[PLUG] Oscillation coming from computer

[Keith Lofstrom]

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

-- 
Keith Lofstrom          keithl at keithl.com