How to repair the circuit board?

How to repair the circuit board?
Online measurement
Step 1: Power on the board. In this step, it should be noted that some boards are not single, they may need 5V, and they need positive and negative 12V, 24V, etc. Don't add the added power. It is. After the board is powered, touch the components on the board to see if there are any hot components. Check the 74 series chips. If the components are hot, the components may be damaged. After replacing the component, check if the board failure has been resolved.
Step 2: Measure the gate on the board with an oscilloscope and see if it is logical. If the output does not conform to the logic, it needs to be treated separately in two cases. One is that the output should be low level, the actual measurement is high level, and the chip can be directly judged to be damaged; the other is that the output should be high level. The actual measurement is low, and it is not possible to determine that the chip has been damaged. It is also necessary to disconnect the chip from the latter circuit, measure again, observe whether the logic is reasonable, and determine whether the chip is good or bad.
Step 3: Use an oscilloscope to measure the crystal in the digital circuit to see if it has an 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 replace the removed chips one by one, install a piece of the test piece, and find out the fault.
The fourth step: digital circuit with bus structure, generally includes three channels of digital, address and control bus. Use an oscilloscope to measure the three-way bus, compare the schematics, observe whether the signal is normal, and find out the problem.
                 From Allicdata