Question: My PC's ATX power supply stopped working. I opened it and noticed that it makes a high pitch sound when turned on. I can't figure out which component in the power supply makes that sound, so that I can replace it.

Answer: 
**Instant Death:**

**WARNING - PC Power supplies have lethal voltages internally both when mains voltage is applied and also for a variable period of time after being turned off.**

**Lurking death**  

**A power supply that has been turned off for many minutes MAY contain voltages that can kill you.**

If you don't know what you are doing and what precautions to take then if you "play" with such devices you may not live long enough to learn. 

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The most likely source of the noise is "magnetostriction" of the switched mode power supply transformer core. The core is driven at excessive currents and/or with an unusual waveform and acts as an audio transducer as the core is magneticallly squeezed thn released by the alternating current. 

It is likely that the fault is **not** in the core itself but is caused by either oscillations being at a lower than usual frequency and/or the supply being overloaded by a fault.

Be aware that outputs MAY be non mains isolated. They are usually but ... . 

Turn supply OFF.
Connect DC meter to a psu output.
Turn on psu.
Measure voltage.
Repeat for all outputs.

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Remove mains power.  

Leave to stand for say 30 minutes.  

 Treat the psu as if it is still alive. It may be. 

Examine the pcb, looking for unusual signs.

* Capacitors with ruptured (broken) tops or split cans.
* Charred, brown or blackened resistors.
* Cratering in plastic bodies of power transistors.
* Burnt circuit.

Main power supply caps are a good start to look at - but expensive to replace. 

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**Discharging high voltage capacitors:**

Capacitors can remain charged for hours in some cases.  

 Seconds to minutes is more common. 

If there is a concern that capacitors may have retained charge they can be discharged by "shorting" or (preferably) by using a resistor with two probes. 

Don't try this at home if not competent and confident with such things. Eye damage (wear goggles) and electric shock are a possible danger. 

* Turn off the power !!!
* Wear safety goggles. 

Spark discharge can throw very small pieces of metal further than you may be happy with. Usually only a danger to eyes unless the capacitor is seriously huge.
* Be SURE the power is off !!!!!!
* Use an insulated wire or test lead with 2 bare ends. Ideally have a resistor in the middle large enough to limit current and small enough to discharge the largest cap quickly enough. 

Most pepole just use a piece of insulated wire or a test probe. Be aware of the exciting nature of spark discharge.
* Is the power really really off ? !!!!!!!!!!!!!!!!!!!
* Short between earth (chassis etc) and possible high voltage point. 

Note that power MUST be removed ! :-( 

and that in very rare cases damage may be caused (not usual) 

AND that the "zap" and spark of a substantial discharge can make you jump and hurt yourself on something sharp. This is more likely than you'd credit until you've discharged a really large capacitor this way. 

**Note obvious shock hazards if doing this!**

**WARNING** - **Capacitors that have been discharged can "regrow" some charge** after a while. This can be enough to give you a shock. Two dischargings a few minutes apart is usually enough to remove most of the charge but be aware of the possibility.
The sound could come from a capacitor or a transformer, though that is not necessarily the part that is defective.

Personally I wouldn't mess with a switched power supply (it is very dangerous and a repair is unlikely to succeed) and buy a new one.