How to repair circuit boards

One, resistance damage characteristics and discrimination

To understand the resistance damage characteristics, there is no need to spend much 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 resistance increases are rare and resistance decreases are rare. Common carbon film resistance, metal film resistance, wire wound resistance and safety resistance. The first two kinds of resistance are the most widely used, the damage characteristics of one is low resistance value (below 100Ω) and high resistance value (above 100 kω) damage rate is high, the middle resistance value (such as hundreds of euros to tens of thousands of euros) rarely damage; Second, when the resistance of low value is damaged, it is often burnt black and easy to find, while when the resistance of high value is damaged, there are few traces. Wire-wound resistance is generally used as a large current limiting, resistance value is not large. When cylindrical wire wound resistance is burned out, some will be blackened or surface blasting, cracks, and some have no trace. Cement resistance is a kind of wire wound resistance, it may break when burned out, otherwise there is no visible trace. When the resistors burn out, some surface will blow off a piece of skin, others will not have any trace, but will never burn black. According to the above characteristics, when checking the resistance can be focused on, quickly find out the damage of the resistance.

According to the characteristics listed above, 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 and high resistance value easy to damage characteristics, we can use a multimeter on the circuit board first direct value of the resistance of both ends of the high resistance value, if the resistance value is larger than the nominal resistance value, Then the resistance must be damaged (pay attention to the resistance value is stable before concluding, because there is a possibility of parallel capacitor components in the circuit, there is a charge-discharge process), if the resistance value is smaller than the nominal resistance value, generally do not ignore it. In this way, every resistance on the PCB board is measured again, even if the "wrong kill" one thousand, also won't let go of a.

Two, operational amplifier is good or bad discrimination method

The ideal operational amplifier has the characteristics of "virtual short" and "virtual break", which are very useful for the analysis of linear operational amplifier circuits. To ensure linear operation, the op amp must operate in a closed loop (negative feedback). Without negative feedback, the open-loop amplifier becomes a comparator. If you want to judge the good or bad of the device, it should be clear that the device in the circuit is used for amplifier or comparator.

As can be seen from the figure, no matter what type of amplifier, there is a feedback resistance Rf, so we can check the feedback resistance from the circuit during maintenance, and use a multimeter to check the resistance between the output end and the reverse input end. If it is too big, such as more than a few MΩ, then we can probably be sure that the device is used as a comparator. If the resistance value is smaller 0Ω to dozens of kΩ, then check whether there is no resistance connected between the output and the reverse input end, if there is a must be used for amplifier.

According to the principle of virtual short amplifier, 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 circuit, the internal resistance of the multimeter will have a little influence on the voltage test, but generally will not exceed 0.2V, if there is more than 0.5V difference, The amplifier will be broken!

If the device is used as a comparator, the same input and reverse input terminals are allowed to be different, the same voltage > If the voltage is reversed, the output voltage is close to a positive maximum; Codirectional voltage < Reverse voltage, the output voltage is close to 0V or negative maximum (depending on dual or single power supply).

If the voltage detected does not conform to this rule, the device will be broken! So you don't have to use substitution, you don't have to remove the chip from the circuit board to see if the op amp is good or bad.

Three, multimeter test SMT components of a tip

Some patch components are very small, with ordinary multimeter pen test and maintenance is very inconvenient, one is easy to cause short circuit, two is coated with insulation coating circuit board inconvenient to contact the metal part of the component pin. Here tells you a simple method, will bring a lot of convenience to the detection.

Take two of the smallest needle, (depth of industrial control maintenance technical column) and the multimeter pen close, and then take a multi-wire cable in the fine copper wire, the pen and needle tied together with the fine copper wire, and then solder firmly. In this way, there is no danger of short circuit when measuring SMT components with a stylus with a small tip, and the tip can Pierce the insulation coating and go straight to the key parts without the bother of scraping the film.

Four,printed circuit board public power short circuit fault maintenance method

Circuit board maintenance, if encountered public power short circuit fault often head big, because many devices share the same power supply, each of the devices with this power supply are suspected of short circuit, if the board components are not much, using the "hoe earth" way can find the short circuit point, if too many components, "hoe earth" can be hoe to the situation will rely on luck. In this recommend a more useful method, using this method, often can quickly find the fault point.

To have a voltage and current are adjustable power supply, voltage 0-30V, current 0-3A, this power is not expensive, 300 yuan or so. The open-circuit voltage is adjusted to the power supply voltage level of the device. Firstly, the current is adjusted to the minimum, and the voltage is applied to the power supply voltage point of the circuit, such as 5V and 0V terminals of the 74 series chip. Depending on the degree of short circuit, the current is gradually increased. Of course, the operating voltage must not exceed the working voltage of the device, and can not be reversed, otherwise it will burn other good devices.

A small eraser solves a big problem

Industrial control of the use of more and more board cards, many board with a golden finger into the slot. Due to the harsh environment of the industrial site, dusty, humid, corrosive gas environment is easy to make the board card contact bad fault, many friends may solve the problem by replacing the board card, but the cost of buying the board card is very considerable, especially some imported equipment of the board card. In fact, you may wish to use an eraser on the gold finger repeatedly rub a few times, the gold finger after cleaning up the dirt, and then try the machine, maybe solve the problem! The method is simple and practical.

Analysis of intermittent electrical failures

All kinds of good and bad electrical failure from the probability of the size of the following types of situations:

1, poor contact

The board and slot contact is bad, the cable is broken when the internal through, the wire plug and terminal contact is not good, components welding, etc.

2, the signal is interfered

For digital circuit, under certain conditions, the fault will present, it is possible that the interference is too much influence on the control system to make it wrong, there are also individual component parameters on the circuit board or the overall performance parameters have changed, so that the anti-interference ability tends to the critical point, resulting in failure;

3. The thermal stability of components is not good

From a large number of maintenance practices, the first electrolytic capacitor thermal stability is not good, followed by other capacitors, triode, diode, IC, resistance, etc.;

4, there is moisture on the PCB circuit board, dust, etc

Moisture and dust will conduct electricity, with resistance effect, and in the process of thermal expansion and cold contraction resistance value will change, the resistance value with other components have parallel effect, this effect is strong will change the circuit parameters, so that the fault occurs;

5. Software is also a factor

Many parameters in the circuit use software to adjust, the margin of some parameters is too low, in the critical range, when the machine operating conditions in line with the software to determine the fault, then the alarm will appear.