How to detect and repair circuit board?

Firstly, fault distribution
1, the incomplete statistics for circuit board fault :
The chip is damaged by 30%;Discrete component is damaged by 30%;copper wire (PCB) is fractured by 30%;Program is damaged or lost by 10% (rising).
2, By the known,if there is a problem with the program or influence in the fault PCB board when you don’t own a workable board ,then it has no possibility to repair,because we are not familiar with its attachment, neither the original program.

Secondly, the crystal oscillator
1,Usually, we can only use the oscilloscope (it need to plus electric for crystal vibration) or frequency meter to test,but Multimeter can not to measure.Otherwise, we can only use the substitution method.
2,Common faults of vibration:the internal leakage;Internal open;Metamorphic deviation;Peripheral connected capacitor leakage.The leakage phenomenon should be able to be measured by < tester > VI curve.
3,For the whole plate test ,we can use two methods of judgment:the chip around crystals is not through when we test it ;except the crystals ,we couldn't find the other point of failure.
4,There are two common crystals:with two feet ;with four feet.Notice:it is added the electric to the one with two feet,so do not casually short circuit.

Thirdly, function and parameter test
1,What the < tester > can show are cut-off area, enlarged area and saturated area,but can not test the concrete numerical value for Working frequency and speed.
2,In the same way,for TTL digital chips,We can only know that there are output changes of high and low level , but can not find out its the speed along which it rises or falls.

Fourthly, reset circuit
 1,When the fault circuit boards is with large scale integrated circuit, we should pay attention to reset problem.
2,Before the test ,it’s better to best it back to the device,and to turn on and off the machine again and again,to press the reset button more times.

Fifthly, the chip with program
1,An EPROM chip usually should not be damaged.The chip need ultraviolet light to wipe away the program, so it will not be damaged in the test procedure.But it is better to prepare backup to avoid damage because of idle for a long time .
2,EEPROM、SPROM and RAM chips with a battery are both easy to damage the program.It is not confirmed whether it can damage the program or not after scanning With the < tester > VI curve .But we’d better to be careful when we meet such situation.
3,For the chip with battery, please do not easily detach it from the circuit board .