There are 5 processes in the history of hardware testing, and it is difficult to make one step less than one!

After a circuit board is soldered, when checking whether the circuit board can work normally, usually do not directly supply power to the circuit board, but follow the steps below to ensure that there are no problems in each step and then it is not too late to power on.
1. Whether the connection is correct. It is very important to check the schematic diagram. The focus of this check is whether the power supply of the chip and the label of the network node are correct, and at the same time, pay attention to whether the network nodes overlap. Another important point is the package of the original, the package model, and the pin sequence of the package; the package cannot be a top view, remember! Especially for non-pin packages. Check whether the wiring is correct, including wrong wiring, less wiring, and multiple wiring.
There are usually two ways to check the line: 1) Check the installed lines according to the circuit diagram, and check the installed lines one by one in a certain order according to the circuit connection; 2) Check the actual line against the schematic diagram, with one component as the center. line. Check the wiring of each component pin, and check whether each place exists on the circuit diagram. In order to prevent errors, the wires that have been checked should usually be marked on the circuit diagram, and the buzzer of the pointer multimeter should be tested, and the component pins can be directly measured, so that the bad wiring can be found at the same time.
2. Whether the power supply is short circuited. Do not power on before debugging, use a multimeter to measure the input impedance of the power supply, this is a necessary step! If the power supply is short-circuited, it will cause the power supply to burn out or even more serious consequences. When it comes to the power supply part, a 0 ohm resistor can be used as a debugging method. Do not solder the resistors before power-on. Check that the voltage of the power supply is normal before soldering the resistors to the PCB to supply power to the subsequent units, so as not to burn the chips of the subsequent units due to the abnormal voltage of the power supply. Increase the protection circuit in the circuit design, such as the use of restoration fuses and other components.
3. Component installation status. It is mainly to check whether the components with polarity, such as light-emitting diodes, electrolytic capacitors, rectifier diodes, etc., and whether the pins of the triode correspond to each other. For the triode, the pin sequence of different manufacturers with the same function is also different. Use a multimeter to test it.
Do the open circuit and short circuit test first to ensure that there will be no short circuit after power on. If the test point is set up, you can get twice the result with half the effort. The use of 0 ohm resistors is sometimes conducive to high-speed circuit testing.
The power-on test can only be started after the above hardware test before power-on is completed.
Power-on detection
1. Power-on observation: Do not rush to measure electrical indicators after power-on, but observe whether the circuit is abnormal, such as whether there is smoke, whether there is abnormal odor, touch the outer package of the integrated circuit, whether it is hot, etc. If an abnormal phenomenon occurs, turn off the power immediately, and then turn on the power again after troubleshooting.
2. Static debugging: Static debugging generally refers to a DC test performed under the condition of no input signal or only a fixed level signal. The potential of each point in the circuit can be measured with a multimeter and compared with the theoretical estimate. , Combined with the analysis of the circuit principle, judge whether the DC working state of the circuit is normal, and find the components in the circuit that have been damaged or are in critical working state in time. By replacing components or adjusting circuit parameters, the DC working state of the circuit can meet the design requirements.
3. Dynamic debugging: Dynamic debugging is carried out on the basis of static debugging. Appropriate signals are added to the input of the circuit, and the output signals of each test point are checked in sequence according to the flow direction of the signals. If abnormal phenomena are found, they should be analyzed. Reasons and troubleshooting, and then debug until the requirements are met.
Do not rely on feelings during the test, but always use instruments to observe. When using an oscilloscope, set the signal input mode of the oscilloscope to the "DC" block. Through the DC coupling mode, the AC and DC components of the measured signal can be observed at the same time. Through debugging, check whether the various indicators of the function block and the whole machine (such as signal amplitude, waveform shape, phase relationship, gain, input impedance and output impedance, etc.) meet the design requirements, and if necessary, further propose reasonable circuit parameters Correction.
Other work in electronic circuit debugging
1. Determine the test point: Develop debugging steps and measurement methods according to the working principle of the system to be adjusted, determine the test point, mark the position on the drawing and the board, and make the debugging data record table.
2. Set up a debugging workbench: the workbench is equipped with the required debugging equipment, and the equipment should be easy to operate and easy to observe. Special note: When making and debugging, the workbench must be arranged clean and tidy.
3. Selection of measuring instruments: For hardware circuits, the system to be adjusted should be selected for measuring instruments, and the accuracy of the measuring instruments should be better than that of the system under test; for software debugging, a microcomputer and development device should be equipped.
4. Debugging sequence: The debugging sequence of electronic circuits is generally carried out according to the signal flow. The output signal of the circuit that has been debugged before is used as the subsequent input signal to create conditions for the overall adjustment.
5. Overall debugging: When selecting digital circuits implemented by programmable logic devices, complete the input, debugging and programming of the programmable logic device source files, and connect the programmable logic devices and analog circuits into a system for overall debugging and result testing.
During the debugging process, the experimental phenomena should be carefully observed and analyzed, and records should be made to ensure the integrity and reliability of the experimental data.
