The main look: 1. whether there is a broken line and short-circuit; in particular, the circuit board on the printed circuit board connecting line whether there is a break in the phenomenon of adhesion; 2. components such as resistors capacitors diodes transistors, etc. Whether or not there is the phenomenon of disconnecting; 3. whether or not someone has repaired? What components have been moved? Whether there is welding leakage welding insertion reverse insertion error and other issues. Rule out the above conditions after this time the first multimeter to measure the circuit board power supply and the resistance between the ground usually circuit board resistance should not be less than 70 Ω. If the resistance is too small, only a few or a dozen ohms. Components on the circuit board is broken or partially broken must take measures to be broken components to find out. Specific approach is to be repaired to the board power (Note! At this time, we must find out the value of the board's operating voltage voltage and positive and negative polarity can not be connected to the wrong and join higher than the value of the operating voltage. Otherwise, the board to be repaired will be harmed! Old faults are not eliminated and increase new problems!) With a thermometer to measure the temperature of each device on the circuit board, the temperature rises faster and higher as a key object of suspicion. If the resistance value is normal, and then use a multimeter to measure the capacitance resistance devices on the board diode, transistor field effect tube as well as peeling section switch and other components. The purpose is to first ensure that the measured components are normal. Can use general test tools (such as multimeter, etc.) to solve the problem do not complicate it. Principle two: first outside and then inside if the situation permits it is best to have a good board with the same board to be repaired as a reference. Then use the tester's double bang VI curve scanning function on the two boards for good and bad comparison test. The beginning of the comparison test point can start from the board's ports; and then from the table to the inside, especially on the capacitor comparison test. This can make up for the multimeter online is difficult to measure the lack of capacitance leakage. Principle 3: the first easy and then difficult to improve the test results in the circuit board for online functional testing should be repaired before the board to do some technical processing to try to weaken the impact of a variety of interference on the test process. Specific measures are as follows: 1. Preparation before the test will be shorted crystal (note that the four-pin crystal to figure out the two feet for the signal output foot can be shorted to these two feet. Remember that in general the other two feet for the power supply feet must not be shorted!!!) For large-capacity electrolytic capacitors should also be welded down a foot to make it open-circuit. Because of large-capacity capacitor charging and discharging will also bring interference. 2. using the exclusion method to test the device for online testing or comparative testing of the device where the test passes (or compare the normal) of the device, please confirm the test results directly to the records. For the test failed (or comparison of super poor) can be tested again. If it still does not pass, you can also confirm the test results. This will continue until the devices on the board have been tested (or compared). Then we will deal with those devices that do not pass the test (or compare poorly). Failure to pass the functionality of the device online test some test instruments also provide a less formal but more practical way to deal with: because of the test instrument on the board's power supply can also be applied to the device through the test clips to the corresponding power supply and ground feet if the device's power supply feet to implement the edge of the cut will be out of the device board power supply system. At this point then the device for online functional testing; due to other devices on the circuit board will not be powered to work to eliminate the interference effect. At this time the actual test results will be equivalent to "quasi-offline test" measurement accuracy will be greatly improved. Article from: /great_newshow.asp?id=2789&BigClass=Technical Documents