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#1
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Electrolytic filter cap replacement
In my understanding the conventional rule of thumb is to replace
filter caps after some number of years (and the number varies depending of who you ask). The same thinking says caps 30+ years really should replaced on the grounds that if they're not failing now they will soon. I've seen a number of old caps (especially the cans in Ampeg V -series amps but in other old tube amps as well) where there is no appreciable ripple in the supply. Testing the caps on a Sencore meter shows them to be well with leakage for their value. Sencore's dieletric absorption test is to run the leakage test until the cap is showing leakage in the acceptable range and then switch quickly back to measuring the cap value. If the value creeps up slowly the DA is too high, if it rises up quickly, it's fine. It doesn't define "slowly" but the caps under test easily match the newer ones I have in stock. So here's my question: If the caps are meeting these two tests and there's no ripple problem in the supply, is there any advantage to replacing them? Thanks, Andy |
#2
Posted to rec.audio.pro
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Electrolytic filter cap replacement
apa wrote:
In my understanding the conventional rule of thumb is to replace filter caps after some number of years (and the number varies depending of who you ask). The same thinking says caps 30+ years really should replaced on the grounds that if they're not failing now they will soon. This is not really true... there are some kinds of electrolytic caps that are very prone to failing, and there are some that are not. And for the most part, ones that are in hot places will tend to fail faster than ones that are kept cold. I've seen a number of old caps (especially the cans in Ampeg V -series amps but in other old tube amps as well) where there is no appreciable ripple in the supply. Testing the caps on a Sencore meter shows them to be well with leakage for their value. Sencore's dieletric absorption test is to run the leakage test until the cap is showing leakage in the acceptable range and then switch quickly back to measuring the cap value. If the value creeps up slowly the DA is too high, if it rises up quickly, it's fine. It doesn't define "slowly" but the caps under test easily match the newer ones I have in stock. Yes, but that doesn't mean that they aren't going to fail prematurely. So here's my question: If the caps are meeting these two tests and there's no ripple problem in the supply, is there any advantage to replacing them? Well, the thing is, sooner or later you're going to have a failure. You don't really know where you are on the failure curve, but you DO know that the failure curve is a bathtub distribution and sooner or later you get to the end of it. If it's mission critical equipment and someone is going to die if it fails, I'd replace the caps, and I'd probably use twist-lock caps that match the originals as well as possible (Antique Electronics Supply will stock them). If it's a guitar amp and the consequences of failure are having to play an acoustic set.... sheesh, just wait until something goes wrong. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#3
Posted to rec.audio.pro
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Electrolytic filter cap replacement
On Nov 17, 3:28 pm, (Scott Dorsey) wrote:
apa wrote: In my understanding the conventional rule of thumb is to replace filter caps after some number of years (and the number varies depending of who you ask). The same thinking says caps 30+ years really should replaced on the grounds that if they're not failing now they will soon. This is not really true... there are some kinds of electrolytic caps that are very prone to failing, and there are some that are not. And for the most part, ones that are in hot places will tend to fail faster than ones that are kept cold. I've seen a number of old caps (especially the cans in Ampeg V -series amps but in other old tube amps as well) where there is no appreciable ripple in the supply. Testing the caps on a Sencore meter shows them to be well with leakage for their value. Sencore's dieletric absorption test is to run the leakage test until the cap is showing leakage in the acceptable range and then switch quickly back to measuring the cap value. If the value creeps up slowly the DA is too high, if it rises up quickly, it's fine. It doesn't define "slowly" but the caps under test easily match the newer ones I have in stock. Yes, but that doesn't mean that they aren't going to fail prematurely. So here's my question: If the caps are meeting these two tests and there's no ripple problem in the supply, is there any advantage to replacing them? Well, the thing is, sooner or later you're going to have a failure. You don't really know where you are on the failure curve, but you DO know that the failure curve is a bathtub distribution and sooner or later you get to the end of it. If it's mission critical equipment and someone is going to die if it fails, I'd replace the caps, and I'd probably use twist-lock caps that match the originals as well as possible (Antique Electronics Supply will stock them). If it's a guitar amp and the consequences of failure are having to play an acoustic set.... sheesh, just wait until something goes wrong. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." Ok. So failure is my only worry. There's not a subtle functional difference I'm not privy to. Thanks as usual Scott. -Andy |
#4
Posted to rec.audio.pro
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Electrolytic filter cap replacement
apa wrote: In my understanding the conventional rule of thumb is to replace filter caps after some number of years (and the number varies depending of who you ask). The same thinking says caps 30+ years really should replaced on the grounds that if they're not failing now they will soon. NO. Electrolytics fail due to loss of electrolyte. The primary cause of this is heat, either in operation or storage. There is no simple answer. But 30 years is going some. Plus, if they're very old, they will need gradual 're-forming' to recreate an Al2O3 layer on the aluminium film. Graham |
#5
Posted to rec.audio.pro
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Electrolytic filter cap replacement
apa wrote:
Ok. So failure is my only worry. There's not a subtle functional difference I'm not privy to. If the series resistance of the capacitor has changed or the value has changed, it will change the sound of the amp. But it COULD change it for the better, too. Whatever that means. If the cap checks out correctly out of circuit on a dynamic tester, it's going to sound the same as a new one, though. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#6
Posted to rec.audio.pro
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Electrolytic filter cap replacement
On Nov 18, 3:24 pm, (Scott Dorsey) wrote:
apa wrote: Ok. So failure is my only worry. There's not a subtle functional difference I'm not privy to. If the series resistance of the capacitor has changed or the value has changed, it will change the sound of the amp. But it COULD change it for the better, too. Whatever that means. If the cap checks out correctly out of circuit on a dynamic tester, it's going to sound the same as a new one, though. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." What's a dynamic tester exactly? The Sencore I have tests for value and leakage at rated voltage. It also has a vague method for testing for dielectric absorption which is the only test it does which I'd think could be considered dynamic. Is there more to it than that? |
#7
Posted to rec.audio.pro
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Electrolytic filter cap replacement
apa wrote:
What's a dynamic tester exactly? The Sencore I have tests for value and leakage at rated voltage. It also has a vague method for testing for dielectric absorption which is the only test it does which I'd think could be considered dynamic. Is there more to it than that? There used to be two kinds of testers. One was a bridge that measured the capacitance by comparison with a reference capacitance, and the other was a relaxation oscillator combined with a frequency measuring device. The bridge type devices are generally better at finding electrolytics with problems. The older tube-type Sencores were like this. The newer ones I don't know about. The capacitance function on my cheap B&K VOM is actually an oscillator and counter type. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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