Step 1: energize the circuit board. In this step, it should be noted that some circuit board power supplies are not single, may need 5 V, plus or minus 12 V, 24 V, etc, don't leak the power supply.
After the circuit board is electrified, touch the components on the circuit board by hand to see if there are components with Perm heat. Focus on checking the 74 series CHiPs. If the components are hot, it means that this component may have been damaged.
After replacing components, check whether the circuit board fault has been resolved.
Step 2: Measure the gate circuit on the circuit board with an oscilloscope to observe whether it conforms to the logical relationship.
If the output does not conform to the logic, it needs to be treated separately in two situations. 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 at a high level, and the actual measurement is low, so it cannot be judged that the chip has been damaged. It is also necessary to disconnect the chip from the following circuit and measure it again, observe whether the logic is reasonable and judge whether the chip is good or bad.
Step 3: Measure the crystal oscillator in the digital circuit with an oscilloscope to see if it has 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 determined that the crystal oscillator has been damaged;
If there is output, it is necessary to install the removed chips one by one and install them one by one to find out the fault.
Step 4: digital circuit with bus structure generally includes digital, address and control bus.
Measure the three-way bus with an oscilloscope, compare the schematic diagram, observe whether the signal is normal, and find out the problem.
The on-line measurement method is mainly used for comparing two good and bad circuit boards. Through comparison, problems can be found and solved. So as to complete the maintenance of the circuit board.