Just found following in wikipedia on "Capacitor". I pretty much knew about limited lifespan and what shortens it. But failure from disuse was new to me -- like so many other things.
"Electrolytic failure from disuse
Aluminum electrolytic capacitors are
conditioned when manufactured by applying a voltage sufficient to initiate the proper internal chemical state. This state is maintained by regular use of the equipment. If a system using electrolytic capacitors is unused for a long period of time it can
lose its conditioning. Sometimes they fail with a short circuit when next operated."
But this applies only to older caps, right ? Not new ones, I suppose. -- new as in cap bought ten years ago for stock but never used ?
Just putting it a different way, but see if this helps: As you know, capacitors are two conductive surfaces separated by an insulator. The main advantage of an electrolytic capacitor is high capacitance for a given size. It does this by having essentially one metal (foil) plate—what serves as the other "plate" is an electrolyte (from which we get the name of the capacitor), usually liquid, the cathode. But it would be a dead short with nothing else, so an oxide layer is grown on the foil to separate them. This is done by limiting voltage during forming during manufacture. Obviously (maybe), running the voltage in the other direction would undo, the oxide. So, the oxide is a dielectric, the capacitor is inherently polarized.
And the basic problem is that the oxide goes back in solution with the electrolyte over time, the capacitor turns into a dead short, or close enough that the insulator burns up when you power it up, or the chemical reaction causes the capacitor to burst (most often seen with power supply caps). If you suspect this in advance, you can reform the capacitors. That's usually only worthwhile for large caps, where you either want the vintage look for the device, or it's a matter of fit (though modern electrolytics are typically smaller), or just plain hard to find.
So, given enough time, it's likely that it doesn't matter whether the cap is more modern or not. It's just the nature of electrolytics. (There's "capacitor plague" from ~20 years ago, but that usually resulted in the capacitors rupturing from gas build-up within a couple of years I think. Believed to be from a mis-copied stolen formula, the electrolyte caused corrosion.) Big caps seem to last longer, particularly big power supply caps, but I don't know for sure, worth powering up an old amp with a dim bulb tester or variac and check current.
I've recapped a number of devices over the past year or two, stuff I bought in the '80s that were no longer working. Basically every one of them worked after replacing the electrolytics (cheaper than troubleshooting which were bad). Lexicon PCM-70 (had a clearly "blown" cap, 5v supply), NAD integrated amp, Oberheim OB-8 (AC line filter also smoked, also battery dead), PS Audio IVH preamp...
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