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I. Visual inspection method: A technique that does not require any testing instruments, and relies on human senses (eyes, ears, nose, hands) to observe and identify faults. The visual inspection method is divided into external inspection and startup inspection. 1. The contents of the visual inspection mainly include: ① Whether the casing of the instruments and the dial glass are in good condition, whether the pointers are deformed or in contact with the scale, whether the mounting fasteners are secure, whether the positions of the various switches and knobs are correct, whether the moving parts rotate smoothly, and whether there are any obvious changes in the adjustable parts ； ②Check whether the connections are intact, whether all connectors are properly connected, and whether the reeds in the circuit board sockets lack sufficient elasticity or have poor contact. For instruments assembled from individual units, pay special attention to ensuring that the screws connecting the various unit boards are tightened properly ； ③Check the contacts of all relays and contactors for any signs of misalignment, jamming, oxidation, or burning and sticking ； ④Is the power supply fuse blown? Are the electron tubes cracked or leaking (a white powder coats the inner wall of the tube in case of leakage), or damaged? Has the paint on the transistor casing changed color or is there a break in the connections? Are the resistors burned out? Are there any broken wires in the coils? Has the capacitor casing swollen, leaked, or exploded? ； ⑤Check whether the copper strips on the printed circuit board are broken, have solder bridging, or are short-circuited; also verify that the solder joints of each component are in good condition, with no signs of poor soldering, missed soldering, or desoldering ； ⑥Check whether the arrangement and wiring of various components are skewed, misaligned, detached, or in contact with each other. 2. The checks upon startup mainly include: ① Whether the power indicator light inside the device, as well as various vacuum tubes and other light-emitting components, are powered on and lit up ； ②Are there any instances of high-voltage arcing, discharge, or smoking inside the machine? ； ③Is there vibration accompanied by cracking, rubbing, or knocking sounds? ； ④Check whether the temperature rise of components that generate heat easily such as transformers, motors, and power amplifier tubes, as well as resistors and integrated circuits, is normal, and whether they are hot to the touch ； ⑤Is there any unusual odor inside the machine, such as a burnt smell resulting from damaged insulation in transformers, or an ozone-like smell produced by the ionization of air caused by high-voltage leakage and arcing in the oscilloscope tube? ； ⑥Check whether the mechanical transmission part is operating properly, and look for issues such as poor gear engagement, jamming, severe wear, slipping or deformation, and inefficient transmission. Visual inspection must be carried out with great care and attention; carelessness and haste are strictly avoided. When checking components and connections, only gentle shaking is allowed; excessive force should be avoided to prevent breaking the components, connections, and the copper foil on the printed circuit board. When turning the device on for inspection, do not remove your hand from the power switch; if any abnormalities are detected, turn it off immediately. Special attention must be paid to personal safety; it is absolutely necessary to avoid touching live equipment with both hands at the same time. The large-capacity filter capacitors in the power supply circuit carry a charge, so precautions must be taken to avoid electric shock. II. The investigation method is a technique for analyzing and determining the cause of a fault by investigating the fault symptoms and its development process. Generally, there are the following aspects: ① The usage conditions prior to the failure and any preceding signs ； ②Were there any phenomena such as sparking, smoking, or unusual odors when the fault occurred? ； ③Changes in supply voltage ； ④External conditions such as overheating, lightning, humidity, and collisions ； ⑤Has it been affected by external strong electric or magnetic fields? ； ⑥Were there any cases of improper use or misoperation? ； ⑦Are the faults occurring under normal use, or after repairing or replacing components? ； ⑧What faults have occurred in the past and what repairs were done, etc. Use investigation methods to diagnose faults; the investigation should be thorough and careful, especially the feedback from those using the equipment on site must be verified, and there should be no rush to disassemble it for repair. Maintenance experience shows that many of the reports from users are incorrect or incomplete, and upon verification, many issues that do not require maintenance can be identified. III. The cutoff method involves disconnecting the suspected part from the entire device or the individual circuit to see if the fault disappears, thereby determining where the fault lies. When an instrument malfunctions, first preliminarily assess the various possible causes of the fault. Within the fault area, disconnect the suspect circuit section to determine whether the fault occurs before or after the disconnection. If the fault disappears after powering