Instrument malfunctions are a common problem we encounter in our work. So, what are some good methods for diagnosing and identifying these problems? Below are 10 methods for analyzing and diagnosing industrial instrument malfunctions, compiled from years of instrument repair experience, which we hope will be helpful.
Image 1: Visual Inspection Method This method involves observing and identifying malfunctions using human senses (eyes, ears, nose, hands) without any testing instruments. Visual inspection includes both physical inspection and power-on inspection.
Physical inspection mainly includes:
① Checking the instrument casing and dial glass for damage, whether the pointer is deformed or touching the scale, whether the fasteners are secure, whether the positions of switches and knobs are correct, whether moving parts rotate freely, and whether there are any obvious changes in the adjustment parts;
② Checking for disconnections, whether connectors are properly connected, and whether the springs on the circuit board sockets have insufficient elasticity or poor contact. For instruments assembled in modular units, pay special attention to whether the screws connecting each unit board are tightened;
③ Checking the contacts of each relay and contactor... ④ Check for misalignment, jamming, oxidation, burning, or sticking;
⑤ Check for blown power fuses, cracked or leaking electron tubes (leaking a layer of white powder on the inner wall of the tube), or damage; discolored or broken transistor casing paint; burnt resistors; broken coil wires; and swollen, leaking, or burst capacitor casings;
⑥ Check for broken, brittle, or short-circuited copper strips on the printed circuit board; ensure all component solder joints are in good condition, with no cold solder joints, missing solder joints, or detached solder joints;
⑦ Check for skewed, misaligned, detached, or contacting components and wiring.
For any issues with the arrangement and wiring of components, check for misalignment, detachment, or contact.
For any issues with the arrangement and wiring of components, check for misalignment, detachment, or contact.
The question is incomplete and requires further clarification. The main checks during startup include:
① Checking if the power indicator light, all electron tubes, and other light-emitting components are powered on and illuminated;
② Checking for high-voltage arcing, discharge, or smoke inside the machine;
③ Checking for vibration and any crackling, friction, or impact sounds;
④ Checking if the temperature rise of heat-prone components such as transformers, motors, power amplifier tubes, resistors, and integrated circuits is normal, and whether they are hot to the touch;
⑤ Checking for any unusual odors inside the machine, such as the burnt smell from burnt insulation in transformers and resistors, or the smell of oxygen produced by high-voltage leakage arcing in the oscilloscope tubes;
⑥ Checking if the mechanical transmission parts are operating normally, and checking for any gears that are not meshing properly, jammed, severely worn, slipping, deformed, or have malfunctioning transmissions.
Visual inspection must be extremely careful and thorough; carelessness and haste are strictly prohibited. When checking components and wiring, only gently shake or move them; do not use excessive force to prevent breaking components, wiring, or copper foil on the printed circuit board. When powering on for the startup check, do not remove your hand from the power switch; if any abnormality is found, shut it off immediately. Special attention must be paid to personal safety; never touch live equipment with both hands simultaneously. Large-capacity filter capacitors in the power supply circuit carry a charging charge; prevent electric shock.
Image 2. Investigation Method: This method involves investigating the fault phenomena and their development process to analyze and determine the cause of the fault. It generally includes the following aspects:
① Usage conditions before the fault occurred and any warning signs;
② Whether there was sparking, smoke, or abnormal odors when the fault occurred;
③ Changes in power supply voltage;
④ External conditions such as overheating, lightning, humidity, and impact;
⑤ Whether there was interference from strong external electric or magnetic fields;
⑥ Whether there was improper use or misoperation;
⑦ Whether the fault occurred under normal use or after repairing or replacing components;
⑧ Previous faults and repair details, etc.
When using the investigation method to troubleshoot faults, the investigation must be thorough and careful, especially verifying the feedback from on-site personnel. Do not rush to disassemble and repair. Maintenance experience shows that many user reports are incorrect or incomplete; verification can uncover many problems that do not require repair.
3. Circuit Breaker Method: Disconnect the suspected component from the main unit or unit circuit and observe if the fault disappears to determine the location of the fault.
When an instrument malfunctions, first assess several possibilities. Within the fault area, disconnect the suspected circuit to determine whether the fault occurred before or after disconnection. Power on the instrument; if the fault disappears, it indicates the fault is likely in the disconnected circuit. If the fault persists, further circuit breaking and inspection should be performed to gradually eliminate suspicions, narrow down the fault range, and ultimately find the true cause.
The circuit breaker method is particularly convenient for troubleshooting modular, combined, and plug-in instruments and is also effective for some short-circuit faults with excessive current. However, it is not suitable for closed-loop systems with large overall circuits or directly coupled circuit structures.
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4. Short-Circuit Method: Temporarily short-circuit the suspected faulty circuit or component and observe any changes in the fault state to determine the location of the fault.
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4. Short-Circuit Method: Temporarily short-circuit the suspected faulty stage of the circuit or component and observe any changes in the fault state to determine the location of the fault. The short-circuit method is used to check multi-stage circuits. If temporarily short-circuiting a stage or component causes the fault to disappear or significantly decrease, the fault is before the short-circuit point; otherwise, it's after. For example, if the output potential of a stage is abnormal, short-circuiting its input terminal will restore the output potential, indicating the stage is functioning correctly.
The short-circuit method is also commonly used to check component functionality. For instance, short-circuiting the base and emitter of a transistor with tweezers and observing the collector voltage change can indicate whether the transistor has an amplification function. In TTL digital integrated circuits, the short-circuit method is used to determine if gate circuits and flip-flops are functioning correctly. Short-circuiting the control and cathode of a thyristor can determine if it is faulty. Additionally, short-circuiting the input terminals of certain instruments (such as electronic potentiometers) and observing changes in the reading can indicate interference.
