Repairing PCB without drawings, this article is enough

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Repairing PCB without drawings, this article is enough [Copy link]

1. How to repair circuit boards without drawings?

1. Have a plan in mind

You must thoroughly understand the principles of some typical circuits and know them by heart. The drawings are dead, but the ideas in your head are alive. You can make analogies, inferences, and draw inferences from one example.
For example, a switching power supply is always inseparable from an oscillation circuit, a switching tube, and a switching transformer. When checking, you must check whether the circuit has started to oscillate, whether the capacitor is damaged, and whether the transistors and diodes are damaged. No matter what switching power supply you encounter, the operation is almost the same, and you don’t have to insist on having a circuit diagram.
For example, a single-chip microcomputer system, including a crystal oscillator, three buses (address line, data line, control line), input and output interface chips, etc., cannot be repaired without these ranges; another example is an analog circuit composed of various operational amplifiers. Even if it changes in thousands of ways, you can reason on the basis of "virtual short" and "virtual break" to have a clear idea, analyze it in detail, and make it clear. After you have mastered the skills of analysis and reasoning, even if you encounter equipment you have never seen before, you only need to understand it in principle.
2. Pay attention to the order of maintenance

Only by paying attention to the maintenance sequence can we find the shortest path to solve the problem, avoid random poking and dismantling, and avoid repairs that fail and cause the fault to expand. Maintenance is like a doctor treating a patient, and it also requires "looking, smelling, asking and feeling". "Looking" means checking the appearance of the faulty board to see if there are obvious signs of damage on it, whether there are any components that are burnt or cracked, whether the circuit board has broken wires or leakage caused by corrosion, whether the capacitor is leaking, whether the top is bulging, etc.; "smelling" means using your nose to smell if there is any smell of burning, and where the smell comes from; "asking" is very important, and you must ask the person involved in detail about the situation when the equipment failed, and infer the possible faulty parts or components from the situation; "feeling" means using certain testing instruments and means, divided into two situations: powered on and not powered on, to check the resistance, voltage, waveform, etc. of the circuit parts or components, compare and test good and bad circuit boards, and observe the differences in parameters, etc.
In fact, there are many faults that you can solve without even using a multimeter, so circuit diagrams are naturally unnecessary.
3. Be good at summarizing rules

Generally, after accumulating a certain amount of maintenance experience, you should be good at summarizing and analyzing the reasons for each component damage. Is it improper operation? Lack of maintenance? Unreasonable design? Poor component quality? Natural aging? With these analyses, the next time you encounter a similar failure, even if it is not the same circuit board, you will have a better idea.
4. Be good at finding information

Since the advent of the Internet, it has become very easy to find information. You can find almost any device principle or circuit principle you don’t understand on the Internet. You can find all kinds of IC information on the Internet.
5. Necessary testing equipment is required

If you consider maintenance as your career, then a certain amount of equipment investment is necessary. Electric soldering iron, multimeter, common disassembly and assembly tools, the brand should not be too bad. If conditions permit, get a 100M dual-trace oscilloscope, and if conditions permit, get an online maintenance tester.

2. Fault characteristics and repair of damaged capacitors on industrial control circuit boards (Senmu Lei Shi’s experience)
Faults caused by capacitor damage are the highest in electronic equipment, especially electrolytic capacitor damage. Capacitor damage is manifested as: 1. Capacitance decreases; 2. Complete loss of capacity; 3. Leakage; 4. Short circuit.
Capacitors play different roles in the circuit, 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, while capacitors used for signal coupling and oscillation circuits are relatively few. 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 will have logical confusion due to unstable voltage, which is manifested as the machine working sometimes well and sometimes bad or unable to start. 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 obvious on the computer motherboard. Many computers have been used for a few years and sometimes cannot be turned on, and sometimes can be turned on. When the chassis is opened, it is often seen that the electrolytic capacitor is bulging. If the capacitor is removed and the capacity is measured, it is found to be much lower than the actual value.
The life of a 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, you should focus on checking capacitors that are relatively close to the heat source, such as capacitors next to heat sinks and high-power components. The closer they are to the heat sink, the greater the possibility of damage.
Some capacitors have serious leakage and can even burn your hands when you touch them with your fingers. Such capacitors must be replaced. When repairing a fault that sometimes works and sometimes doesn't, 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. Replacing the capacitor is often a pleasant surprise (of course, you should also pay attention to the quality of the capacitor and choose a better brand).

