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1. Determine whether the computer is good or bad.

1) First, connect the host power supply (ATX), press the main switch button, if it can not be powered, then unplug the power supply to the motherboard.

2) Short-circuit the green and black wires of the power supply with tweezers, and check that the fan of the power supply does not turn. If the power supply fan turns, the power supply is good and the fault is on the host side.

3), judge the computer host switch is good or bad. Connect the ATX power cable to the motherboard, pull up the switch pin and reset pin on the motherboard, and use the tweezers to short-circuit the switch pin to trigger the power switch to see if it can be turned on. If it can, it means that the switch of the main box is broken. The switch is removed for cleaning.

4) If the short-circuit switch pin triggers the power or does not turn on, it means that the motherboard can’t trigger the boot, remove the motherboard from the chassis for maintenance.

5), remove the motherboard, first clean the dust on the board, so as not to hinder maintenance. First look at the motherboard above whether there is component burnout, drum kit, there is no burnt or broken line on the circuit board.

6), put the motherboard, plug in the CPU dummy load, plug in the power. Plug in the motherboard test card and prepare for maintenance.

2. Check the trigger circuit

When the motherboard is not powered, firstly, the specific fault circuit of the motherboard is energized by the method of forcing power. That is to say, the green line and the black line are directly short-circuited. If it can be powered up at this time, the fault is in the soft start circuit itself. If power is not available at this time, there is a serious short circuit. ATX power supply internal protection, it does not allow the voltage output by itself to the ground, so the power supply is automatically protected inside.

There may be a short circuit of the red line, a short circuit of the yellow line, a short circuit of the purple line or a short circuit of the main power supply terminal of the CPU. In the above short-circuit phenomenon, any one of the actual motherboard failures will be forced to be powered up and not powered.

Possible reasons for the short circuit of the red line are: a short circuit of the FET on the main board or a short circuit of the power manager, a short circuit of the gate circuit or an I/O short circuit, and a short circuit of the south bridge, or a short circuit of the 5V filter capacitor. Measure the 5VATX ground data or measure the value of the power supply pipe to the ground to see if it is shorted to ground. The normal ground value is about 380U. If the power supply tube is obviously measured to 0 megaohms or close to 0 megaohms, it indicates that the motherboard has ATX protection due to short circuit to ground.

For the 12V short circuit of the yellow line, it is usually the power management itself and the 12V filter capacitor short circuit. For the 12V short circuit, there may be a problem with the serial port chip.

The short circuit for the purple line may be caused by the south bridge, I/O, FET and gate circuit, as well as the purple line filter capacitor and the violet line Zener diode.

The short circuit to the CPU mains supply may be the FET, the power manager and the main supply filter capacitor. For the motherboard after P4, the CPU main power supply short circuit may also be a short circuit of the north bridge. Measure the ATX power line shorted to the pool, and then find the relevant damaged components to change along the line running circuit.

3. Check the soft boot circuit

If the power is applied forcibly, the fault is in the soft start fault itself. At this time, the soft boot circuit itself and other circuits connected with the soft boot circuit should be focused.

(1) COMS battery. Some motherboards, battery power can not be turned on, and most of the motherboard does not have a battery and does not affect the boot. The COMS battery provides a voltage of 2.6V (2.6 to 3.3V) or more under normal conditions.

(2) COMS? Practice. The COMS jumper is not correct and cannot be turned on. Generally, it is correct to jump on one or two pins, and the third pin is grounded. If you jump on the second and third pins, you will not be able to turn it on. After the mainline of argon injection is faulty, it can be turned on because the real-time crystal power supply is provided by the purple line.

(3) Measure whether the POWER switch pin has 3.3V or 5V voltage. One pin of the POWER switch pin is grounded, and one pin is powered by purple 5V. The middle part will pass some electronic components such as circuits and resistors. If there is no 5V or 3.3 voltage to the switch pin, run the circuit, look at the damage from the ATX power supply purple 5V to the components between the POWER, find the damage and then replace it.

(4) Measure the crystal oscillator next to the South Bridge chip to see if it starts to vibrate. The starting voltage is about 0.5V and about 1.6V. If not, replace the filter capacitor and the crystal element next to the crystal. There is also a way to touch the two pins of the real-time crystal by hand. After touching the motherboard, you can power up and work. After the real-time crystal is damaged, you can power up after you touch the real-time crystal oscillator, so the CPU does not work. At this time, continue to touch the two pins of the real-time crystal by hand, so that the power is turned on but the memory is not used, and then the two pins of the real-time crystal oscillator are touched, and the voltage 乂 will pass through the memory. This is a typical phenomenon of damage to the peripheral circuitry of a real-time crystal. Such a motherboard is more difficult to repair. The real-time crystal oscillator’s electric circuit is very strict. After the damage, the real-time crystal oscillator with the same colour and size will be used, and the vibrating capacitor will be replaced. Otherwise, the replacement will be unsuccessful.

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