1, field effect transistor breakdown, resulting in ATX power protection, the phenomenon is that the fan stops as soon as it turns, and the light on the motherboard diagnosis card goes out as soon as it turns on.
Unplug the power of CPU 12V, and it can be started normally.
The specific diagnosis method: set the digital multimeter at the diode position, then short-circuit the three pins of the FET, and then touch two of the three pins of the FET with two probes to measure three groups of data. If two sets of data are 1 and the other set of data is 300-800 ohms, the FET is normal. If a set of data is 0, the field effect transistor will break down.
2. The 2.CPU filter capacitor is damaged, resulting in abnormal power supply or unstable operation of the motherboard.
Specific diagnosis method: observe whether the capacitor is bulging or burnt before measurement, and replace it if it is. Turn the multimeter to the "20K" position, with the red probe connected to the positive pole of the capacitor and the black probe connected to the negative pole of the capacitor. If the displayed value gradually increases from "000" and finally "1" is displayed, the capacitance is normal.
Capacitance problems will lead to motherboard failure to boot or irregular crash, blue screen, black screen and other failures. Replacement principle: the pressure resistance is greater or the same as that of the original factory. Capacity plus or minus 20%.
3. FET is degenerating and aging.
The drain has an input voltage, and the gate has a control voltage (about 3V for high-end transistor; The low-end transistor is about 10V), and the field effect transistor is bad if the source has no output voltage. 85N03L replaces 70N03L and so on
4. The power management chip is damaged.
If the field effect transistor and capacitor are measured normally, and there is no control voltage on the grid of the upper tube, check whether the power supply pin of the power management chip has 5V or 12V voltage; if so, check whether the PG signal pin has voltage; if so, the power management chip is damaged.
For the circuit with driver chip, check whether there is power supply voltage and PWM control signal. If there is an input but no output, the driver chip is damaged. If PWM control signal is input, check whether there is power supply and PG signal in the main control chip. If yes, the main control chip is damaged.
5. The inductance coil becomes discolored, resulting in large current and turn-to-turn short circuit. Replacement principle: the copper coil is the same size, the copper wire is the same thickness and the number of turns is the same.
Extended data:
Working principle of CPU power supply circuit:
Different CPU needs different working current and voltage. P3CPU has two power supply voltages: the core power supply voltage Vcore is 1.2V-2V, and the external power supply voltage is fixed at 2.5V (the external power supply voltage is generally obtained through a three-terminal regulator):
The power supply voltage of P4CPU includes the core power supply voltage Vcore (usually 1. O5V- 1.5V) and AGTL bus terminal voltage VTT (1.8V, 1.5V, 1.25v for different types of CPU, respectively) are generally provided by the north bridge power supply circuit, and the circuit is relatively simple.
The core voltage power supply circuit of CPU is the most easily damaged circuit, so the CPU power supply circuit referred to in maintenance work generally refers to the core power supply circuit (Vcore circuit).
The PWM power management chip used on the motherboard has several voltage identification control pins (usually VIDO-VID4), which are usually connected to the CPU (if the CPU is not connected, these control terminals default to high level). By controlling the level of these pins, the output DC voltage value, that is, the power supply voltage of CPU, can be controlled.