5 Feb
2011
5 Feb
'11
7:53 a.m.
I was checking the capacitors in-circuit and
understand that this is not
correct.
However, I would have thought that a 0 ohm measurement in-circuit would
indicate a shorted capacitor because there would surely not be any point
designing a circuit where both ends of the capacitor are connected together?
Yes and no...
Firstly, there is one very common circuit (common in the world, not in
classic computers) which effectively shorts out a capacitor, at least for
DC tests. And that, of course, is a parallel LC (inductor/capacitor)
tuned circuit. An inductor should have a very low DC resistance, of course.
Secondly, the fact that 2 poitns on a PCB test as being shorted together
meands that _something_ is connecting them. That something doesn't have
to be the obvious compomnent between them. That capacitor is presumably
conencted to other components. A short in one of those could be the problem.
Is there a simple way to find shorted caps in-circuit?
Would an absolutely 0
ohm resistance while in-circuit indicate a shorted cap for the reason I
suggest above?
There's no simple way, it's easier just ot desolder at least one end and
test them effectively out-of-circuit.
The fact that you've found something that's shorted that shouldn't be is
a clue I think. What does that capacitor connect to (it may be easier to
trace this o nthe good PCB where the 2 ends don't test as being shorted
together).
-tony