Electrical circuit troubleshooting can generally be detected by input to the input sequence or by the reverse method of output to input.
All kinds of good and bad electrical faults may include the following situations:
Poor contact: Poor contact between the board and the slot, unreachable when the cable is broken inside, wire plug and wiring terminal contact is not good, component Sky welding and so on are all such.
The solution to this type of failure is to double-check the suspect connector to see if there is any obvious oxidation or poor contact. Scrape the oxidized metal to contact the contact point, and adjust the position of the contact point by dialing. After the process, the test contact is re-dialed.
Second, the signal is interfered with: For the digital circuit, in a certain situation, I am drunk and the fault will be presented. It may be that the interference is too much affecting the control system. The error occurred, and there were also changes in the individual component parameters or the overall performance parameters of the circuit board, which made the anti-interference ability tend to a critical point, thus causing a malfunction.
This type of fault focuses on checking whether the equipment is well grounded. Use the test pencil to check whether the equipment casing is energized, or use the meter to measure whether the equipment casing has a higher voltage on the earth, generally below 1V. If it is above 10V, it is suspected. Whether the grounding is good.
Third, the thermal stability of the components is not good: from the perspective of a large number of maintenance practices, the first electrolytic capacitors The thermal stability is not good, followed by other capacitors, transconductors, diodes, ICs, resistors, etc.
1. This kind of fault generally occurs or disappears with the change of the boot time of the machine. The essence is that the fault changes with the temperature change of a faulty component. For example, the fault caused by the aging of the electrolytic capacitor is generally faulty when it is just energized. After the power is turned on for a period of time, the fault disappears, that is, there is a fault in the cold machine, and the heat engine has no fault. The essence is: the capacity of the aging electrolytic capacitor changes with temperature, and the capacity is small when the temperature is low, resulting in poor filtering, circuit The board does not work properly, and as the power is turned on, the temperature of the electrolytic capacitor rises, the capacity increases, and the filter condition is satisfied, so that the fault disappears.
2, thermal stability fault is a soft fault, it is not easy to directly detect and determine the faulty component during maintenance, but the inspection scope can be narrowed by artificially raising or cooling the suspected component. You can use a hair dryer or a heat gun. For the suspected component heating, use a cotton swab to cool the suspected component. The quality of the capacitor is easily determined using the VI curve test.
Four, there is moisture, dust, etc. on the board: moisture And the dust will be conductive, with a resistance effect, and the resistance value will change during the process of thermal expansion and contraction. This resistance value will be related to other components. When the effect is strong, the circuit parameters will be changed to make the fault occur.
This type of fault can be solved by cleaning the board. It is recommended to wash the board with wash water or directly with clean water and then blow dry thoroughly with a hair dryer. Alcohol is not recommended.Because it is easy to leave some white matter on the board after cleaning with alcohol.
V. Software is also one of the considerations: many parameters in the circuit are adjusted using software, and the margin of some parameters is adjusted too low, in a critical range. When the machine operating conditions meet the reason for the software to determine the fault, then the alarm will appear.
Fault detection judgment method:
Method 1. Direct observation: When the circuit fails, Normally, the instrument will not be used immediately. Instead, the naked eye will be used to find the abnormal parts of the circuit. The direct observation method is divided into non-energized and energized detection
No power-on detection checks whether the power supply voltage level and polarity meet the circuit requirements; The pin position of the sex and the two triodes, and the pin position of the integrated circuit are problems such as the occurrence of solder joints, mis-welding and crossover;Is there any unreasonable place in the wiring; when the printed board is printed, there is no line breakage; the resistor and the capacitor have obvious burning problems.
The power-on check is mainly to observe whether the components are overheated, smoke and obvious burnt smell, electron tube and the filament of the oscilloscope Is there any problem such as high pressure fire?
Method 2, Multimeter detection: Multimeter detection is mainly to check the static working point, where the power supply system, triode, and integrated block of the electronic circuit The resistance value and the DC working state in the line can be detected by a multimeter. Check to see if the value is normal.
Method 3: Signal tracing method: In a complex circuit, you can connect a signal to the input and then pass the oscilloscope. Observe the waveform and amplitude changes from the previous stage to the latter stage or from the latter stage to the previous level, and finally see which level is abnormal.
Method 4: Comparison method: The comparison method is more intuitive, mainly by comparing the parameters of the suspected fault circuit with the same circuit with normal working condition to find out whether it is There is a value with a large difference in parameters, and then the cause of the failure is analyzed, and the fault location is determined at the most.
Method 5: Replacement method: For electronic circuits with inconspicuous faults, the existing identical components can be used when it is impossible to make an intuitive judgment or suspected fault point. Replacement is performed to shorten the fault judgment orientation by replacing whether the observation circuit changes.
Method 6: Bypass check method: If there is parasitic oscillation in the circuit, then you can use a certain capacity of capacitor The capacitor is connected between the place to be inspected or the reference ground point, and then observe whether the oscillation exists. If the oscillation disappears, it means that the oscillation is generated in the circuit of the front stage or nearby. If not, move backwards and continue to look for checkpoints.The choice of capacitors should be careful not to oversize the bypass capacitors, and it is better to eliminate the unfavorable signals.
Method 7: Short-circuit check method: The short-circuit check method is a temporary short circuit made by us to make a short circuit in some circuits.
Method 8: Open circuit check method: The short circuit method mentioned above is used to check the open circuit is the most effective. Similarly, the short circuit check with the open circuit method is also the most effective. The thinking of the open circuit check method is similar to the previous methods, and is used to exclude suspected points and shorten the range. Assume that the regulated power supply is connected to a faulty circuit, and the output circuit is too large. Then, if we turn off a certain part of the circuit in turn, then observe the circuit current. Output the situation to determine the branch where the fault occurred.