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Capacitor Aging: To Replace or not to Replace


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There has been discussion of the need to replace elderly capacitors in well-aged AR speakers. Some of you have been extremely pleased with the improvement in sound quality after replacing old caps. Others questioned whether such a change is cost effective. I am almost through restoring two AR speakers, one a factory AR-3a, the second an AR-3 factory converted to 3a. My materials background says that some will not remain stable and since the caps were both dated and un-matched, I opted for Solen replacements -- simply because it was the brand most often mentioned on this forum. I'll offer my music review after the putty arrives.

While waiting, I measured the electrical properties of the old units. Attached in a small gif file are numerical data that support the musical quality improvements observed by forum members. In summary, the capacitance values of wax-encapsulated, paper-impregnated foil caps showed an increase of 13-15% (1968 mfr), whereas the (1976) non-polar electrolytics showed a 2-3% increase. The serious change was in the dissipation factor. Presumably, all were of order 0.05 when new; however, thirty-six years later their DF's measure as high as 0.8! Note that two caps -- abandoned since the conversion from AR-3 -- were oil-filled, metal-encapsulated units. They still have respectable DFs. Ionic conduction is the problem at low frequencies and this suggests that water, which slowly permeated the wax encapsulation, but not the metal can, is the culprit. The non-polar electrolytics of the seventies also show severe DF degredation. Forming a good vacuum seal between the aluminum can and the cover insulation is not easy.

Based on these measurements, IMO, replacing capacitors of that vintage should not require discussion; it should be automatic. Should they be replaced with the most sophisticated caps available today? I'll not go there :-)...


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