Posted Sun Apr 15, 2012 9:54 AM
Let me see if I can remember this right...
A capacitor is two conductors separated by a dielectric insulator. Its ability to hold charges depends on the dielectric properties, the area of the plates and the distance between them.
Old foil/paper caps are just that - foil and paper. There is very little to 'wear out' in them. There is no polarity and I can believe that they may last forever as long as the internals stay sealed.
Electrolytic caps have the dielectric 'grown' on the plates and have a polarity that you must observe, lest you 'un-grow' the dielectric. When used as power supply filters, high AC currents flow through the cap, generating heat and stress. This deteriorates the dielectric and reduces its effectiveness as a filter. If the cap fails, it may short, blowing a fuse, or be unable to filter out all the AC component, causing hum bars in the video and audio.
Neither of these will normally affect the rest of your computer, other than the fuses and perhaps the rectifiers. The dangerous failure is one where the voltage regulator shorts. This puts high voltages (>8v) on all your ICS without blowing any fuses. Whether the regulator is in an internal power supply, such as an 800/1200XL/1050, or an external supply, such as a 130XE/800XL, makes no difference. If the regulator shorts, you're going to smoke test your computer.
Bottom line: Electrolytics have a high failure rate but cause little damage,
As a side note, newer, large electrolytics usually come in higher values for the same package size. This means that you can replace a 6800/16v cap with a 10,000/25v unit. Don't get crazy with the ufd. Installing a 25,000ufd cap where you had 4,700ufd can cause current in-rush problems. Use a higher voltage unit with less than a 50% increase in ufd.