on for inspection, it indicates that the fault is likely in the circuit that was disconnected. If the fault persists, further checks by disconnecting additional circuits should be carried out to gradually rule out possible causes and narrow down the scope of the fault until the true cause is identified. The open-circuit method is particularly convenient for troubleshooting modular, combinable, and plug-in instruments and meters, and it is also effective for certain short-circuit faults with excessive current. However, it is not suitable for the overall circuit to be a closed-loop system with large loops or a directly coupled circuit structure. IV. The short-circuit method involves temporarily short-circuiting a particular circuit stage or component suspected of having a fault, and observing whether there are any changes in the fault condition in order to determine the location of the fault. When the short-circuit method is used to examine multi-stage circuits, if a certain stage or component is temporarily short-circuited and the fault disappears or is significantly reduced, it indicates that the fault lies before the short-circuit point; if there is no change in the fault, then the fault is located after the short-circuit point. If the voltage at an output stage is abnormal, short-circuit the input of that stage; if the voltage at the output becomes normal in this case, then the circuit of that stage is functioning properly. The short-circuit method is also commonly used to check whether components are functioning properly; for example, the base and emitter of a bipolar transistor can be short-circuited using pliers, and the change in collector voltage is observed to determine whether the transistor has amplifying capability. In TTL digital integrated circuits, the short-circuit method is used to determine whether gate circuits and flip-flops are functioning properly. Shorting the control electrode and cathode of the thyristor is used to determine whether the thyristor is faulty, etc. Additionally, it is also possible to short-circuit the input terminals of certain instruments (such as electronic potentiometers) and observe any changes in the instrument’s readings to determine whether the instrument is affected by interference. V. The replacement method is a technique for identifying where a fault lies by swapping out certain components or circuit boards. Replace the component under suspicion with one of the same specifications and good performance, then power it on for testing; if the fault disappears, it can be determined that the suspected component is the cause of the problem. If the fault persists, the same replacement test can be performed on another suspected component or circuit board until the location of the fault is identified. Before making any replacements, take some time to analyze the cause of the fault, rather than replacing components blindly. If the failure is caused by a short circuit or thermal damage, the replacement component may also be damaged. Another example is when a diode burns out; this may be due to insufficient operating current or reverse peak voltage for that diode. Replacing it with another diode of the same model only provides a temporary solution to the problem, without eliminating it completely. Additionally, when replacing components, the power supply must be turned off; welding and testing while the power is on are not allowed. When installing and soldering the replaced components, it shall comply with the original soldering methods and requirements. As an insulating sheet is generally placed between high-power transistors and heat sinks, be sure not to forget to install it. When replacing it, be careful not to damage other surrounding components to avoid artificial failures. VI. The sectional method is a technique used in fault diagnosis, which involves dividing the circuit and its electrical components into several sections in order to identify the cause of the fault. Generally, the circuit of a detection and control instrument can be divided into three main parts: the external circuit (all the circuits from the instrument’s terminals outward to the detection elements and control actuators), the power supply circuit (all the circuits from the AC power source to the power transformer, etc.), and the internal circuit (all the circuits other than those in the external and power supply circuits). Within the internal circuit, it can be further divided into several smaller parts (based on the characteristics of its internal circuit and the structure of its electrical components). Sectional inspection involves examining each of the divided sections in a method that goes from the outside to the inside, from large to small, and from the surface to the interior, thereby gradually narrowing down the area of suspicion. After identifying which part is faulty through inspection, a thorough check is conducted on that part to locate the exact location of the fault. Partial inspection involves examining, analyzing, and evaluating each component of the instruments in sequence; although it is systematic, it takes a long time, and it is often difficult to focus on the key aspects, resulting in significant time wastage. This method is suitable for situations where maintenance personnel have limited repair experience, are not very familiar with the symptoms of instrument failures, and the failures are