01 LED display circuit board capacitor is damaged
The failure caused by capacitor damage is the highest in electronic equipment, especially the damage of electrolytic capacitor is the most common.
Capacitor damage is manifested as: 1. Capacity becomes smaller; 2. Complete loss of capacity; 3. Leakage; 4. Short circuit.
02 Resistance damage
Often see many beginners in the maintenance of the circuit board in the resistance toss, and dismantle and weld, in fact, much more repair, you only need to understand the resistance damage characteristics, you do not have to spend a lot of time.
Resistance is the most numerous component in electrical equipment, but it is not the component with the highest damage rate. Resistance damage is most common in open circuit, and it is rare for the resistance value to become large, and it is rare for the resistance value to become small. Common carbon film resistor, metal film resistor, wire wound resistor and safety resistor several.
We can first observe the low resistance on the circuit board there is no trace of burning black, and then according to the resistance damage when the vast majority of open circuit or resistance value becomes large and high resistance resistance easy to damage characteristics, we can use a multimeter on the circuit board first directly measure the resistance value of the resistance at both ends of the high resistance value, if the resistance value is larger than the nominal resistance value, Then the resistance is definitely damaged (pay attention to the resistance value is stable before concluding, because there may be a parallel capacitor element in the circuit, there is a charge-discharge process), if the resistance value is smaller than the nominal resistance value, then generally ignore it. In this way, every resistance on the circuit board is measured again, even if the “wrong kill” one thousand, it will not miss one.
03 Operational amplifier good or bad judgment method
Amplifiers have the characteristics of “virtual short” and “virtual break”, which are very useful for analyzing linear operation amplifier circuits. In order to ensure linear operation, the op amp must work in a closed loop (negative feedback). Without negative feedback, the open-loop amplifier becomes a comparator. If you want to judge the quality of the device, 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, if the operational amplifier works normally, the same input and reverse input voltage must be equal, even if there is a difference is mv level, of course, in some high input impedance circuits, the internal resistance of the multimeter will have a little impact on the voltage test, but generally will not exceed 0.2V, if there is a difference of more than 0.5V, The amplifier will be broken!
If the device is used as a comparator, the same input and the reverse input are allowed to differ.
If the same voltage is greater than the reverse voltage, the output voltage is close to the positive maximum;
If the same voltage is < the reverse voltage, the output voltage is close to the maximum of 0V or negative (depending on the dual power supply or single power supply).
If the voltage detected does not meet this rule, the device will be broken!
So you don’t have to use substitution, you don’t have to remove the chip on the circuit board to determine whether the op amp is good or bad.
04 Multimeter test SMT components a tip
Some patch components are very small, it is very inconvenient to test and repair with ordinary multimeter pen, one is easy to cause short circuit, and the other is inconvenient for the circuit board coated with insulation coating to contact the metal part of the component pin. Here I tell you a simple method that will bring a lot of convenience to the detection.
Take two of the smallest sewing needles and hold them close to the multimeter pen. Then take a thin copper wire from a multi-strand cable and tie the pen and needle together with the thin copper wire. In this way, there is no danger of short circuit when using a stylus with a small tip to measure those SMT components, and the tip can Pierce the insulation coating and go straight to the key parts, and there is no need to bother to scrape the film.
05 Maintenance method of circuit board common power short circuit fault
Circuit board maintenance, if the fault of short circuit of the public power supply is often large, because many devices share the same power supply, each device with this power supply is suspected of short circuit, if the board components are not much, the “hoe earth” way can eventually find the short circuit point, if the components are too many, “hoe earth” can not hoe to the situation depends on luck. In this recommend a more effective method, the use of this method, often can quickly find the fault point.
To have a voltage and current adjustable power supply, voltage 0-30V, current 0-3A, this power supply is not expensive, about 300 yuan. The open circuit voltage is adjusted to the device power supply voltage level, the current is adjusted to the minimum, the voltage is added to the circuit power supply voltage point such as 5V and 0V end of the 74 series chip, depending on the short-circuit degree, slowly increase the current, touch the device, when touch a device heat obviously, this is often the damaged component, can be removed for further measurement and confirmation. Of course, the operating voltage must not exceed the operating voltage of the device, and can not be reversed, otherwise it will burn other good devices.
06 A small eraser solves a big problem
More and more boards are used in industrial control, and many boards are inserted into slots by goldfinger. Due to the harsh environment of the industrial site, the dusty, humid, corrosive gas environment is easy to make the board produce poor contact failure, many friends may solve the problem by replacing the board, but the cost of buying the board is very considerable, especially the board of some imported equipment. In fact, you may wish to use an eraser to rub several times on the gold finger, clean up the dirt on the gold finger, and then try the machine, maybe it will solve the problem! The method is simple and practical.
07 Good and bad electrical fault analysis
A variety of good and bad electrical failures from the probability of the size probably includes the following situations:
1. Poor contact
Poor contact between the board and the slot, failure to pass when the cable is broken inside, bad contact between the wire plug and the wiring terminal, and virtual welding of components are all of these.
2. Signal interference
For the digital circuit, under specific conditions, the fault will appear, it is possible that the interference is too much to affect the control system to make it wrong, there are also changes in the parameters of individual components of the circuit board or the overall performance parameters, so that the anti-interference ability tends to the critical point, resulting in failure;
3. The thermal stability of the components is not good
From a large number of maintenance practices, the first is that the thermal stability of electrolytic capacitors is not good, 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, have a resistance effect, and in the process of thermal expansion and contraction resistance value will change, this resistance value with other components have a parallel effect, this effect is relatively strong will change the circuit parameters, so that the fault occurs;
5. Software is also a consideration
Many parameters in the circuit are adjusted by software, and the margin of some parameters is adjusted too low and is in the critical range. When the operating condition of the machine meets the reason for the software to determine the fault, then the alarm will appear.
