Replacing old capacitors

Posted on Feb 6, 2014

Original components, when still good, are stable and not subjected to infant mortality. Of course everybody has his own opinions derived from past experiences. Nevertheless answers may vary depending upon several reasons: quality of the design with proper use of right components, type and quality of materials, environmental conditions

Replacing old capacitors
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during life and storage of equipment. I see frequent discussions tied to specific models. Since almost every collector of old radio equipment may be interested in this topic, I decided to open a generic thread, hoping that readers would add their experiences. A short can be easily identified by low voltage and/or resistance values all around the faulty section. Often shorted caps cause other faults, as burned resistors or blown fuses. Whenever one of the latter conditions is encountered, shorted capacitor should be suspected unless different cause is found. This failure is common in electrolytic capacitors, when fully dried. If faulty capacitor is in the B+ filter section, a loud hum will arise. In some circumstances, such as in the cathode path of audio power amplifier tubes, a dried capacitor could also give benefic effect on sound quality. Opens can also be occasionally found in some polystyrene foil capacitors, due thermal stress on the leads. These may derive from different causes: partial drying of electrolyte in aluminum capacitors; drying or alteration of impregnating oils or waxes in paper capacitors; moisture adsorption in ceramic or in paper dielectric; small cracks in silver coating of some lacquered mica capacitors, partial chipping of ceramic capacitors. A twenty-percent decrease in the capacitance of filter electrolytic capacitors may be tolerated, but a five-percent variation on the value of ceramic...

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