The Most Complete Circuit Test Process (3)

How to Check for Faults

1. Direct observation method: Check whether the selection and use of the instrument are correct, whether the level and polarity of the power supply voltage meet the requirements, whether the polarity component pins are connected correctly, whether there are wrong connections, missed connections, and mutual collisions, whether the wiring is reasonable, whether the printed board is short-circuited and disconnected, whether the resistors and capacitors are burnt or burst, etc. Power on to observe whether the components are hot, smoking, whether the transformer has a burnt smell, whether the filament of the electronic tube and oscilloscope is bright, whether there is high-voltage ignition, etc.

2. Use a multimeter to check the static operating point: The power supply system of the electronic circuit, the DC working state of the semiconductor triode, the integrated block (including components, device pins, power supply voltage), and the resistance value in the line can be measured with a multimeter. When the measured value differs greatly from the normal value, the fault can be found after analysis. Incidentally, the static operating point can also be determined with an oscilloscope “DC” input. The advantage of using an oscilloscope is that the internal resistance is high, and the DC working state and the signal waveform on the measured point, as well as possible interference signals and noise voltages, can be seen at the same time, which is more conducive to analyzing faults.

3. Signal tracing method: For various complex circuits, a signal of a certain amplitude and appropriate frequency can be connected to the input end (for example, for a multi-stage amplifier, a f, 1000 HZ signal can be connected to its input end. Sine signal), use the oscilloscope from the front stage to the back stage (or the opposite), and observe the changes of the waveform and amplitude step by step. If any stage is abnormal, the fault is at that stage.

4. Comparison method: When there is a problem with a certain circuit, you can compare the parameters of this circuit with the same normal parameters (or theoretically analyzed current, voltage, waveform, etc.) to find out the abnormality in the circuit, and then analyze and determine the fault point.

5. Parts replacement method: Sometimes the fault is relatively hidden and cannot be seen at a glance. For example, when you have an instrument of the same model as the faulty instrument, you can replace the parts, components, plug-in boards, etc. in the instrument with the faulty instrument. Corresponding parts in order to narrow the scope of the fault and find the source of the fault.

6. Bypass method: When there is parasitic oscillation, you can use a capacitor with an appropriate capacity, select an appropriate check point, and temporarily connect the capacitor between the check point and the reference ground point. If the oscillation disappears, it means that the oscillation is generated. near this or in the previous circuit. Otherwise, it is in the back, and then move the checkpoint to find it. The bypass capacitor should be appropriate and should not be too large, as long as it can better eliminate harmful signals.

7. Short-circuit method: It is a method of temporarily short-circuiting a part of the circuit to find the fault. The short-circuit method is most effective for checking open circuit faults. However, it should be noted that the short-circuit method cannot be used for the power supply (circuit).

8. Open circuit method: The open circuit method is the most effective for checking short-circuit faults. The open circuit method is also a method of gradually narrowing down the suspected points of failure. For example, when a regulated power supply is connected to a circuit with a fault, the output current is too large. We take the method of disconnecting a certain branch of the circuit in turn to check the fault. If the current returns to normal after disconnecting the branch, the fault occurs in this branch.