Hi,
How can I test a smt capacitor in "live" circuit with a oscilloscope?
As Jamie notes, testing in an operating circuit requires knowledge of
what to expect to see if the component is operating normally.
For in-circuit testing on a de-energized circuit, however, it is
possible to get some idea of what's "going on" with an o-scope and a
stimulator circuit. There are commercial versions (see Huntron
Tracker) but a home-made rig is pretty simple. See a discussion at
<
http://octopus.freeyellow.com/octopus.html> and similar.
Using an octopus isn't magical and is not likely to yield useful
results unless the effects of the surrounding components can be
determined. It's most usefully employed when there is a known good
card against which the suspected failure can be compared.