Question about measuring capacitors.

[Jonk]

How can you tell if a capacitor is good? Do you just put a DMM across it (in DC?) with the positive lead touching the positive leg of the cap and the negative lead touching the negative leg?
And look for the cap's appropriate voltage on your DMM. i.e., 50V's should show up if you're measuring a 50V cap?

One more question: Is there a way to measure that the path from the leg of a capacitor to say a transistor or a resistor or another capacitor is intact? How would you do this?

[JonCraig]

a capacitor blocks dc, so if you set your dmm to continuity test (often with the omega symbol) and put your leads on either side of the cap, you should read an open circuit. now, as there is some time required for the cap to do its thing, you might get a reading very quickly at first, then measure an open (the dielectrics are charging), that's okay.

--jon

[Jonk]

Thanks Jon,

Do you have to be applying power to the cap? Or is what you're describing, measuring w/out power? I realize measuring with the power on could be dangerous and actually impossible considering the caps are on a PCB of a channel strip in a mixer.

Also, what does an open circuit look like on the DMM?

[JonCraig]

power off!!! your dmm has a battery in it, so that's what it uses in all tests. very rarely should you be poking around inside things w/ it powered up, at least until you get more familiar & comfortable with what you're doing.

there are common household devices that produce voltages that can very easily kill you (read: computer crt, tv, microwave).

--jon

[Cojo]

Originally Posted by JonCraig

a capacitor blocks dc, so if you set your dmm to continuity test (often with the omega symbol) and put your leads on either side of the cap, you should read an open circuit. now, as there is some time required for the cap to do its thing, you might get a reading very quickly at first, then measure an open (the dielectrics are charging), that's okay.

--jon

This can be hard to do when it's still connected on the board because you probably get readings from all surrounding circuits as well. You have to desolder it. You will also need a tonegenerator and an oscilloscope to see if it's operating correctly but you can do a "quick test" that it's not short connected with an ohm meter.

The voltage that is printed on the capacitator is the maximum voltage it can handle not the same that's passing through it.

Hope this will be to any help.

/Cojo

[Jonk]

Thanks guys,

Cojo, are you saying I can do a quick test that it's not short connected while it's still in the PCB? If so, how do I do this?

The reason I'm asking these questions is because I've been replacing caps on my board, and I was using a little Weller soldering needle that wasn't temp. controlled before I switched to the Hakko one I have now.

Anyway, with the weller needle I'd have to wait longer to melt solder and sometimes I'd do it when it wasn't hot enough, so there are some places where points on the PCB started to pull up. Just a few, but to remedy it, I made jumpers between the point and whatever else it connected to.

I'm still skeptical as to whether all the paths on the PCB are working, and I fear some caps might be damaged from overheating.

I don't really want to take out the new caps I've already put it though, so I can test them and then put them back in.

[Cojo]

No you can't messure if it's short curcuit when it's still connected, you have to lift one end. Then when you messure you should get a quick reading on the display that then fade away depending on the capacitance. If it doesn't goes away then you have a short circuit.

/Cojo

[r0ck1r0ck2]

what about pulling one of the legs to test...same thing...

[JonCraig]

Originally Posted by Cojo

you can't messure if it's short curcuit when it's still connected, you have to lift one end.

Originally Posted by r0ck1r0ck2

what about pulling one of the legs to test...same thing...

you two think very alike. (and one doesn't read the other's posts, )

--jon

[David Kulka]

If you suspect that a capacitor is shorted, 99% of the time you won't need to unsolder anything to confirm this. Just put your DMM on a low resistance range and measure away. If you see anything under 100 ohms or so, the capacitor is probably bad. You can confirm this by disconnecting one lead (which you'd be doing anyway, in order to replace it) and measuring again.

A bad tantalum capacitor will usually be shorted. Electrolytics typically go under value, but sometimes short. Old paper capacitors, as used in tube gear, usually become leaky (they go low resistance, without actually shorting). Bad film and disk capacitors will usually be shorted.

The most common cap in audio gear is the electrolytic. To test one you really need a DMM that measures capacitance, or a capacitor checker, or an ESR meter. The first two usually require that one lead be disconnected, with an ESR meter this step can often be skipped.

[Cojo]

Originally Posted by JonCraig

you two think very alike. (and one doesn't read the other's posts, )

--jon

Yeah, but I was thinkin' it half an hour faster!

Originally Posted by David Kulka

If you suspect that a capacitor is shorted, 99% of the time you won't need to unsolder anything to confirm this. Just put your DMM on a low resistance range and measure away. If you see anything under 100 ohms or so, the capacitor is probably bad. You can confirm this by disconnecting one lead (which you'd be doing anyway, in order to replace it) and measuring again.

Yes this is true and I was actually going to suggest this but I didn't know how to put it in print easily enough for one to understand. It was a late night, sorry my bad!

/Cojo

[michaels]

Sorry to grave dig an older post but I have some bacon frying noises on a few channels and suspect I too overheated a few caps putting them in my 416b when I recapped it.

Would an ESR meter be what I'd need to check them out to be absolutely sure and if so, any recommendations on which one to purchase or any to stay away from?

Also as I understand it I would *not* have to remove the cap or desolder one end using an ESR meter - is that correct?

Thanks!

[nosebleedaudio]

You can overheat the leads on a cap.