Electrolytic capacitors

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Failure modes of old aluminum electrolytic capacitors include:

  • Short Circuit
  • Open Circuit
  • Capacitance Drop
  • Leakage Current Increase
  • Open Vent
  • Electrolyte Leakage

Old capacitors that have not been powered for a long time will have their dielectric (the aluminum oxide layer) degrade. When powered up abruptly at the rated voltage, the oxide layer may be too weak and fail. To avoid this, such capacitors should be reformed by gradually applying voltage through a suitable limiting resistor, then letting them sit at the rated voltage for several hours.

Some old capacitors have experienced a loss or "drying out" of electrolyte; the observed effect is typically a partial to almost full loss of capacitance. In case of partial loss, ESR may be highly increased. Such capacitors need to be replaced. During troubleshooting, small modern capacitors can be paralleled under the chassis. For a cosmetically pleasing restoration, the replacements can be hidden in the emptied original can, see below.


Rebuilding metal can electrolytics using modern replacements