[PLUG] Oscillation coming from computer

Denis Heidtmann denis.heidtmann at gmail.com
Thu Jun 12 21:53:29 UTC 2014


On Thu, Jun 12, 2014 at 2:08 PM, Keith Lofstrom <keithl at gate.kl-ic.com> wrote:
> On Thu, Jun 12, 2014 at 07:28:37AM -0700, Dick Steffens wrote:
>> My new computer is generating what sounds like a sine wave of about 975
>> Hz. It is fairly quiet, but noticeable. It is not constant, but nearly
>> so. It fades in and out, and sometimes is not there at all. If I'm not
>> using the running machine for awhile it goes quiet. I'm guessing it's a
>> fan, but wonder what other folks might have experienced like this. I can
>> take the box back to ENU for analysis and warranty repair, but it's my
>> production machine, so I'd need to transfer some stuff to a backup and
>> live with that while it's out.
>>
>> Any thoughts?
>
> If "new computer" == desktop, open it up and listen for the
> precise area the sound is coming from.  CPU fan speed varies
> with CPU load, so I would expect a fan to vary in pitch as
> well.
>
> Constant pitch, I would guess either the main power supply,
> or the switching power supply on the motherboard.  Power
> supplies operate at ultrasonic frequencies, you wouldn't
> be able to hear that.  However, they have feedback loops
> that control the output voltage, and if the feedback loop
> is wonky, it will cycle around the setpoint, perhaps at an
> audio frequency.  That would be highly dependent on the
> specific motherboard design, and the specific failure of
> the feedback loop (connections? component?  bad design?).
>
> Identify the source - power supply or motherboard (or maybe
> even the disk or cdrom drive) - then google for that model
> number and "noisy".
>
> If the problem is the disk drive, use "dd" in single user
> mode to copy it to another similar disk drive NOW.  If not,
> put in another disk drive with a scratch distro and let
> ENU have that.   Disk drives are cheap, your time is not.
>
> Clinic is Sunday.  You can bring it in and we can take a
> look.  If I remember, I can bring an oscilloscope and we can
> probe around (and risk shorting something out, what fun!).
>
> Keith

Is it possible it is a beat frequency of two synchronous oscillators?

-Denis



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