1. Fault Characteristics and Repair of Circuit Board Capacitor Damage
Failures caused by capacitor damage are the most common in electronic devices, particularly with electrolytic capacitors.
Capacitor damage manifests as:
Reduced capacitance;
Complete loss of capacitance;
Leakage;
Short circuit.
The role of capacitors in circuits varies, leading to different failure characteristics. In industrial control circuit boards, digital circuits dominate, and capacitors are primarily used for power filtering, with fewer used for signal coupling and oscillation circuits. If an electrolytic capacitor in a switching power supply fails, the power supply may not oscillate, resulting in no voltage output; or the output voltage may be poorly filtered, causing voltage instability and logical confusion in the circuit, which manifests as intermittent operation or failure to start. If the capacitor is between the positive and negative terminals of the digital circuit power supply, the failure symptoms are the same.
This is particularly evident on computer motherboards, where many computers exhibit intermittent startup issues after a few years of use. Upon opening the case, it is often possible to see bulging electrolytic capacitors. If the capacitors are removed and measured, their capacitance is often found to be significantly lower than the rated value.
The lifespan of a capacitor is directly related to the ambient temperature; the higher the temperature, the shorter the lifespan. This rule applies not only to electrolytic capacitors but also to other types of capacitors. Therefore, when searching for faulty capacitors, it is essential to focus on those located near heat sources, such as near heat sinks and high-power components, as the closer they are, the higher the likelihood of damage.
I once repaired a power supply for an X-ray flaw detector, where the user reported smoke coming from the power supply. Upon opening the case, I found a large 1000uF/350V capacitor leaking a viscous substance. After removing it and measuring, I found its capacitance was only a few tens of uF. I also noticed that this capacitor was the closest to the heat sink of the rectifier bridge, while others further away were intact and had normal capacitance. Additionally, I found ceramic capacitors that had shorted, which were also located near heat-generating components. Therefore, it is important to focus on these areas during troubleshooting.
Some capacitors exhibit severe leakage, and touching them can even result in a burn. Such capacitors must be replaced.
When troubleshooting intermittent faults, if poor contact has been ruled out, most cases are generally due to capacitor damage. Therefore, when encountering such faults, it is advisable to check the capacitors closely; replacing them often yields surprising results (of course, attention should also be paid to the quality of the capacitors, opting for reputable brands like Rubycon or Nichicon).
2. Characteristics and Identification of Resistor Damage (Surface Mount Resistors are Common in Modern Variable Frequency Air Conditioning Circuit Boards)
Many beginners often struggle with resistors during circuit repairs, disassembling and soldering them repeatedly. However, with experience, understanding the characteristics of resistor damage can save a lot of effort.
Resistors are the most numerous components in electrical devices, but they are not the most failure-prone components, with the exception of surface mount resistors. Resistor failures are most commonly open circuits, with increased resistance being less common and decreased resistance being very rare. Common types include carbon film resistors, metal film resistors, wire-wound resistors, and fusible resistors.
The first two types of resistors are widely used, and their failure characteristics are as follows: 1) Low resistance (below 100Ω) and high resistance (above 100kΩ) have a higher failure rate, while mid-range resistances (such as a few hundred ohms to tens of kilohms) rarely fail; 2) Low resistance failures often result in charring and blackening, making them easy to identify, while high resistance failures usually leave little to no visible trace.
Wire-wound resistors are generally used for high current limiting and have low resistance values. When cylindrical wire-wound resistors fail, some may turn black or develop surface peeling or cracks, while others may show no visible signs. Cement resistors, a type of wire-wound resistor, may break when burned, otherwise showing no visible signs. Fusible resistors may have a portion of their surface blown off, while others may show no signs, but they will never be charred or blackened. Based on these characteristics, we can focus our inspection on resistors to quickly identify the damaged ones.
Based on the characteristics listed above, we can first check for any signs of burning on low resistance resistors on the circuit board. Then, based on the fact that most resistor failures are open circuits or increased resistance, and that high resistance resistors are prone to failure, we can use a multimeter to measure the resistance across high resistance resistors on the circuit board. If the measured resistance is greater than the rated value, that resistor is definitely damaged (be sure to wait for the resistance reading to stabilize before concluding, as there may be parallel capacitive elements in the circuit that require a charging and discharging process). If the measured resistance is less than the rated value, it is generally safe to ignore it. By measuring each resistor on the circuit board, even if we “wrongly” identify a thousand, we will not miss a single one.
3. Methods for Determining the Quality of Operational Amplifiers
Determining the quality of operational amplifiers can be challenging for many electronic repair technicians, not only due to educational background (many of my students are undergraduates who struggle without guidance, and even a graduate student specializing in variable frequency control has similar difficulties!). Here, I would like to discuss this with everyone, hoping to provide some assistance.
An ideal operational amplifier has the characteristics of “virtual short” and “virtual open,” which are very useful for analyzing linear applications of operational amplifier circuits. To ensure linear operation, the operational amplifier must work in a closed-loop (negative feedback) configuration. If there is no negative feedback, the open-loop operational amplifier becomes a comparator. To determine the quality of the device, one must first clarify whether the device is used as an amplifier or as a comparator in the circuit.
