Firstly, power on the circuit board, in this step, pay attention that some printed circuit board power supply is not single, it may require 5V, also may require plus or minus 12V, 24V, etc, do not add the power leakage. After circuit board power on, touch the components by hand to see if there are hot components of fever, focusing on checking 74 series chips, if the components has a hot situation, then it may have been damaged. After replacing the components, check whether the circuit board failure is resolved or not.
Secondly, measured circuit board gate with the oscilloscope to see if it is accord with logical relationship.
Thirdly, using the crystal in the digital circuit by oscilloscope to see if it has an output. If there is no output, you need to measure it after removing the chip which connected with the crystal, if there still no output, then then we determinate initially the crystal has been damaged. If there has output, you need to take the removed chip back one by one, install one, test one, to indentify the fault.
Fourthly, digital circuits with bus structure include digital, address and control bus. Compared with schematic diagram, use oscilloscope to test the three-way bus to observe whether the signal is normal or not, then find out the problem.