3. Characteristics and identification of resistor damage (from Senmu Lei Shi’s experience)
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 often, and the resistance value decreases very rarely. 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 high 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 limiting and have low resistance. When cylindrical wirewound resistors burn out, some will turn black or have peeling and cracking on the surface, while some will have no traces. Cement resistors are a type of wirewound resistor. They may break when burned out, otherwise there will be no visible traces. When the fuse resistor is burned, some of the surface will explode with a piece of skin, and some will not have any traces, but it will never be burnt black. According to the above characteristics, we can focus on the inspection of the resistor and quickly find the damaged resistor. According to the characteristics listed above, we can first observe whether the low-resistance resistor on the circuit board has traces of burning black, and 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 of the high-resistance resistors on the circuit board.

If the measured resistance is larger than the nominal resistance, the resistor is definitely damaged (note that you should wait until the resistance value is stable before drawing a conclusion, because there may be a capacitor in parallel in the circuit, and there is a charging and discharging process). If the measured resistance is smaller than the nominal resistance, generally ignore it. In this way, every resistor on the circuit board is measured, even if you "mistakenly kill" a thousand, you will not miss any.

4. Methods for judging the quality of operational amplifiers (from Senmu Lei Shi's experience)
Ideal operational amplifiers have the characteristics of "virtual short" and "virtual disconnection", which are very useful for analyzing linear operational amplifier circuits. 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! 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.

5. A little trick for testing SMT components with a multimeter (from Senmu Leishi’s experience)

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 need to worry about short circuits, and the needle tip can pierce the insulating coating and directly hit the key parts, so you don’t have to bother to scrape those membranes.

6. Inspection and repair methods for short-circuit faults of common power supply of circuit boards (experience of Senmu Lei Shi)
In 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 earth". If there are too many components, whether "hoeing the earth" can hoe the situation depends on luck. Here I recommend a more effective method. With 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 power supply voltage level of the device, 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, during operation, the voltage must not exceed the working voltage of the device, and it cannot be connected in reverse, otherwise other good devices will be burned out.

7. A small eraser can solve big problems (from Senmu Lei Shi’s experience)

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 field environment, dusty, humid, and corrosive gas environments, boards are prone to poor contact failures. Many friends may solve the problem by replacing boards, but the cost of purchasing boards is very considerable, especially for boards of some imported equipment. In fact, you might as well use an eraser to repeatedly wipe the gold fingers several times to clean the dirt on the gold fingers, and then test the machine, maybe the problem will be solved! The method is simple and practical.

8. Analysis of electrical faults that are sometimes good and sometimes bad (from Senmu Lei Shi’s experience)

Various electrical faults that sometimes work and sometimes don't work include the following situations in terms of probability:
1. Poor contact

This includes poor contact between the board and the slot, broken cables, poor contact between the plug and the terminal, and poor soldering of components.
2. Signal interference

For digital circuits, faults will only occur under specific conditions. It is possible that the interference is too great and affects the control system, causing it to go wrong. There may also be changes in the parameters of individual components of the circuit board or the overall performance parameters, causing the anti-interference ability to approach the critical point, thus causing a fault.
3. The thermal stability of components is not good.
From a large number of maintenance practices, the first thing to consider 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 relatively 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 using software. The margin of some parameters is adjusted too low and is within the critical range. When the machine operating conditions meet the reasons for the software to determine a fault, an alarm will occur.

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