Author: zhangtao2:
Testing equipment is an indispensable part of our daily production and life. Once these devices malfunction, how should we inspect and diagnose them? Here are some summarized experiences to share with everyone.
1. Observation Method Utilize visual, olfactory, and tactile senses. Sometimes, damaged components may change color, bubble, or show signs of burning; burnt devices may emit peculiar odors; short-circuited chips may become hot; visible soldering issues can also be detected with the naked eye.
2. Tapping Method It is common to encounter instruments that work intermittently. This phenomenon is mostly caused by poor contact or cold solder joints. For this situation, the tapping and hand-pressing methods can be used. The so-called “tapping” involves gently tapping the suspected faulty area with a small rubber hammer or other objects on the plug-in board or component to see if it causes errors or downtime. The “hand-pressing” method involves firmly pressing the plugged components and connectors after turning off the power when a fault occurs, then turning the power back on to see if the fault is eliminated. If tapping the casing makes it work normally, but tapping again causes it to malfunction, it’s best to reinsert all connectors firmly before trying again; if that doesn’t work, then alternative methods must be considered.
3. Replacement Method This requires having two identical instruments or sufficient spare parts. Replace a good spare part with the same component on the faulty machine to see if the fault is eliminated.
4. Elimination Method The elimination method involves removing some plug-in boards or components within the machine to determine the cause of the fault. If removing a certain plug-in board or component restores normal operation, it indicates that the fault lies there.
5. Temperature Cycling Method Sometimes, instruments may malfunction after long periods of operation or in high-temperature environments during summer. They may work normally after being turned off and left for a while, but malfunction again after some time. This phenomenon is caused by certain ICs or components having poor performance and not meeting thermal specifications. To identify the fault, the temperature cycling method can be employed. The cooling process involves applying anhydrous alcohol with a cotton swab to the suspected faulty area to lower its temperature and observe if the fault disappears. The heating process involves artificially increasing the ambient temperature, such as bringing a soldering iron close to the suspected area (care must be taken not to raise the temperature too high to damage normal components) to see if the fault appears.
6. Comparison Method This requires having two identical instruments, one of which is functioning normally. This method also necessitates having essential equipment like multimeters and oscilloscopes. There are various types of comparisons, such as voltage comparison, waveform comparison, static impedance comparison, output result comparison, and current comparison. The specific method is to run the faulty instrument and the normal instrument under the same conditions and then check the signals at certain points to compare the two sets of measured signals. If there are differences, it can be concluded that the fault lies there. This method requires maintenance personnel to possess considerable knowledge and skills.
7. Shoulder Riding Method Also known as the parallel method. Place a good IC chip on top of the chip being inspected, or connect good components (resistors, capacitors, diodes, transistors, etc.) in parallel with the components being checked, maintaining good contact. If the fault is due to internal open circuits or poor contacts within the component, this method can help eliminate it.
8. Isolation Method The fault isolation method does not require identical equipment or spare parts for comparison and is safe and reliable. According to the fault detection flowchart, the fault search range is gradually narrowed by dividing and surrounding, supplemented by signal comparison and component swapping methods, which usually leads to quickly locating the fault.
9. Capacitor Bypass Method When a circuit exhibits strange phenomena, such as a chaotic display, the capacitor bypass method can be used to determine which part of the circuit is likely faulty. Connect a capacitor across the circuit.
10. State Adjustment Method Generally, before the fault is identified, do not casually touch the components in the circuit, especially adjustable components like potentiometers. However, if prior reference measures are taken (for example, marking the position or measuring voltage or resistance values before touching the instrument), it is permissible to make adjustments when necessary. Sometimes, changing the state may eliminate the fault. The capacitor is connected across the power and ground terminals of the IC; for transistor circuits, it is connected across the base input or collector output, observing the effect on the fault phenomenon. If the input of the capacitor bypass does not work but bypassing its output causes the fault phenomenon to disappear, it confirms that the fault lies in that level of the circuit.
