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These came to us as part of a lot of surplus stock from a large Milspec type OEM and have been kept in storage for some years we have therefore classified them as ‘new other’. They are however in good serviceable condition, though they may require reforming…

Reforming Electrolytic Capacitors

Manufacturers claim that most old electrolytics can be saved if the correct procedure is followed, regardless of how long they have been unused. Such capacitors must be “reformed”. This process consists of applying rated voltage through a resistance (about 30,000 ohms, five watt) for five minutes plus one minute for each month of storage (see diagram below). As the capacitor reforms, the voltage across the resistor will drop (measured at the Xs). If that voltage will not drop below 10% of applied voltage after one hour, the capacitor is probably beyond help.

The circuit shown in the sketch above works reasonably well. Apply the rated voltage through a 5W resistor. Anything from 20K-50K will do, as this is far from a precision process. The meter is used to measure the voltage drop across the resistor; when no current is flowing, there will be no drop. Obviously, when there is a large voltage drop (more than 20% of the applied voltage), there must be a significant current flow through the capacitor. The nature of a proper capacitor is to impede DC current flow, so when there is such flow, something must be wrong.

Note: Apply the appropriate D.C. voltage to the capacitor with a D.C. power supply. An old Kepco, Lamba etc. tube regulated lab power supply rig works great. Be sure to observe the proper polarity!