1. FANUC circuit board failure characteristics and maintenance

Faults caused by capacitor damage are particularly high in electronic equipment, especially the damage of electrolytic capacitors is common

The performance of capacitor damage is as follows: 1. Capacity becomes smaller; 2. Complete loss of capacity; 3. Leakage; 4. Short circuit.

Capacitors play different roles in the circuit, and the faults they cause have their own characteristics. In the maintenance of FANUC circuit boards, digital circuits account for the vast majority. Capacitors are mostly used for power supply filtering, and less capacitors are used for signal coupling and oscillation circuits. If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not vibrate, and there is no voltage output; or the output voltage is not filtered well, and the circuit is logically chaotic due to voltage instability. No matter the machine, if the capacitor is connected between the positive and negative poles of the power supply of the digital circuit, the failure will be the same as above. This is especially obvious on computer motherboards. Many computers sometimes fail to turn on after a few years, and sometimes they can be turned on. Open the case, you can often see the phenomenon of electrolytic capacitors bulging. If you remove the capacitors to measure the capacity , 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 the capacitors that are close to the heat source, such as the capacitors next to the heat sink and high-power components. The closer you are to it, the greater the possibility of damage. I have repaired the power supply of an X-ray flaw detector. The user reported that smoke was coming out of the power supply. After disassembling the case, I found that there was a 1000uF/350V large capacitor with oily things flowing out. Removed a certain amount of capacity. It is only tens of uF, and it is found that only this capacitor is the closest to the heat sink of the rectifier bridge, and the others far away are intact and have normal capacity. In addition, there was a short circuit in the ceramic capacitor, and it was also found that the capacitor was relatively close to the heating component. Therefore, there should be some emphasis in the inspection and search.

Some capacitors have serious leakage current, and even burn your hands when touched with your fingers. This type of capacitor must be replaced.

In the case of ups and downs during maintenance, except for the possibility of poor contact, most of the failures are generally caused by capacitor damage. Therefore, when encountering such failures, you can focus on checking the capacitors. After replacing the capacitors, it is often surprising (of course, you must also pay attention to the quality of the capacitors, and choose a better brand, such as Ruby, Black Diamond, etc.).

 

2. Characteristics and discrimination of resistance damage

 It is often seen that many beginners are tossing on the resistance when repairing the circuit, and it is dismantled and welded. In fact, there are a lot of repairs. As long as you understand the damage characteristics of the resistance, you don't have to spend a lot of time.

        Resistance is the largest number of components in electrical equipment, but it is not the component with the highest damage rate. Resistance damage is most common as an open circuit. It is rare for the resistance to become larger, and it is very rare for the resistance to become smaller. Common ones include carbon film resistors, metal film resistors, wire wound resistors and insurance resistors. The first two types of resistors are the most widely used. One of the characteristics of their damage is that the damage rate of low resistance (below 100Ω) and high resistance (above 100kΩ) is higher, and the middle resistance value (such as hundreds of ohms to tens of kiloohms) ) Is rarely damaged; second, when low-resistance resistors are damaged, they are often burnt and blackened, which is easy to find, while high-resistance resistors are rarely damaged. Wirewound resistors are generally used for high current limiting, and the resistance is not large. When cylindrical wire wound resistors are burnt, some will turn black or the surface will burst or crack, and some will have no traces. Cement resistors are a kind of wire wound resistors, which may break when burned out, otherwise there will be no visible traces. When the fuse resistor burns out, a piece of skin will be exploded on the surface, and some have no trace, but it will never be burnt or blackened. According to the above characteristics, you can focus on checking the resistance and quickly find out the damaged resistance.

  According to the characteristics listed above, we can first observe whether the low-resistance resistors on the circuit board have any traces of burning black, and then according to the characteristics that most of the resistors are open or the resistance becomes larger when the resistors are damaged, and the high-resistance resistors are easily damaged. We can use a multimeter to directly measure the resistance at both ends of the high-resistance resistor on the circuit board. If the measured resistance is greater than the nominal resistance, the resistance must be damaged (note that the resistance is stable after the display is stable. In conclusion, because there may be parallel capacitance elements in the circuit, there is a charge and discharge process), if the measured resistance is smaller than the nominal resistance, generally ignore it. In this way, every resistance on the circuit board is measured once, even if one thousand is "wrongly killed", one will not be missed.

 

Three. How to judge the quality of operational amplifiers

 It’s difficult to judge the quality of operational amplifiers for quite a lot of electronic repairers, not only because of their educational level (there are many undergraduates undergraduates, if they don’t teach, they won’t. It takes a long time to understand. There is also a special follower The same is true for graduate students whose tutors are learning inverter control!), I would like to discuss with you here, and I hope it will be helpful to everyone.