Matters needing attention in circuit debugging
Whether the debugging result is correct is largely affected by whether the test volume is correct or not and the test accuracy. In order to ensure the results of the test, it is necessary to reduce the test error and improve the test accuracy. For this reason, you need to pay attention to the following points:
1. Use the ground terminal of the test instrument correctly. Use an electronic instrument with a ground terminal connected to the chassis for testing. The ground terminal should be connected to the ground terminal of the amplifier. Otherwise, the interference introduced by the instrument chassis will not only change the working status of the amplifier, but also cause errors in the test results. . According to this principle, when debugging the emitter bias circuit, if you need to test Vce, you should not directly connect the two ends of the instrument to the collector and emitter, but should measure Vc and Ve respectively to the ground, and then the two phases Less. If you use a dry battery-powered multimeter to test, since the two input ends of the meter are floating, it is allowed to connect directly between the test points.
2. The input impedance of the instrument used to measure the voltage must be much greater than the equivalent impedance of the place being measured. If the input impedance of the test instrument is small, it will cause shunting during the measurement, which will bring great errors to the test results.
3. The bandwidth of the test instrument must be greater than the bandwidth of the circuit under test.
4. Select the test point correctly. When measuring with the same test instrument, the error caused by the internal resistance of the instrument will be very different if the measuring point is different.
5. The measurement method should be convenient and feasible. When it is necessary to measure the current of a certain circuit, it is generally possible to measure the voltage without measuring the current, because it is not necessary to change the circuit to measure the voltage. If you need to know the current value of a branch, you can get it by measuring the voltage across the resistance of the branch and converting it.
6. In the process of debugging, not only must observe and measure carefully, but also be good at recording. The recorded content includes experimental conditions, observed phenomena, measured data, waveforms and phase relationships, etc. Only by comparing a large number of reliable experimental records with theoretical results, can the circuit design problems be discovered and the design plan can be improved.
Troubleshoot during debugging
It is necessary to carefully find the cause of the fault, and must not remove the line and reinstall it once the fault cannot be solved. Because if it is a problem in principle, even reinstallation will not solve the problem.
1. General method of fault inspection
For a complex system, it is not easy to accurately find faults in a large number of components and circuits. The general fault diagnosis process starts from the fault phenomenon, makes analysis and judgments through repeated tests, and gradually finds out the fault.
2. Failure phenomenon and cause of failure
1) Common failure phenomenon: The amplifying circuit has no input signal, but has an output waveform. The amplifier circuit has input signal, but no output waveform, or the waveform is abnormal. The series regulated power supply has no voltage output, or the output voltage is too high to be adjusted, or the output voltage regulation performance deteriorates, the output voltage is unstable, etc. The oscillation circuit does not produce oscillation, the counter waveform is unstable, and so on.
2) The cause of the failure: the finalized product fails after being used for a period of time, which may be due to damage to the components, short circuit or open circuit in the connection, or changes in conditions, etc.
3. General method to check the fault
1) Direct observation method: Check whether the selection and use of the instrument are correct, whether the power supply voltage level and polarity meet the requirements; whether the pins of the polar components are connected correctly, whether there are wrong connections, missing connections, and mutual collisions. Whether the wiring is reasonable; whether the printed board is short-circuited, whether the resistors and capacitors are burnt or exploded, etc. Power on and observe whether the components are hot or smoke, whether the transformer has a burning smell, whether the filament of the electron tube and oscilloscope tube is on, whether there is high-voltage ignition, etc.
2) Check the static operating point with a multimeter: the power supply system of the electronic circuit, the DC operating status of the semiconductor transistor, the integrated block (including components, device pins, power supply voltage), and the resistance value in the circuit can all be measured with the multimeter. When the measured value differs greatly from the normal value, the fault can be found after analysis.
By the way, the static operating point can also be measured with the oscilloscope "DC" input mode. The advantage of using an oscilloscope is that the internal resistance is high, and the DC working status and the signal waveform on the measured point can be seen at the same time, as well as possible interference signals and noise voltages, which is more conducive to fault analysis.
3) Signal tracing method: For a variety of more 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, f, 1000 HZ can be connected to its input end. Sine signal), use an oscilloscope from the front stage to the back stage (or vice versa) to observe the changes in the waveform and amplitude level by level. If something is abnormal, the fault is at that level.
4) Comparison method: When you suspect that 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. Circumstances, then analyze and determine the point of failure.
5) Component replacement method: Sometimes the fault is concealed and cannot be seen at a glance. For example, if 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 of appropriate capacity to select an appropriate check point, and temporarily connect the capacitor between the check point and the reference ground point. If the oscillation disappears, it indicates that the oscillation is occurring Near this or in the previous circuit. Otherwise, it will be at the back, and then move the checkpoint to find it. The bypass capacitor should be appropriate and not too large, as long as it can eliminate harmful signals well.
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 effective for checking open-circuit faults. But it should be noted that the short-circuit method cannot be used for the power supply (circuit).
8) Open circuit method: Open circuit method is used to check the effectiveness of short-circuit faults. The circuit breaking method is also a method to gradually narrow the scope of the suspected fault point. For example, because a certain regulated power supply is connected to a circuit with a fault, the output current is too large, we take the method of disconnecting a branch of the circuit in turn to check the fault. If the current returns to normal after the branch is disconnected, the fault has occurred in this branch.
In actual debugging, there are many ways to find the cause of the failure, and only a few commonly used methods are listed above. The use of these methods can find the fault point with one method for simple faults, but for more complicated faults, it is necessary to adopt a variety of methods to complement and cooperate with each other to find the fault point.