From the images, we can see that regardless of the type of amplifier, there is a feedback resistor Rf. Therefore, during repairs, we can check this feedback resistor on the circuit. Using a multimeter, measure the resistance between the output and the inverting input. If the resistance is excessively high, such as several MΩ, we can reasonably conclude that the device is being used as a comparator. If this resistance is relatively low (0Ω to several tens of kΩ), we should check for any resistors connected between the output and the inverting input; if present, it is definitely being used as an amplifier.
According to the principle of virtual short in amplifiers, if this operational amplifier is functioning correctly, the voltages at the non-inverting and inverting inputs must be equal, with any difference being in the millivolt range. Of course, in some high input impedance circuits, the internal resistance of the multimeter may slightly affect the voltage measurement, but generally, it should not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier is undoubtedly faulty! (I use a FLUKE 179 multimeter)
If the device is used as a comparator, the non-inverting and inverting inputs are allowed to be unequal:
If the non-inverting voltage > inverting voltage, the output voltage approaches the positive maximum value;
If the non-inverting voltage < inverting voltage, the output voltage approaches 0V or the negative maximum value (depending on whether it is a dual or single power supply).
If the detected voltages do not conform to these rules, the device is undoubtedly faulty!
Thus, you do not need to use substitution methods or remove the chip from the circuit board to determine the quality of the operational amplifier.
4. A Small Tip for Testing SMT Components with a Multimeter
Some surface mount components are very small, making it inconvenient to test them with a standard multimeter probe. This can easily cause short circuits, and it is difficult to access the metal parts of the component leads on insulated circuit boards. Here is a simple method that can greatly facilitate testing.
Take two of the smallest sewing needles, and attach them to the multimeter probes. Then, take a thin copper wire from a multi-strand cable and bind the probes and needles together, securing them with solder. This way, when using the probes with small needle tips to test SMT components, there is no risk of short circuits, and the needle tips can pierce the insulation coating, allowing access to critical areas without the need to scrape off the coating.
5. Troubleshooting Methods for Short Circuit Faults in Common Power Supplies on Circuit Boards
In circuit board repairs, encountering short circuit faults in common power supplies can be daunting, as many components share the same power supply, and each component using this power supply is a suspect for short circuits. If there are few components on the board, using a “broad search” approach can eventually locate the short circuit point. However, if there are too many components, relying on luck may not yield results. Here, I recommend a more effective method that can quickly identify the fault point.
You need a power supply that can adjust both voltage and current, with a voltage range of 0-30V and a current range of 0-3A. This type of power supply is inexpensive, around 300 yuan. Set the open-circuit voltage to the level of the device’s power supply voltage, and first adjust the current to the minimum. Apply this voltage to the power supply points of the circuit, such as the 5V and 0V terminals of the 74 series chips. Depending on the severity of the short circuit, gradually increase the current. When you touch a component that becomes noticeably hot, that is often the damaged component, which can then be removed for further measurement and confirmation. Of course, during operation, the voltage must not exceed the working voltage of the components, and it must not be reversed; otherwise, other good components may be damaged.
6. A Small Eraser Solving Big Problems
With the increasing use of circuit boards in industrial control, many boards utilize gold fingers inserted into slots. Due to the harsh industrial environment, which is often dusty, humid, and filled with corrosive gases, circuit boards can develop contact failure issues. Many friends may have resolved problems by replacing circuit boards, but the cost of purchasing new boards can be significant, especially for certain imported equipment. In fact, you might try using an eraser to clean the gold fingers a few times; after cleaning the dirt off the gold fingers, try the machine again, and you might just solve the problem! This method is simple and practical.
7. Analysis of Intermittent Electrical Faults
Various intermittent electrical faults can be categorized by their probability as follows:
1. Poor contact
Poor contact between the board and the slot, intermittent breaks in internal cables, poor contact at connectors and terminals, and cold solder joints all fall into this category;
2. Signal interference
For digital circuits, faults may only manifest under specific conditions, possibly due to excessive interference affecting the control system, or changes in the parameters of individual components or overall performance may reduce the circuit’s immunity to interference, leading to faults;
3. Poor thermal stability of components
From extensive repair experience, the poor thermal stability of electrolytic capacitors is the most notable, followed by other capacitors, transistors, diodes, ICs, and resistors;
4. Moisture and dust on the circuit board
Moisture and dust can conduct electricity, creating resistance effects, and during thermal expansion and contraction, the resistance value may change. This resistance can have a parallel effect with other components, and when this effect is strong, it can alter circuit parameters, causing faults;
5. Software is also a consideration
Many parameters in circuits are adjusted using software, and if certain parameters are set too close to their limits, they may fall within a critical range. When the machine operates under conditions that meet the software’s fault criteria, alarms will trigger.
Some Screenshots from Electronic Books
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