Power on the CPU fan to spin and then turn off

When the power on the mainboard is turned on, the fan spins a few times and then turns off, or the CPU fan just wobbles. Those are the cases of touching the power on the PC mainboard. To fix the problem of triggering the CPU fan to rotate and then turn off, you need to follow the following methods.

1. Check 12v circuit for CPU

Completely remove the 12V power supply jack (4 pin or 8 pin) for the VRM circuit (CPU voltage circuit) and try again. If the fan is removed, the mainboard is being touched by the VRM circuit. Note that some good sources like Acbel when the power jack on the mainboard is shorted, then you need to completely remove the 20 pin jack and connect the blue and black pins to discharge all the power and then attach it to the jack to take effect (turn off the power switch) ).

When removing the 4-pin jack of the 12v source but the machine is normal, you proceed to measure the mosfet on the mainboard to see which one is shorted by. Use an analog clock to adjust the X1 scale and measure on the D – S pins of the mosfet with two measuring directions (red rod to D, black rod to S and vice versa, black rod to D, red to S).

+ If both directions, measure the resistance from a few tens to a few hundred ohms, that mosfet is normal.

+ On the contrary, both measurement directions, or only 1 direction with resistance up to = 0 ohms, the mosfet has been shorted. You need to replace the equivalent mosfet taken from another main’s VRM  circuit.

After replacing the mosfet, you should also check if the 12v line for the oscillator circuit is shorted. By using the meter to turn on the X1 scale, the red rod is set to mass, and the black rod is placed at the  12v pin (the yellow wire has to go through) of the 20 pin or 24 pin ATX power jack.

+ If the clock needle only moves up a little => deduce normal impedance, not touched, short.

+ If the clock needle is up to the end of the scale and has an impedance of 0 ohms, the mainboard is shorting the 12V load line. This load line is usually supplied to the oscilloscope IC, you should try another oscillator ic (70% short of mosfet will kill the oscilloscope ic) the same or equivalent. To replace the equivalent IC, please refer to: IC  pinout.

Most CPU fan power sources rotate and then turn off due to short circuit of CPU voltage regulator. You can measure if there is a short by testing by removing the 12v jack as above or measuring the 12v impedance supplied to the VRM circuit by using an X1 scale meter, the red rod is placed on mass, the black rod is placed at the 12v pin of jack 4. battery or 8 battery. If the meter has an impedance of 0 ohms, it is short, otherwise it is normal. When removing the 12v jack, the source does not rotate, then we measure the line impedance of 5v and 3.3v.

2. Check circuit 3.3v and 5V supply for IC

ATX power orange connector  = 3.3 V
Yellow connector ATX power = 5 V

Measure the 3.3v and 5v loads by adjusting the meter to the X1 scale, hugging the red rod to place the mass, the black rod to enter the orange leg (3.3V) then the yellow leg (5v).

+ If the meter only moves up a bit, about 500 – 1k ohms, the impedance is normal.

+ If the clock reaches the end of the needle or is close to 0 ohms, the circuit load is shorted.

Shorting 3.3 V, one of the following components is shorted: IC SIO, male chip, ROM BIOS, IC clock gen, IC LAN onboard.
If you short circuit 5 V, one of the following components is shorted:  IC SIO, male chip, ROM BIOS.

Usually short on this load line, most of them have short circuit of male chip, short circuit of chipset voltage stabilizer, dead SIO ic, short of LAN IC. To fix the 3v5 and 5v load short, you plug in the power for a moment and touch the SIO chip, male chip and onboard LAN to see which one is hot, if it’s hot, 90% of it will die. If there is a current meter (used to repair the phone) then we can determine which IC is short by.

Reduce the voltage of the meter to 0v, put the black rod to ground, the red rod to the shorted line (3.3x or 5v line). Then adjust the voltage gradually until the current reaches 0.8A. Let the current flow through the mainboard for about 1 minute and touch the above components. If the child is hot, the child is shorted, proceed to replace the other one.