How to determine if capacitor has gone bad ?

[sizzle]

I know the obvious way if radial is to check its top but can it be bad even if obvious difference hasn't come up yet ?

 

[davenn]

Now I'm assuming you are mainly refering to electrolytic capacitors, yes, they can be bad without any outwardly visible signs.
Electro caps will quite happily dry out without bulging. Its generally the ones that are in hi temperature situations that bulge and blow their top.

so any over ~ 10 years of age that are in low temp enviroment couls easily be faulty and testing with a meter is you best option

Dave

 

[(*steve*)]

And to complicate matters, the same capacitor in two different circuits may be considered failed in one and perfectly OK in another.

As an example, a filter capacitor in a switchmode power supply might be considered failed if it's ESR rises above an ohm. However, the same capacitor might be used in an audio circuit and be perfectly OK even if the ESR rose to 100 ohms.

That means there's no simple test (other than for the most gross failures) that apply everywhere.

 

[GreenGiant]

You want to test them out of circuit if you suspect they are bad

I generally test in circuit and if they read close to what they are supposed to then I leave them, but if they are a little off I will pull them out and test that way

But like steve said circuits vary, for example I was fixing a Bose surround sound which the LCD screen did not work at all, there were two 100uF caps, one read 89uF out of circuit and one read 90uF, replacing them with new ones fixed the screen, in some cases a slight difference can make all the difference