Solution to the failure of debugging the board debugging skills

To find out the cause of the fault carefully, you must remove the line and reinstall it if the fault cannot be solved. Because there may still be various problems with the reinstalled line, if it is a problem in principle, even if it is reinstalled, it will not solve the problem. We should look for faults and analyze the causes of failures as a good learning opportunity, through which we can continuously improve our ability to analyze and solve problems.

(1) General method for checking faults

Failure is an unexpected but inevitable circuit abnormality. Analysis, troubleshooting, and troubleshooting are essential skills for electrical engineers. For a complex system, it is not easy to find faults quickly and accurately in a large number of components and circuits. The general fault diagnosis process is to start from the fault phenomenon, through repeated tests, make analysis and judgment, and gradually find out the cause of the fault.

(2) Fault phenomenon and cause of malfunction

1) Common fault phenomenon: The amplifier circuit has no input signal and has an output waveform. The amplifier circuit has an input signal, but there is no output waveform, or the waveform is abnormal. The series regulated power supply has no voltage output, or the output voltage is too high and cannot be adjusted, or the output voltage regulation performance is deteriorated, and the output voltage is unstable. The oscillating circuit does not oscillate. The counter output waveform is unstable or cannot be counted correctly. In the radio, there are "嗡嗡" hum and "啪啪" steamboat sounds. These are some of the most common failures, and there are many strange phenomena that are not listed here.

2) Causes of failure: There are many reasons for the failure, and the situation is very complicated. Some are simple faults caused by one cause, and some are complex faults caused by interactions of various reasons. Therefore, the cause of the malfunction is difficult to classify simply. Only a few rough analysis can be done here.

If the product is faulty after using it for a period of time, the cause of the failure may be component damage, short circuit or open circuit (such as solder joint solder joint, poor connector contact, variable resistor, potentiometer, semi-variable resistor, etc.). Poor, oxide plating on the surface of the contact surface, etc., or changes in the use conditions (such as grid voltage fluctuations, super-cold or overheated working environment, etc.) affect the normal operation of electronic equipment.

For newly designed and installed circuits, the cause of the failure may be: the actual circuit does not conform to the schematic of the design; the component is soldered incorrectly, the component is improperly used or damaged; the designed circuit itself has some serious shortcomings that do not meet the technical requirements. ; short circuit or open circuit, etc.

Faults caused by incorrect use of the instrument, such as waveform anomalies or no waveforms caused by improper use of the oscilloscope, and interference caused by improper handling of grounding problems.

Faults caused by various disturbances.

(3) General method for checking faults

The order in which faults are found can range from input to output, and from output to input. The general methods for finding faults are:

1) Direct observation method: Direct observation method refers to the use of human visual, listening, smelling, touching, etc. as a means to find problems without any instrument, and to find and analyze faults. Direct observation includes no power check and power on observation.

Check whether the selection and use of the instrument are correct; whether the grade and polarity of the power supply voltage meet the requirements; the polarity of the electrolytic capacitor, the pins of the diode and the triode, and the pins of the integrated circuit are misconnected, missed, and touched. Whether the wiring is reasonable; whether the printed board is broken or not; whether the resistor and capacitor are burnt or cracked.

Power on to observe whether the components are hot or smoke, whether the transformer has a burnt smell, whether the electron tube or the oscilloscope tube is bright, and whether there is high-voltage ignition.

This method is simple and effective, and can be used for preliminary inspections, but it is powerless for relatively hidden faults.

2) Check the static working point with a multimeter

The power supply system of the electronic circuit, the DC operating state of the semiconductor transistor and the integrated block (including the element, the device pin, the power supply voltage), and the resistance value in the line can be measured by 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 measured using the oscilloscope "DC" input method. The advantage of using an oscilloscope is that the internal resistance is high, and the DC working state and the signal waveform at 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 the fault.

3) Signal tracing method

For a variety of more complex circuits, a signal of a certain amplitude and frequency can be connected to the input (for example, for a multi-stage amplifier, a sinusoidal signal of f=1000 Hz can be connected to its input). From the front to the back (or vice versa), observe the waveform and amplitude changes step by step, such as which level of abnormality, then the fault is at that level. This is the way to check the circuit in depth.

4) Comparison method

When you suspect that there is a problem with a certain circuit, you can compare the parameters of the circuit with the parameters of the normal circuit (or the theoretical analysis of current, voltage, waveform, etc.) with the same working state, and find out the abnormal conditions in the circuit. Then analyze the cause of the failure and determine the point of failure.

5) Component replacement method

Sometimes the fault is hidden and cannot be seen at a glance. If you have the same type of instrument as the faulty instrument at this time, you can replace the components, components, and boards in the instrument with the corresponding components in the faulty instrument to facilitate reduction. Fault range to further find faults.

6) Bypass method

When there is parasitic oscillation, you can use a capacitor of appropriate capacity to select an appropriate checkpoint and temporarily connect the capacitor between the checkpoint and the reference ground. If the oscillation disappears, it indicates that the oscillation is generated nearby or the previous stage. In the circuit. Otherwise, just behind, move the checkpoint to find it. It should be pointed out that the bypass capacitor should be appropriate and should not be too large, as long as the harmful signal can be eliminated better.

7) Short circuit method

It is a method of temporarily shorting a part of the circuit to find the fault. The short circuit method is most effective for checking for 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 most effective for checking short circuit faults. The open circuit method is also a way to gradually narrow down the suspected point of failure. For example, if a regulated power supply is connected to a faulty circuit and the output current is too large, we take a way to disconnect one of the circuits in turn to check the fault. If the current returns to normal after the branch is disconnected, the fault occurs in this branch.