Re: gEDA-user: buzzing board

[Dave McGuire <mcguire@xxxxxxxxxxxxx>]

On Sep 13, 2007, at 12:34 AM, DJ Delorie wrote:
> 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.

   Is it convenient to try putting an choke in series with the OLED's  
power supply line, with the OLED's bypass capacitor on the OLED side  
of the choke?

> 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.

   I've only used those two methods, aside from probing around with  
an oscilloscope to look for the noise.  One other trick that I've not  
tried (if anyone can comment on this I'd appreciate it) is to use an  
inductive pickup probe to look at the *current* waveforms on the  
supply lines to different parts of the circuit, and see what  
corresponds to the noise you're hearing.

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

   Inductors and PCB traces, mainly...but I suppose nearly anything  
probably could, to one degree or another.

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

   Hot glue, molten wax, conformal coating..

           -Dave

-- 
Dave McGuire
Port Charlotte, FL
Farewell Ophelia, 9/22/1991 - 7/25/2007





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