gEDA-user: buzzing board

[DJ Delorie <dj@xxxxxxxxxxx>]

Ok, help me out with this one.

The PCB for my alarm clock buzzes.

It's really hard to tell which part is doing it, just from listening.

The sound may be a combination of a high pitch whine and a 100hz buzz
(the oled is 100hz).

The buzzing is related to what's displayed on the OLED; the +12v to
the oled has "noise" on it (0.5vpp) that corresponds to current draw.
If I erase the screen, or run without the oled, or unplug the power
jumper for it, the buzzing stops.  However, adding more bypass on it
(which gets rid of the noise) doesn't stop the buzzing, nor does
shorting out the enabling transistor, nor bypassing the LDO.  Removing
the bypass cap doesn't make it worse.  The OLED power supply is on the
opposite end of the board (a few inches away) from the 3.3v switcher.

I *think* the sound may be coming from the switching power supply (it
has three power inductors in it), but neither the +15v power input nor
the 3.3v rail have any noise on them (outside of the 150KHz from the
switcher itself).  Bypassing two of the inductors (the filter ones)
does nothing.  Pressing on any component does nothing.

So, three questions:

1. How do you find out where such a noise is coming from, when
   everything is so close together?  Neither a stethoscope nor a straw
   were helpful.

2. What kind of components *can* make that kind of noise?

3. If you know what's causing it, how do I fix it?



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