The red test lead is grounded, the pins in the red circle are all locations, and the negative poles of the capacitors are all locations. Put the black test lead on the IC pin to be measured, and then the multimeter will display a diode value, and judge the quality of the IC based on the diode value. What is a good value? It depends on experience. Either you have a motherboard and do comparison measurements.
How to quickly detect faults
1 Look at the status of the component
Get a faulty circuit board, first observe whether the circuit board has obvious component damage, such as electrolytic capacitor burnout and swelling, resistor burnout, and power device burnout.
2 Look at the soldering of the circuit board
For example, whether the printed circuit board is deformed or warped; whether the solder joints fall off or are obviously weakly soldered; whether the copper clad skin of the circuit board is warped, burned and turned black.
3 Observation component plug-in
Such as integrated circuits, diodes, circuit board power transformers, etc. are inserted correctly.
4 Simple test resistance\capacity\induction
Use a multimeter to perform a simple test on suspected components such as resistance, capacitance, and inductance within the range to test whether the resistance value increases, capacitor short circuit, open circuit and capacitance change, inductance short circuit and open circuit.
5 Power-on test
After the above-mentioned simple observation and testing, the fault cannot be eliminated, and the power-on test can be performed. First test whether the power supply of the circuit board is normal. Such as whether the AC power supply of the circuit board is abnormal, whether the voltage regulator output is abnormal, whether the switching power supply output and waveform are abnormal, etc.
6 brush program
For programmable components such as single-chip microcomputer, DSP, CPLD, etc., you can consider brushing the program again to eliminate circuit failures caused by abnormal program operation.
How to repair circuit boards？
1 Observation
This method is quite intuitive. Through careful inspection, we can clearly see the burnt traces. When this problem occurs, we must pay attention to the rules during maintenance and inspection to ensure that no more serious injuries occur when the power is turned on. When we use this method, we need to pay attention to the following issues:
1. Observe whether the circuit board is damaged by man.
2. Observe carefully the related components of the circuit board, and observe every capacitor and resistance to see if there is any blackening. Since resistance cannot be viewed, it can only be measured with an instrument. Related bad parts should be replaced in time.
3.Observation of circuit board integrated circuits, such as CPU, AD and other related chips, should be modified in time when observing related conditions such as bulging and burning.
The cause of the above problems may be in the current. Excessive current can cause burnout, so check the relevant circuit diagram to see where the problem is.
2. Static measurement
In circuit board repairs, it is often difficult to find some problems by observation method, unless it is obvious that it is burned or deformed. But most of the problems still need to be measured by a voltmeter before conclusions can be drawn. Circuit board components and related parts should be tested one by one. The repair procedure should be operated according to the following procedure.
Detect the short circuit between the power supply and the ground and check the reason.
Check if the diode is normal.
Check whether there is a short circuit or even an open circuit in the capacitor.
Check the circuit board-related integrated circuits, and resistance and other related device indicators.
We can use observation method and static measurement method to solve most of the problems in circuit board maintenance. This is unquestionable, but we must ensure that the power supply is normal during measurement and no secondary damage can occur.
3 Online measurement
The online measurement method is often used by manufacturers. It is necessary to build a general debugging and maintenance platform for the convenience of maintenance. When measuring with this method, you need to follow the steps below.
Power on the circuit board and check whether the components are overheated. If so, check it out and replace the related components.
Check the gate circuit corresponding to the circuit board, observe whether there is a problem with the logic, and determine whether the chip is good or bad.
Test whether the output of the digital circuit crystal oscillator is normal.
The online measurement method is mainly used to compare two good and bad circuit boards. Through the comparison, the problem is found, the problem is solved, and the repair of the circuit board is completed.