Eric Smith wrote:
"Michael J. Mahon" <mjmahon@aol.com> writes:The problems of "starting up" long-idle electronics is an interesting problem, but not the one that I was addressing. In fact, I have simply turned on a lot of monitors that had not been operated for at least ten years, and have never (yet!) experienced a failure when doing so.The five year timeframe I mentioned was the *minimum* that I've seen this happen, for a capacitor that isn't physically damaged. Most of them take longer to deform. But there are no guarantees.The problem I *was* referring to is the loss of capacitance brought on by prolonged operation at a relatively high temperature. This causes electrolytics--which are all non-hermetic to allow for ventingActually they are usualy hermetic. The top of the can generally has two score lines so that in an overpressure condition the can will somewhat gracefully open at the scoring before the overpressure gets too serious, rather than exploding violently. Once the seal is broken, the electrolyte starts to dry out, reducing the capacitance quickly. Any electrolytic that has leaked, opened at the scoring, or burst needs to be replaced.
Most of the 1980's vintage electrolytics I've seen have been sealed at the bottom of the can with a plastic-fiber-rubber seal that seems to be the usual locus of leaks. Perhaps these are only the defective ones. ;-) The slow-drying-out theory could be easily proven if we had the initial weights of these capacitors, but I don't have that data. Of course, I agree with the purpose of the top-of-can scoring, which I have seen exercised most often when an electrolytic is installed with polarity reversed (messy, isn't it--and with a characteristic odor ;-). The issue is leakage over the long term, much accelerated by high temperatures (over 60 C), versus explosive leakage caused by *very* high internal dissipated power, usually the result of incorrect polarity. We recently had a spate of shorter-term capacitor leakage, of *liquid* not vapor, from capacitors with incorrect electrolyte formulations. All of these leaked from the bottom, too. I must conclude that these capacitors are not hermetic above a relatively low positive internal pressure. -michael Music synthesis for 8-bit Apple II's! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it is seriously underused."