Well electrolytic caps have a gel in them that dries out over time and it can leak out. It is probably corrosive too. Like the original post mentioned, you have to install an electrolytic cap one way. If the cap has a long line on one side like the one in the pic then that is the negative side. Sometimes they will have a + on them. The +, of course denotes the positive side. So just look at which way the cap is installed that you take out. Electrolytics will also have a max voltage stamped on them like 25V or 16V. Make sure you buy caps that have a max voltage at least as high as the ones you are pulling out. If you take out one that says 25V then you can put one back in that says 25V or higher. Just as an FYI, if you install and electrolytic backwards the metal cap on the top of it will blow off like a little light weight bullet when it is energized. Just a little fun with electronics 101.
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