5. Replacement Method: This method involves replacing certain components or circuit boards to pinpoint the location of the fault.
Replace the suspected component with a component of the same specifications and with good performance, then test the circuit. If the fault disappears, the suspected component is the source of the problem. If the fault persists, perform the same substitution test on another suspected component or circuit board until the faulty part is identified.
Before replacing components, take some time to analyze the cause of the fault, rather than blindly replacing components. If the fault is caused by a short circuit or thermal damage, the replaced component may also be damaged. For example, if a diode burns out, it may be due to insufficient operating current and reverse peak voltage. Replacing it with another diode of the same model only temporarily addresses the problem, not eliminates it.
Furthermore, the power should always be disconnected when replacing components. Do not test while soldering with power on. When installing and soldering the replaced components, follow the original soldering method and requirements. For example, high-power transistors and heat sinks usually have insulating sheets between them; do not forget to install these. Take care not to damage other surrounding components during replacement to avoid human-caused malfunctions.
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6. Sectional Method: This method involves dividing the circuit and electrical components into several parts during fault diagnosis to identify the cause of the fault.
Generally, the circuit of a testing and control instrument can be divided into three main parts: the external circuit (all circuits from the instrument's terminals outwards to the sensing element and control actuator), the power supply circuit (all circuits from the AC power supply to the power transformer, etc.), and the internal circuit (all circuits excluding the external and power supply circuits). The internal circuit can be further divided into several smaller parts (based on its internal circuit characteristics and the structure of its electrical components). Sectional inspection involves checking each part from the outside in, from large to small, and from the surface inwards, gradually narrowing down the scope of suspicion. Once the fault is identified, a comprehensive inspection of that part is conducted to locate the faulty component.
While sectional inspection involves checking and analyzing each part of the instrument sequentially, it is time-consuming and often misses key points, wasting considerable time. This method is suitable for maintenance personnel with limited experience, unfamiliarity with the instrument's fault symptoms, and situations involving complex faults.
7. Human Body Interference Method: When a person is in a chaotic electromagnetic field (including the electromagnetic field generated by an AC power grid), a weak low-frequency electromotive force (tens to hundreds of microvolts) will be induced. When a person's hand touches certain circuits of an instrument, the circuit will react. This principle can be used to easily determine certain fault locations in the circuit.
When using the human body interference method, attention must be paid to the environment. In areas with few electrical devices and lines, basements, or some reinforced concrete buildings, the interference signal will be weaker. In these cases, a long wire can be used instead of a hand to obtain a stronger interference signal. Additionally, when using this method to check high-voltage parts of instruments or instruments with live base plates, extreme caution must be exercised to avoid electric shock.
8. Voltage Method: The voltage method involves using a multimeter (or other voltmeter) at an appropriate range to measure the suspected component. It can measure both AC and DC voltage. AC voltage measurement mainly refers to AC power supply voltage, such as AC 220V mains voltage, AC voltage regulator output voltage, transformer coil voltage, and oscillation voltage. DC voltage measurement refers to DC power supply voltage, the operating voltage of each electrode of vacuum tubes and semiconductor components, and the voltage to ground of each lead of integrated circuits.
The voltage method is one of the most basic methods in maintenance work, but its scope of fault diagnosis is still limited. Some faults, such as minor short circuits in coils, broken capacitors, or minor leakage, are often not reflected in DC voltage readings. For some faults, such as short circuits in components, smoke, or sparking, the power must be turned off, rendering the voltage method ineffective; in these cases, other methods must be used for inspection.
9. 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, measuring the current value, and comparing it with the data from the instrument's normal operating condition to determine the fault. If any part of the current is found to be outside the normal range, it can be assumed that this part of the circuit is faulty or at least affected. Indirect measurement does not require disconnecting the circuit. It measures the voltage drop across the resistor and calculates an approximate current value based on the resistance value. It is often used for measuring the current of transistor components.
The current method is more complicated than the voltage method, generally requiring the circuit to be disconnected before connecting the ammeter in series for testing. However, it is more effective at diagnosing faults in certain situations. The current and voltage methods, used together, can detect and diagnose most circuit faults.
Image 10: Resistance Method. The resistance method involves using a multimeter in resistance mode without power to check the input and output resistances of the instrument's entire circuit and some circuits; whether each resistor is open-circuited, short-circuited, or has a change in resistance value; whether capacitors are broken down or leaking; whether inductors and transformers have broken wires or short circuits; the forward and reverse resistance of semiconductor devices; the resistance of each integrated circuit lead to ground; and a rough assessment of the transistor's beta value; whether vacuum tubes and oscilloscope tubes have inter-electrode short circuits, and whether the filament is intact, etc.
When using the resistance method to troubleshoot, the following points should be noted:
① Because circuits often contain nonlinear components, such as transistors and large-capacity electrolytic capacitors, when measuring the resistance between two points using the resistance method, pay attention to the red and black polarities of the multimeter, as different polarities will produce different results.
② Avoid using the Ω×1 range (for higher current) and the Ω×10K range (for higher voltage) to directly measure ordinary small currents and low-voltage transistors and integrated circuits, as this may cause damage.
③ The component being measured in the instrument is often connected (in series or parallel) to many other components in the circuit. Therefore, for cases where there is leakage or a relatively high resistance value, the component being measured should be disconnected before inspection and measurement. For components with only two leads, such as resistors and capacitors, disconnecting one lead is sufficient. However, for components with three leads, such as transistors, two leads should be disconnected.
