Re: gEDA-user: buzzing board

[DJ Delorie <dj@xxxxxxxxxxx>]

> The PCB for my alarm clock buzzes.

Having poked and prodded, and getting a piezo unit that did nothing, I
finally figured it out (the trick was to use a nail, not a toothpick,
to poke and prod - I was using that as a better sound conductor for
the piezo).

There are not one but TWO buzzing caps.  If I replace both of them
with electrolytics, the buzzing goes away.  If I replace them both
with two new ones of the same type/size, it still buzzes.

The caps are 4.7uF 16v.  They're decoupling/bypass for a +12v rail
(well, one is, the other is VcomH, which seems to be 7v).  They're the
biggest 0805's with that voltage rating.

I can fit a 1206 in that spot.  If I replace those with 10uF 25v, do
you think they'll not buzz?

Another alternative is 22uF 16v.  I don't know if the buzz is due to
insufficient capacitance, or damage due to being too close to the
voltage rating.

Alternatively, I can stack two 10uFs on top of each other to make 20uF
25v, but that's ugly.  1210's won't fit.

Note that adding a 47uF 35v electrolytic across the 4.7u reduces the
noise greatly.  In fact, a 4.7u electrolytic helps much more than a
second 4.7u ceramic.

If I replace just the +12v cap with a 22u elec, the +7 cap still
buzzes.  I suspect the elec alone can't react fast enough to get rid
of the ripple.


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