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Vintage threads from the first ten years
Filter Caps Death mechanisms
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 4/17/2003 2:58 PM |
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| R.G. | Filter Caps Death mechanisms Filter caps die from: - being used hard (high ripple currents causing temperature rise) - over voltage - over temperature, including nearby components heating them - *not* being used - time The optimum conditions for a filter cap is to be polarized near its rated voltage at about 25C. Anything other than that lets them age faster. Except for the temperature, this is the approximate condition for non-first-capacitor filter caps. Shelf life is 5-10 years without reforming. Reforming can reclaim some life for NOS devices. Any sign of liquid leakage is a death sentence - rip out any cap that's leaked any noticeable amount of gook. Worse yet, that fluid is conductive, and it can start things like charring ciruit boards. Plan on replacing all electros at about 15-20 years. A very sensitive indicator of capacitor condition before any external symptoms show up is a rise in Equivalent Series Resistance (ESR). There are some special meters that measure this. I have some articles I've been collecting for a DIY ESR meter, but you can do a simple one with some low-value resistors, a 555, and an oscilloscope. As soon as the ESR on a cap starts rising, it's life is ending. Interestingly, you can fake an end-of-life cap in a tube amp by inserting external resistances. This may make those vintage-at-all-costs customers happy because the new caps in their amp don't remove its vintage hum and hash 
R.G. |
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| 4/18/2003 2:20 AM |
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KB |
I remenber kg showed me a way to do a series test with a 10k series resistor and measure the charge across the resistor while applying it's working and close to maximum rating voltage. I think this was a millivolt reading and if the cap was leaking it would start discharging quickly. I forgot the test (damn-it) but I think I could figure it out again. I had a Solid State power board today that the Manufacturor had sealed the bottom of 2 -4700uf 80v E-Radials with hard sealer and there was a white powder on the board around the two and a 10uf 100 volter. I don't know which one it came from or even if it was them because the tops weren't puffed out at all but I'm swapping both even though the amp is only 4 years old. I think the output chips fried and caused some temperature heating but a ESR test would tell the story., KB |
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| 4/18/2003 2:44 PM |
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| R.G. | I think that was measuring the leakage current. That *is* another indicator of cap condition. If the leakage current is up, the cap's toast.
The white powder is the remains of leaking electrolyte that's dried out to a powder. They're dead or dying. Good choice to swap them out. R.G. | |
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