complex. VII. Human body interference: When a person is in a chaotic electromagnetic field (including those generated by AC power grids), a weak low-frequency electromotive force is induced (ranging from several tens to several hundred microvolts). When a person touches certain circuits of the instruments, those circuits respond, and this principle can be used to simply identify the locations of faults in the circuits. When using the human interference method, attention should be paid to the surrounding environment. In situations where there are few electrical devices and wiring, as well as in basements or some reinforced-concrete buildings, the signals generated by interference are weaker; in such cases, a long wire can be used in place of a hand to obtain stronger interference signals. Additionally, when using this method to inspect instruments with high-voltage components or those whose chassis is live, it is essential to be extremely careful to avoid electric shock. VIII. Voltage method: The voltage method involves using a multimeter (or another voltage meter) with an appropriate range to measure the suspected part, with measurements taken for both AC voltage and DC voltage. Measuring AC voltage mainly refers to the AC supply voltage, such as the 220V AC mains voltage, the output voltage of an AC voltage regulator, the voltage across transformer coils, and oscillating voltages, etc ； Measuring DC voltage refers to the DC supply voltage, the operating voltages at the various terminals of electron tubes and semiconductor components, as well as the voltage between each lead of an integrated circuit and ground. The voltage method is one of the most basic techniques in maintenance work, but the range of faults it can resolve remains limited. Some faults, such as slight coil short circuits, broken capacitors, or minor leakage currents, often do not show up under direct current voltage. For some faults, such as component short circuits, smoking, or arcing, it is necessary to turn off the power supply; in such cases, the voltage method is not effective, and other methods must be used for inspection. IX. Current method: The current method is divided into direct measurement and indirect measurement. Direct measurement involves disconnecting the circuit and connecting an ammeter in series to measure the current value; this value is then compared with the data obtained when the instrument is operating normally, in order to determine the fault. If it is found that a certain portion of the current is not within the normal range, it can be assumed that there is a problem with that part of the circuit, or at least it has been affected. Indirect measurement does not require disconnecting the circuit; it involves measuring the voltage drop across the resistor, and an approximate current value is calculated based on the resistance value. It is commonly used for measuring the current in transistor components. The current method is a bit more complicated than the voltage method; generally, it is necessary to disconnect the circuit and connect an ammeter in series for testing. But in some situations, it is easier to detect faults than the voltage method. By combining the current method with the voltage method, it is possible to detect and identify the vast majority of faults in a circuit. X. Resistance method: The resistance check method involves using the resistance setting on a multimeter, without applying power, to verify whether the input and output resistances of the entire circuit as well as certain individual circuits of the instrument are within normal ranges ； Whether each resistance element is open-circuited, short-circuited, or if its resistance value has changed ； Whether the capacitor is broken down or leaking ； Check for any broken wires or short circuits in the inductor coils and transformers ； Forward and reverse resistance of semiconductor devices ； Resistance of each integrated circuit pin to ground ； And make a rough estimate of the transistor’s β value ； Check for inter-electrode short circuits in the vacuum tubes and oscilloscope tubes, as well as whether the filaments are in good condition. When using the resistance method to detect faults, the following points should be noted: ① Since there are many nonlinear components in the circuit, such as transistors and large-capacity electrolytic capacitors, when measuring the resistance between two points using this method, it is necessary to pay attention to the red and black polarities of the multimeter, as different polarities will yield different measurement results ； ②To avoid direct measurement of ordinary transistors and integrated circuit modules with low current levels and low voltage tolerance using the Ω×1 range (which involves high current) or the Ω×10K range (which involves high voltage), in order to prevent damage ； ③In instruments, the components being measured are usually connected in a circuit to many other components, either in series or in parallel. Therefore, in cases where it is not a direct breakdown but rather leakage or a high resistance value, the component under test must be disconnected before inspection and measurement. For components such as resistors and capacitors that have only two leads, disconnecting one lead is sufficient to turn them off; whereas for components with three leads such as bipolar transistors, both leads need to be disconnected.