Capacitor Failure
Capacitor damage causes the highest number of failures in electronic equipment, especially electrolytic capacitor damage is the most common. Capacitor damage manifests itself in the following ways: reduced capacity, complete loss of capacity, leakage, short circuit.
Capacitors play a different role in the circuit, the failure caused by the characteristics: in the industrial control circuit board, digital circuits account for the vast majority of capacitors are mostly used for power supply filtering, signal coupling and oscillator circuits used for less capacitance. Used in the switching power supply electrolytic capacitors, if damaged, the switching power supply may not vibrate, no voltage output;
Or the output voltage filtering is not good, the circuit due to voltage instability and the logic of chaos, manifested in the machine works well or can not turn on the machine, if the capacitor and in the digital circuit between the positive and negative poles of the power supply, the fault performance as above.
This is particularly evident in the computer motherboard, many computers with a few years on the sometimes can not turn on the machine, and sometimes can be turned on the phenomenon, open the chassis, you can often see the phenomenon of electrolytic capacitors bulging, if the capacitance will be removed to measure the capacity, and found that the actual value is much lower than the value.
Capacitor life and ambient temperature is directly related to the higher the ambient temperature, the shorter the life of the capacitor. This law applies not only to electrolytic capacitors, but also to other capacitors. So in the search for faulty capacitors should focus on checking and heat sources closer to the capacitor, such as heat sinks and high-power components next to the capacitor, the closer it is, the greater the possibility of damage. So in the maintenance should focus on finding.
Some capacitors leakage is more serious, with a finger touch will even burn your hand, this capacitor must be replaced. In the maintenance of good and bad failure, excluding the possibility of poor contact, generally most of the capacitor damage caused by the failure. So when it comes to such failures, you can focus on checking the capacitance, replace the capacitance is often a surprise.
Resistor failure
Often see many beginners in overhauling the circuit in the resistor on the toss, and is to tear down and weld, in fact, repair more, you just need to understand the characteristics of the damage to the resistor, there is no need to go to great lengths.
Resistors are the largest number of components in electrical equipment, but not the highest damage rate components. Resistor damage to the most common open-circuit, resistance value is less common, resistance value becomes small very rare. Commonly, there are carbon film resistors, metal film resistors, wirewound resistors and insurance resistors.
The first two types of resistors are the most widely used, the damage is characterized by a low resistance (100Ω or less) and high resistance (100kΩ or more) of the damage rate is high, the intermediate resistance (such as hundreds of ohms to dozens of kilo-ohms) of the damage is rare; two is often low resistance resistance damage is burnt black, it is easy to find, and high resistance resistance damage is seldom traces.
Wirewound resistors are generally used as high-current current limiting, resistance value is not large; cylindrical wirewound resistors when burned, some will be black or the surface of the burst skin, cracks, and some have no trace; cement resistor is a kind of wirewound resistor, burned, it may be broken, otherwise there is no visible traces; insurance resistor burned when some of the surface of a piece of skin will be blown off, and some do not have any traces, but will never be scorched black. According to the above characteristics, you can focus on when checking the resistor, and quickly find the damaged resistor.
According to the characteristics listed above, we can first observe the circuit board on the low resistance resistor there is no trace of burnt black, and then according to the vast majority of resistor damage or resistance value of the majority of open-circuit and the characteristics of the high resistance resistance is easily damaged, we can use a multimeter in the circuit board first directly measure the resistance of the high resistance resistance of the resistance of the two sides of the resistance value.
If the measured resistance is larger than the nominal resistance, the resistor must be damaged (pay attention to wait until the resistance is stable before concluding, because the circuit may be connected in parallel with capacitive elements, there is a charging and discharging process), if the measured resistance is smaller than the nominal resistance, it is generally not necessary to pay attention to it. In this way, each resistor on the circuit board are measured once, even if the "wrong" thousand, will not let go of one.
Operational amplifier failure
Operational amplifier good or bad judgment of a considerable number of electronic repairers have a certain degree of difficulty, not only the relationship between the degree of culture, here to discuss with you, I hope to help you.
Ideal operational amplifier has a "false short" and "false break" characteristics, these two characteristics are very useful for analyzing the linear use of the op amp circuit. In order to ensure linear operation, the op-amp must operate in closed loop (negative feedback). Without negative feedback, an op amp under open loop amplification becomes a comparator. If you want to judge the device is good or bad, you should first distinguish whether the device is used as an amplifier or a comparator in the circuit.
