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How to tell if an old electrolytic cap is good


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I was reading another thread where this subject came up. I was wondering what would be a good way to test if an old electrolytic capacitor was dried out or not?

Maybe "dried out" is not the correct description, as I have heard of being able to "re-form" electrolytic capacitors
I am specifically interested in out-of-circuit testing because I have a number of Panasonic large value electrolytic caps that have never been used but are over 10 years old (TSU series).

One of my computers is 8 years old and I still use it with no problem, I even have one that is 12 years old and still runs with no problems, maybe these caps last longer than we think or is it just the quality of the capacitors used?

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As electrolytic capacitors getting old their ESR (Equivalent Series Resistor) is rising and that may or not cause problem to your circuit.

You can measure ESR using special ESR meters (Elektor has published one before) or using low resistance meters. An alternative method if you don't like any of the above is described here:


The quality it's for sure a reason for a capacitor to fail but also enviromental conditions or excess heat can dry out a capacitor. I think it's difficult to predict the lifetime of an electrolytic capacitor so some may last much more than others.

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There have been a number of discussions on this site about electrolytic caps going bad from drying out with age. From my experience, it is only in extreme conditions or in very old equipment that one would have this worry. There have been several instances of bad caps entering the market from cheap companies. Poor quality caps are the greatest danger for this problem.
I have tube amps which were built in the 50's and 60's which run without problem. Scopes a little newer than that, and a multitude of equipment purchased at various times between then and now. Never have replaced caps in any of them. If this were a real problem, manufacturers would consider the cap a consumable to protect them from lawsuit. Electrolytic caps are listed nowhere as a consumable.
I think the discussions have provoked much unneeded alarm.


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The number one reason for electrolytic caps to fail is heat, at least this is my experience. I have changed a number of bad caps and all of them have been in a high temperature environment. Broken switch mode power supplys is one place where I often have encountered bad caps and I always replace an 85C with a 105C. Often the manufacturer is to blame, badly constructed circuit, bad choice of components and poor cooling contributes to the breakdowns. There have been some motherboard manufacturers with bad luck who made series of boards with low-grade caps and lived to regret it. I agree the problem is exaggerated.

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My 'scope is about 40 years old and still works every day with its original electrolytic caps, but I replaced its electrically-leaky foil and paper caps years ago with good metalized poly ones.
My old TV is nearly 40 and also still works but its biggest electrolytic caps are dripping.
I worked for a major manufacturer and was told not to use their smallest electrolytic caps because they dried-out and failed after only about 2 years. The rubber seal developed many cracks.
I know guys who made a very good living about 4 or 5 years ago by replacing bad Chinese caps in new equipment.

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