance resistors on the circuit board. Then, according to the characteristics that most of the resistors are open circuit or the resistance value increases when they are damaged, and the high-resistance resistors are easy to be damaged, we can use a multimeter to directly measure the resistance value of the high-resistance resistors on the circuit board. If the measured resistance value is larger than the nominal resistance value, the resistor must be damaged (pay attention to wait until the resistance value display is stable before drawing a conclusion, because there may be a capacitor component in parallel in the circuit, and there is a charging and discharging process). If the measured resistance value is smaller than the nominal resistance value, it is generally ignored. In this way, every resistor on the circuit board is measured once, even if a thousand are “killed by mistake”, one will not be missed.
3.Methods for judging the quality of operational amplifiers
The judgment of the quality of operational amplifiers is difficult for quite a few electronic repairers, not only because of the cultural level (there are many undergraduates under my command, and they will definitely not know how to do it if they are not taught, and it will take a long time to understand it after being taught. There is also a graduate student who specializes in variable frequency control with a tutor, and it is also the same!). Let’s discuss it with you here, hoping to help you.
The ideal operational amplifier has the characteristics of “virtual short” and “virtual open”, which are very useful for analyzing the operational amplifier circuit for linear application. In order to ensure linear application, the operational amplifier must work in a closed loop (negative feedback). If there is no negative feedback, the operational amplifier under open-loop amplification becomes a comparator. If you want to judge the quality of a device, you should first distinguish whether the device is used as an amplifier or a comparator in the circuit.
From the figure, we can see that no matter what type of amplifier, there is a feedback resistor Rf.
When repairing, we can check this feedback resistor from the circuit and use a multimeter to check the resistance between the output terminal and the reverse input terminal. If it is ridiculously large, such as more than a few MΩ, we can probably be sure that the device is used as a comparator. If this resistance is small, from 0Ω to tens of kΩ, then check whether there is a resistor connected between the output terminal and the reverse input terminal. If there is, it must be used as an amplifier.
According to the principle of virtual short of amplifier, if the operational amplifier works normally, the voltage of its same direction input terminal and reverse input terminal must be equal, even if there is a difference, it is at the mV level. Of course, in some high input impedance circuits, the internal resistance of the multimeter will have some influence on the voltage test, but generally it will not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier must be broken! (I use FLUKE179 multimeter)
If the device is used as a comparator, the same direction input terminal and reverse input terminal are allowed to be different.
If the same direction voltage > reverse voltage, the output voltage is close to the maximum positive value;
If the same direction voltage < reverse voltage, the output voltage is close to 0V or the maximum negative value (depending on dual power supply or single power supply).
If the voltage detected does not meet this rule, the device must be broken!
In this way, you don’t have to use the substitution method, and you don’t have to remove the chip on the circuit board to judge the quality of the operational amplifier.
4.A little trick for testing SMT components with a multimeter
Some SMD components are very small, and it is inconvenient to use ordinary multimeter probes to test and repair them. First, it is easy to cause short circuits, and second, it is inconvenient to contact the metal part of the component pins on the circuit board coated with insulating coating. Here is a simple method that will bring a lot of convenience to the detection.
Take two smallest sewing needles, (In-depth Industrial Control Maintenance Technology Column) put them close to the multimeter probe, and then take a thin copper wire from a multi-strand cable, use the thin copper wire to tie the probe and sewing needle together, and then solder it with solder. In this way, when using a probe with a fine needle tip to test those SMT components, there is no risk of short circuit, and the needle tip can pierce the insulating coating and directly hit the key parts, so there is no need to bother to scrape those membranes.
5.Inspection and repair methods for short circuit faults of common power supply of circuit boards
During circuit board maintenance, if you encounter a short circuit fault of common power supply, you will often have a headache, because many devices share the same power supply, and every device using this power supply is suspected of short circuit. If there are not many components on the board, you can find the short circuit point by “hoeing the ground”. If there are too many components, whether “hoeing the ground” can find the problem depends on luck. Here is a more effective method. Using this method, you can get twice the result with half the effort, and you can often find the fault point quickly.
You need a power supply with adjustable voltage and current, voltage 0-30V, current 0-3A, this power supply is not expensive, about 300 yuan. Adjust the open circuit voltage to the device power supply voltage level, first adjust the current to the minimum, and apply this voltage to the power supply voltage point of the circuit, such as the 5V and 0V ends of the 74 series chip. Depending on the degree of short circuit, slowly increase the current, touch the device with your hand, and when you touch a device that is obviously hot, this is often a damaged component, which can be removed for further measurement and confirmation. Of course, the voltage must not exceed the working voltage of the device during operation, and it cannot be connected in reverse, otherwise other good devices will be burned.
6.A small eraser solves big problems
There are more and more boards used in industrial control, and many boards use the method of inserting gold fingers into slots. Due to the harsh industrial site environment, dusty, humid, and corrosive gas environments, boards are prone to poor contact failures. Many friends may solve the problem by replacing the board, but the cost of purchasing the board is very considerable, especially for some imported equipment. In fact, you might as well use an eraser to repeatedly wipe the gold finger several times, clean the dirt on the gold finger, and then test the machine, maybe the problem will be solved! The method is simple and practical.
7.Analysis of intermittent electrical faults
In terms of probability, various intermittent electrical faults include the following situations:
(1)Poor contact
Poor contact between the board and the slot, broken cables, poor contact between the plug and the terminal, and poor soldering of components all belong to this category;
(2)Signal interference
For digital circuits, faults will only appear under certain conditions. It is possible that the interference is too great and affects the control system, causing errors. There are also changes in the parameters of individual components of the circuit board or the overall performance parameters, which makes the anti-interference ability tend to the critical point, resulting in faults;
(3)Poor thermal stability of components
From the big From the maintenance practice of a large number of circuit boards, the first choice is the poor thermal stability of electrolytic capacitors, followed by other capacitors, transistors, diodes, ICs, resistors, etc.;
(4)There is moisture and dust on the circuit board
Moisture and dust will conduct electricity and have a resistance effect, and the resistance value will change during the process of thermal expansion and contraction. This resistance value will have a parallel effect with other components. When this effect is strong, it will change the circuit parameters and cause a fault;
(5)Software is also a factor to consider
Many parameters in the circuit are adjusted by software. The margin of some parameters is adjusted too low and is in the critical range. When the machine operating conditions meet the reasons for the software to determine the fault, an alarm will appear.
