1. Investigation method

When we receive a circuit board to be repaired, our first priority is to carefully investigate its appearance. If the circuit board has been burned, it is necessary to carefully check whether the power circuit is normal before powering on the circuit board, and ensure that it will not cause secondary damage before powering on. Survey method is a type of static viewing method, and when using survey method, the following processes are generally followed.

The first step is to investigate whether the circuit board has been damaged by human factors, mainly from the following aspects:

① Check if the circuit board has been dropped, causing deformation of the board corners, or if the chips on the board have been deformed or damaged by the drop.

② Investigate the socket of the chip to see if it was forcibly pried open due to the lack of specialized equipment.

③ Investigate the chips on the circuit board. If there is a socket, the first thing to investigate is whether the chip has been inserted incorrectly. This is primarily to prevent the operator from inserting the chip in the wrong position or direction when repairing the circuit board themselves. If the fault is not corrected in a timely manner, when the circuit board is powered on, it may burn out the chip and cause unnecessary loss.

④ If there are short-circuit terminals on the circuit board, investigate whether the short-circuit terminals are plugged in incorrectly.

The repair of circuit boards requires a solid theoretical foundation and meticulous work. Through careful investigation by the repairman, sometimes the cause of the problem can be identified at this step.

The second step is to investigate whether the components on the circuit board have been burned out. For example, whether resistors, capacitors, and diodes are blackened or burnt. Under normal circumstances, even if the resistor is burnt, its resistance value will not change, its function will not change, and it will not affect normal use. In this case, a multimeter is needed to assist in measurement. But if capacitors and diodes are burnt, their functions will be altered, and they will not be able to perform their intended function in the circuit, which will affect the normal operation of the entire circuit. At this time, new components must be replaced.

The third step is to investigate the integrated circuits on the circuit board, such as the 74 series CPU、 Are there any bulges, cracks, burning, or blackening of chips such as coprocessors and ADs. If such a situation occurs, it can be basically confirmed that the chip has been burned out and must be replaced.

The fourth step is to investigate whether the wiring on the circuit board has peeling, burning, or broken circuits. Has the copper hole detached from the solder pad.

Step 5: Investigate the safety of the circuit board (including the safety tube and thermistor) to see if the safety wire is blown. Sometimes, because the safety wire is too thin to see clearly, an auxiliary tool - a multimeter - can be used to determine whether the safety tube is damaged.

The occurrence of the above four conditions is mostly the result of excessive current in the circuit. But the detailed reasons for the excessive current need to be analyzed in detail. But the overall idea of finding a problem is to carefully analyze the schematic of the circuit board, then based on the location of the burned component circuit, search for its superior circuit, step by step deduction, and then based on some experience accumulated in the work, analyze the most prone areas to problems and find the cause of the problem.

Static measurement method for repairing circuit boards

In circuit board repair, it is often difficult to find some problems by investigation, unless it is obviously burned or deformed. However, most problems still require measurement with a voltmeter in order to draw a conclusion. Circuit board components and related parts should be inspected one by one.

The repair process should be operated according to the following procedure, and the most useful thing is a multimeter.

Step 1: Check for short circuits between the power supply and ground to determine the cause.

Step 2: Check if the diode is functioning properly.

Step 3: Check if the capacitor shows a short circuit or even an open circuit condition.

Step 4: Check the indicators of integrated circuits and resistors related to the circuit board.

We can use survey and static measurement methods to deal with most of the problems in circuit board repair, which is beyond doubt. However, when measuring, we must ensure that the power supply is normal and there is no secondary damage.

3 Online Measurement Methods

The online measurement method is generally applied to manufacturers of mass-produced circuit boards. In order to facilitate repair, manufacturers usually establish a relatively universal debugging and repair channel, which can conveniently supply the power required by the circuit board and some necessary initial signals. The online measurement method primarily addresses two issues. One is to break down the problems discovered in the previous two processes and ultimately identify the components that present the issues. Secondly, after the above two steps of inspection, the problem has not been resolved, and it is necessary to conduct online measurement to identify the cause of the problem. The online measurement method primarily goes through the following processes.

Step 1: Power on the circuit board. It should be noted that some circuit boards do not have a single power supply, and may require 5V, as well as positive and negative 12V, 24V, and so on. Do not miss the power supply that should be added. After the circuit board is powered on, touch the components on the circuit board by hand to see if there are any hot or overheated components, with a focus on the 74 series chips. If the components are hot to the touch, it indicates that they may have been damaged. After replacing the components, check if the circuit board issue has been resolved.

Step 2: Use an oscilloscope to measure the gate circuit on the circuit board and investigate whether it conforms to the logical relationship. If the output does not conform to logic, there are two situations that need to be treated separately. One is that the output should be low level, but the actual measurement is high level, which can directly determine chip damage; Another approach is that the output should be at a high level, but the actual measurement is low, which does not necessarily indicate that the chip is damaged. It is necessary to disconnect the chip from the subsequent circuit, measure again, investigate whether the logic is reasonable, and determine the quality of the chip.

Step 3: Measure the crystal oscillator in the digital circuit with an oscilloscope to see if it has output. If there is no output, it is necessary to remove as many chips connected to the crystal oscillator as possible before measuring. If there is no output yet, it begins to conclude that the crystal oscillator is damaged; If there is output, the removed chips need to be installed one by one, tested one by one, and the problem location found.

Step 4: A digital circuit with a bus structure generally includes three paths: digital, address, and control bus. Measure the three bus lines with an oscilloscope, compare with the schematic diagram, investigate whether the signals are normal, and identify the problem.

The online measurement method is primarily used for comparing two good and bad circuit boards. Through comparison, problems can be identified and resolved. Thus completing the repair of the circuit board.

Article sourced from Jiangmen single and double-sided circuit boardhttp://www.yonghongpcb.com