Restuffing Electrolytic Capacitor Cans

[jshorva65]

Recently, I discovered the key to restuffing Twist-Lock electrolytic capacitors without shattering the phenolic terminal wafer.

There is usually a small gap at each mounting tab between the right-angle bend and the wafer, so a small screwdriver blade can be slipped in between the tab ring and the wafer to begin loosening the base crimp under each of the four tabs (three tabs on the smaller 1" can diameter). Once loosened under the four tabs, gently work around the circumference toward the next tabs in both directions. With patience and practice, the entire circumference of the crimp can be opened to clear the wafer assembly.

Next, place the uncrimped inverted can in an old electric skillet which is no longer suitable for kitchen use. A temperature setting of about 350 degrees is usually adequate to soften the potting material inside the can, and takes 15 to 20 minutes. An old oven mitt and pair of long-nose pliers will be needed for the next step. Holding the can with the oven mitt, grip one of the mounting tabs with the pliers and gently pull the mounting ring out. The foil strip which makes the common "-" connection will probably tear, but the old insides will be discarded so this doesn't matter. Keep the mounting ring since it will be used later. Grasp one of the "+" terminals with the pliers and GENTLY begin to remove the internal structure from the can. Lifting gently on each terminal one at a time and setting the can in the skillet periodically to reheat may be needed. The entire structure should slide out easily after it has been loosened by gentle pulling. Cut the aluminum strips which connect the capacitor structrure to the terminals and discard the inner structure. Keep the terminal disc. Build the new capacitor by connecting the "-" terminals of new and pre-tested electrolytics together, carefully covering all leads and connections with heatshrink tubing. Do not pack the capacitors together too tightly, but leave gaps of at least 1/16" at the top and bottom of each capacitor. Drill a 1/16" hole beside each of the "+" terminals on the terminal disc. Arrange the "+" leads of the new capacitors in the proper configuration, solder extensions to them as needed and cover with shrink tubing, leaving about 1" of exposed wire on each to protrude through the disc.

Loosely wrap the capacitors with a soft, non-conductive, flame-retardant filler material such as soft cloth to fill out the diameter of the assembly to about 1/16" to 3/32" less than the inside diameter of the can. This will fill voids while providing some air space for steam to reach the vent port in the terminal disc in the event of future failure and overheating.

Fill the can about 1/4 full with hot-melt glue and lower the new capacitors into the can until the leads protrude about 1" above the rim, fit the terminal disc in place so each "+" lead passes through the hole beside the corresponding terminal and the "-" lead exits through the notch in the edge of the disc. Enlarge the notch with a 1/16" or 3/32" needle file if needed. Add hot glue to almost fill the gap under the disc, note the location of the vent port and take care to use a very small thickness of hot glue in this area, then press the disc firmly in place. Solder the "+" leads to their terminals, solder the "-" lead to the mounting ring, trim excess lead length, place the mounting ring to align the tab with the notch. Test with an ohmmeter and capacitance meter to reveal any shorted wiring or poorly-soldered connections, correcting any problems before crimping. Re-crimp the rim with pliers to securely lock the mounting ring and terminal disc in place. Re-test all sections of the finished unit for ESR, Capacitance, and Leakage. Reinstall in chassis.

Regarding how much hot glue to use, a small amount in the top of the can to hold the new capacitor assembly in place, the previously-mentioned soft, non-conductive, and flame-retardant filler material around the new capacitors' sides (inserted loosely to just fill voids and thus provide some expansion room around the new capacitors), along with leaving the vent area of the terminal disc clear with a very small thickness of glue would be adequate to permit venting. Avoiding the packing of the new capacitors too tighly inside the can would provide a means for steam to escape in the event of overheating, which was the purpose of the provision of a vent port in the terminal disc.

In the original construction, the venting was provided by making the thickness of potting material small at the location of the vent port. Since the re-crimping at the base is done using hand tools, it will have less strength than the machine-made factory crimp and will result in any excessive steam pressure being more likely to loosen the crimp and thus escape around the loosened edges than to cause violent rupturing of the can.

[bolly]

nice!