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Today, editor is here to walk you through how to determine if power supply is bad and fix motherboard not being turned on. Let's study with me.

1. Assessing health of computer's power supply

1) Connect host power (ATX) first, press host switch button if power does not turn on, then unplug power plug connecting power supply to motherboard.

2) Use tweezers to short green and black wires of power supply and check if power supply fan can rotate. If power supply fan turns on, power supply is good and problem lies with host computer.

3) to determine if host computer switch is OK. Connect ATX power cable to motherboard, pull out switch pin and reset pin on motherboard, use tweezers to short switch pin to activate power switch, and see if it can be turned on. means main unit switch is broken. Remove switch for cleaning.

4) If short circuit switch contact starts power and still cannot be turned on, it means that motherboard really cannot start start, and motherboard should be removed from chassis to check.

5), remove main board, and first clean board of dust so as not to interfere with maintenance. First check for burnt components on motherboard, blisters, and for burnt or broken wires on board.

6), install motherboard, connect dummy load of processor and connect power supply. Plug in motherboard test card and get ready for service.

2. Check trigger circuit

If main board is not receiving power, first locate specific faulty circuit in which main board is receiving power by forcibly turning power on. That is, short-circuit green wire and black wire. If power can be turned on at this time, then fault is in soft start circuit itself. If power cannot be turned on at this time, it means that a serious short circuit has occurred. The internal protection of ATX power supply prevents output voltage from shorting to ground, so internal protection of power supply operates automatically.

There may be a short circuit in red, yellow, purple lines, or main CPU power supply terminal. If any of above short circuit phenomena occur when motherboard actually fails, a forced power-on will occur, but power will not turn on.

Possible causes of a shorted red line are: motherboard FET shorted or power manager shorted, gate circuit shorted or I/O shorted, southbridge shorted, or 5V filter capacitor shorted. Measure 5VATX data to ground or power supply tube value to ground to see if there is a short to ground. ButThe normal ground value is about 380 U. If it is obvious that power supply tube has a resistance of 0 MΩ or close to 0 MΩ against ground, this means that motherboard has a IC short to ground, which causes ATX protection.

A 12V yellow line short usually occurs between power management system itself and 12V filter capacitor. A 12V short may also be due to a problem with serial port chip.

A shorted purple line can be caused by southbridge, I/O, FET and gate circuit, as well as purple line filter capacitor and purple line zener diode.

A short circuit in main CPU power supply can be caused by FET, power management device, and main power filter capacitor. For motherboards after P4, short circuit of main CPU power can also be short circuit of north bridge. Measure ATX power line that has a short to battery, then run a circuit along line to find corresponding damaged components and replace them.

3. Check soft start circuit

If power can be forced on, problem is with soft start itself. At this time, soft start circuit itself and some other circuits related to soft start circuit should be emphasized.

(1) COMS battery. Some motherboards can't turn on even if battery is low, but most motherboards won't affect startup without batteries. Under normal conditions, COMS battery provides voltages above 2.6V (2.6-3.3V).

(2) COMS? Practice. If COMS jumper is incorrect, it cannot be enabled. In general, it is correct to jump to first and second pins, and third pin to ground. If he jumps on second and third needle, he cannot be turned on. Please note that after some main lines jump incorrectly, it can be turned on because real-time power of crystal oscillator is provided by purple line.

(3) Check for 3.3V or 5V voltage at POWER switch terminal. go through middle. If there is no voltage of 5V or 3.3 on switch contact, start circuit, check damaged components from ATX purple 5V power supply to POWER, find damaged ones and replace them.

(4) Measure crystal oscillator near southbridge chip to see if it starts to vibrate. The initial voltage is about 0.5V and 1.6V. If it is not, replace filter capacitor next to crystal oscillator and crystal oscillator component. There is also a way to touch two pins of real-time crystal oscillator with your hands. After touching main board with your hands, you can turn on power and work. This means that after damaging real time crystal oscillator, you can turn it on by touching thearts generator in real time, but CPU is not working. At this time, keep touching two pins of real-time crystal oscillator with your hands so that power does not leak memory, and then use it to touch two pins of real-time crystal oscillator and voltage will flow through memory. This is a typical phenomenon of damage to peripheral circuit of a real-time crystal oscillator. Such motherboards are more difficult to repair. The electrical circuit requirements of a real-time crystal oscillator are very strict. After damage, try to replace it with a real-time crystal oscillator of same color and size, as well as a vibration capacitor, otherwise replacement will fail.

The above is an introduction to how to judge quality of a computer's power supply. I hope it will be useful to you.