1: Power on the circuit board. It should be noted in this step that some circuit boards do not have a single power supply, and may need 5V, plus or minus 12V, 24V, etc. Don't omit the added power supply. After the circuit board is powered on, touch the components on the circuit board to see if there are any heating components, especially the 74 series chips. If the component is hot, it means that the component may have been damaged. After replacing components, check whether the circuit board fault has been solved.
2. Measure the gate circuit on the circuit board with an oscilloscope to see if it conforms to the logical relationship. If the output is illogical, it needs to be handled separately in two cases. One is that the output should be low level and the actual measurement is high level, which can directly judge the chip damage; The other is that the output should be high and the measured value is low, so it is impossible to judge that the chip has been damaged. It is also necessary to disconnect the chip from the back circuit and measure it again to observe whether the logic is reasonable and judge whether the chip is good or not.
3. Measure the crystal oscillator in the digital circuit with an oscilloscope to see if there is any output. If there is no output, the chip connected to the crystal oscillator needs to be removed as much as possible before measurement. If there is no output, it is preliminarily judged that the crystal oscillator has been damaged; If there is output, we should put the removed chips back one by one and test them one by one to find out the fault.
4. Digital circuits with bus structure, usually including digital, address and control buses. Measure three buses with oscilloscope, compare the schematic diagram, observe whether the signal is normal, and find out the problem.
On-line measurement is mainly used to compare the quality of two circuit boards, through which problems can be found and solved. Thereby completing the maintenance of the circuit board.