According to the principle of the amplifier virtual short, that is to say, if the operational amplifier is working properly, its isotropic input and inverse input voltage must be equal, even if there is a difference is also mv level, of course, in some high input impedance circuits, the internal resistance of the multimeter will be a little bit of influence on the voltage test, but generally will not be more than 0.2V, if there is a difference of 0.5V or more, the amplifier must be bad.
If the device is used as a comparator, it is allowed to have unequal isotropic and inverse inputs. If the isotropic voltage > the inverse voltage, the output voltage is close to a positive maximum; if the isotropic voltage < the inverse voltage, the output voltage is close to 0V or a negative maximum (depending on the dual or single power supply). If a voltage is detected that does not conform to this rule, the device must be bad! This way you don't have to use the substitution method and you don't have to remove the chip on the board to determine if the op-amp is good or bad.
SMT component failure
Some SMD components are very small, with an ordinary multimeter pen test service is very inconvenient, one is easy to cause a short circuit, the second is coated with an insulating coating on the circuit board is inconvenient to contact the metal part of the component pins. Here to tell you a simple method, will bring a lot of convenience to the test.
Take two of the smallest number of sewing needles, will be close to the pen with a multimeter, and then take a multi-stranded cable in the thin copper wire, with a thin copper wire will be tied to the pen and sewing needles, and then welded firmly with solder. So that with a fine tip of the pen to measure those SMT components when there is no longer a short-circuit risk, and the tip of the needle can pierce the insulating coating, hit the key parts, and no longer have to bother to scrape the film membrane.
Public power short-circuit fault
Circuit board maintenance, if you encounter a public power supply short-circuit failure is often a big head, because many devices share the same power supply, every device with this power supply is suspected of short-circuit.
If not many components on the board, the use of "hoeing" approach can eventually find a short-circuit point; if too many components, "hoeing" can not hoe to the situation depends on luck. Recommended here is a more useful method, using this method, twice the effort, often quickly find the fault point.
To have a voltage and current are adjustable power supply, voltage 0-30V, current 0-3A, this power supply is not expensive, about 300 yuan. Open circuit voltage to the device supply voltage level, the first current to the minimum, the voltage will be added to the circuit's power supply voltage points such as 74 series chip 5V and 0V terminal, depending on the degree of short-circuit, slowly increase the current.
Touch the device by hand, when a device touched the heat is obvious, this is often damaged components, can be removed for further measurements to confirm. Of course, the operation of the voltage must not exceed the device's operating voltage, and can not be reversed, otherwise it will burn other good devices.
Board failure
More and more boards are used for industrial control, and many boards are inserted into slots with gold fingers. Due to the harsh environment of the industrial site, dusty, humid, corrosive gas environment prone to poor contact failure of the board, many friends may be through the replacement of the board to solve the problem, but the cost of purchasing the board is very substantial, especially some of the imported equipment board.
In fact, you may wish to use an eraser on the gold finger repeatedly rubbed a few times, the gold finger on the dirt cleaned up, and then try the machine, may solve the problem, the method is simple and practical.
Electrical faults
A variety of good and bad electrical failure from the probability of the size of the following cases:
Poor contact: poor contact between the board and the slot, cable internal fracture when through when not pass, wire plug and terminal contact is not good, component soldering, etc. are such;
Signal interference: for digital circuits, in specific situations under the conditions of the failure will be presented, it may indeed be too much interference affects the control system to make it wrong, there are circuit boards individual component parameters or the overall performance of the parameters appear to be a change in the anti-jamming ability tends to the critical point and thus failures;
Components thermal stability is not good: from a large number of maintenance practices, which is the first electrolytic capacitor thermal stability is not good, followed by other capacitors, transistors, diodes, ICs, resistors and so on;
Circuit board with moisture, dust, etc.: moisture and dust will be conductive with a resistive effect, and in the process of thermal expansion and contraction of the resistance will also change, the resistance value will have a parallel effect with other components, the effect is relatively strong when it will change the circuit parameters so that the failure occurs;
Software is also one of the factors to consider: many parameters in the circuit to adjust the use of software, some parameters of the margin adjusted too low in the critical range, when the machine operating conditions in line with the software to determine the reasons for failure, then the alarm will appear.