The ideal operational amplifier has the characteristics of "virtual short" and "virtual break", these two characteristics are very useful for analyzing the linear application of the operational amplifier circuit. In order to ensure linear application, the op amp must work in a closed loop (negative feedback). If there is no negative feedback, the op amp under open-loop amplification 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 as a comparator in the circuit.

        From the figure, we can see that no matter what type of amplifier, there is a feedback resistance Rf, then we can check this feedback resistance from the circuit when repairing, and use a multimeter to check the resistance between the output terminal and the reverse input terminal. If the value is outrageous, such as a few MΩ or more, we can be sure that the device is used as a comparator. If the resistance is small from 0Ω to tens of kΩ, then check whether there is a resistor connected to the output terminal and reverse input Between the terminals, if there are some, it must be used as an amplifier.

According to the principle of virtual shortness of the amplifier, that is to say, if the operational amplifier works normally, the voltage at the same direction input terminal and the reverse input terminal must be equal, and even if there is a difference, it is at the mv level. Of course, in some high input impedance circuits, a multimeter The internal resistance will have a little effect on the voltage test, but generally it will not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier will undoubtedly be broken! (I am using FLUKE179 multimeter)

If the device is used as a comparator, the same direction input terminal and reverse input terminal are allowed to be different,

Same direction voltage>reverse voltage, the output voltage is close to the maximum positive value;

If the same direction voltage is less than the reverse voltage, the output voltage is close to 0V or the negative maximum value (depending on dual power supply or single power supply).

If it is detected that the voltage does not meet this rule, the device must be broken!

In this way, you can judge whether the operational amplifier is good or bad without using the substitution method or removing the chip on the circuit board.

 

Four. A small trick to test SMT components with a multimeter

Some SMD components are very small, and it is inconvenient to use ordinary multimeter test leads to test and repair. One is easy to cause a short circuit, and the other is that it is inconvenient for the circuit board coated with an insulating coating to touch the metal part of the component pin. Here is an easy way to tell everyone, it will bring a lot of convenience to the detection.

Take two small sewing needles (in-depth industrial control maintenance technology column) and close them with the multimeter pen, then take a thin copper wire from a multi-strand cable, and use the thin copper wire to sew the pen and the clothes The pins are tied together and then soldered firmly. In this way, there is no risk of short circuit when measuring those SMT components with a test pen with a small needle tip, and the needle tip can pierce the insulating coating and directly ram the key parts without having to bother to scrape the film.

 

Five. Repair methods for short circuit faults of circuit board public power supply

In circuit board maintenance, if you encounter a short circuit of the public power supply, the fault is often serious, 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, use "hoe the earth" After all, you can find the short-circuit point. If there are too many components, it will depend on luck whether the "hoe the earth" can hoe the situation. A more effective method is recommended here. Using this method will get twice the result with half the effort, and the point of failure can often be found quickly. It is necessary to have 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, add this voltage to the power supply voltage point of the circuit, such as the 5V and 0V terminals of the 74 series chip, and slowly increase the current depending on the degree of short circuit. Large, touch the device with your hands. When you touch a device that heats up significantly, 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 the connection cannot be reversed, otherwise it will burn out other good devices.

 

6. A small eraser can solve big problems

More and more boards are used in industrial control, and many boards use golden fingers to insert into the slots. Due to the harsh industrial site environment, dusty, humid, and corrosive gas environment, the board may have poor contact failures. Friends may have solved the problem by replacing the board, but the cost of buying the board is very considerable, especially the boards of some imported equipment. In fact, you might as well use an eraser to rub the golden finger a few times, clean up the dirt on the golden finger, and try the machine again, maybe the problem will be solved! The method is simple and practical.

 

Seven, the analysis of electrical faults in good times and bad times

In terms of probability, various electrical faults with good times and bad times may include the following situations:

1. Poor contact, poor contact between the board and the slot, failure of the cable when the cable is broken, poor contact between the plugs and terminals, and false soldering of components, etc., all belong to this category;

2. The signal is interfered. For digital circuits, faults will only appear under certain conditions. It is possible that too much interference affects the control system and causes errors. There are also individual component parameters or overall performance parameters of the circuit board. Changes have made the anti-interference ability to the critical point, and thus the failure;

3. The thermal stability of components is not good. From a large number of maintenance practices, the thermal stability of electrolytic capacitors is the first to be poor, followed by other capacitors, transistors, diodes, ICs, resistors, etc.;

4. Moisture, dust, etc. 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 malfunctions occur;

5. Software is also one of the consideration factors. Many parameters in the circuit are adjusted by software. The margins of some parameters are adjusted too low and are in the critical range. When the machine operating conditions meet the reason for the software to determine the failure, then the alarm will be issued. appear.