Can a capacitor that's faulty still show a reasonable reading on a multimeter?

I have bought these vintage used capacitors that the seller tested to show pretty close readings within their tollerance range. What I was wondering was whether these tests prove that the capacitors are still in good condition or on the way out?

Can a capacitor still produce good readings even if they are close to failure or faulty?

How do you tell if a capacitor is on it's way out?

You can measure any number of capacitor variables, but even a perfect vintage capacitor will only be as good as an old capacitor was way back when it was new. Capacitors are not wine.

If this is in pursuit of some classic sound or to save money, I'd at least confirm capacitance, leakage (at rated voltage), and maybe esr at 100Hz or so.

If the caps are really old, you might need to re-form them (bring them up to voltage slowly with an R in series).

In general modern capacitors are better in all regards.

JR

EDIT- I notice that I didn't really answer your question. "can a capacitor measure good and be bad" the answer is it no if you measure the exact way it is faulty, but you would have to measure everything.  I encountered one production run of caps that had a faulty swage (mechanical connection) to one lead. We only discovered the problem when a percentage of the test lot measured almost no capacitance.  So units from this run could actually measure good and be bad, but this was a pretty rare scenario (swage tool broke at cap factory). You are not likely to find this kind of problem with "vintage" units out in the field, most likely loss of electrolyte, which should show up in ESR or simple C.

 

Well, once I was told that old caps could behave very different when using higher voltages.

And from my own experience : I had a faulty V72 micpre, and after a really long search for the fault, I discovered that it had a couple of faulty electrolyt caps (8 uF/525v and a 400v one) which gave the right capacitance reading on my multimeter, but in circuit they were malfunctioning.

But I think you're going to use them for your 1176 ? I suppose you won't have a problem using old caps, but still, I'd buy new ones just to be sure.

helterbelter said:

Well, once I was told that old caps could behave very different when using higher voltages.

And from my own experience : I had a faulty V72 micpre, and after a really long search for the fault, I discovered that it had a couple of faulty electrolyt caps (8 uF/525v and a 400v one) which gave the right capacitance reading on my multimeter, but in circuit they were malfunctioning.

But I think you're going to use them for your 1176 ? I suppose you won't have a problem using old caps, but still, I'd buy new ones just to be sure.

Click to expand...

Il be using the vintage caps for the Drips La2a.

The first post needs more information.

  What were the caps tested for Cap value, leakage, ESR, DA?

What kind of caps?

Absolutely.  I have seen plenty of old coupling caps that measure perfectly, and even perform perfectly for a few minutes, then exhibit a large increase in DC leakage after enough time passes.  Short answer is you can only tell in circuit in question, even when passing all the simple tests.  Meaning, some of the tests aren't as simple as testing in circuit, especially for the layman. 

http://www.groupdiy.com/index.php?topic=31110.msg377665#msg377665
build an esr meter.

Easy answer - never use "vintage" or old used caps = dried out and useless !

Buy new good quality caps, the modern production process mean they will be better anyway.

MM

MartyMart said:

Easy answer - never use "vintage" or old used caps = dried out and useless !

Buy new good quality caps, the modern production process mean they will be better anyway.

MM

Click to expand...

Its for my Drips La2a. It sais to experiment with vintage caps. I just received the caps and they are HUGE.
I have rarely seen caps this huge. I definitely want to install these mothers!

If you are considering installing 'vintage' caps in an LA-2a, be warned.

I think that the vast majority of techs who've been around a bit have had some experience of the output DC-blocking capacitors failing, and when they go, they take the (now ÜBER-expensive) output transformers with them.

Installing 'vintage' caps in an LA-2a? -Personally I think it's a fool's goal. -Any 'gain' is utterly outweighed by the horriffic risk, in my experience. Newer caps are more reliable, and unless you have a capacitance tester which tests caps AT THEIR RATED VOLTAGE, then yes, absolutely. Your hand-held multimeter can read a good value, while that same cap can still be a mini-grenade ready to pop as soon as it gets hit with a hundred volts or two.

Caps can read correct capacitance on any number of test meters, and the dielectric can fail when high voltage is applied. -Old electrolytics are the WORST for doing this, and in any case, there are LOTS of different ways that a cap can "go bad". -High ESR can lead to excessive heat, which can in turn either dry out the cap once it's in use, resulting in low capacitance once it's been in use for a while, or occasionally dielectic failure or leakage. High leakage again frequently leads to temperature rise etc.

Summary: -In an LA-2a, NEVER use "vintage" capacitors. -There's a thread over on PSW where this exact thing was discussed, and a couple of techs (who between have repaired more LA-2a's than I've possibly even seen) chimed in to share their collective opinion as to the foolishness of such an endeavour.

Don't do it. -Non-elects are much less risky, but electrolytics... you'd have to be mad. -For the PSU especially, modern caps are *SO* much better in every respect.

-But if you must persist with your investigations, may I respectfully offer the following advice:

NEVER APPLY POWER with the case open, unless you are wearing a FULL-FACE MASK.

-And no, I'm not kidding.

Keith

The two white caps are ceramic though and they all are non polarized?

Since these are caps rated at 600-1000 DC volts, would they be O.K to use for ground lifts like in Drips La2a ground lift for the input and output XLR.