Rebuilding High Voltage (EHT) Capacitors
Often the high voltage (EHT) capacitors in prewar sets are bad. They can be rebuilt using modern parts inside the old cans. Here is the process I use.
Can Style
First, the capacitor is opened up. In this case the can is mounted to the chassis with a bracket around its base. The can is cut so that the cut will be hidden by the bracket when installed. For screw mounted capacitors the procedure described in the section on rebuilding electrolytics can be used to open the can.
Next, the guts are removed. In this case, two holes were drilled in the material inside the can, then the jaws of needlenose pliers were used to remove the material. A heat gun melted the wax at the end.
Then, modern capacitors are grouped together to provide the values needed. This capacitor had a .05/3000 volt and a .15/3000 volt section. For the .05 section, 5 - .01/4000 volt capacitors were used. For the .15 section, 3 - .05/4000 volt capacitors were used.
The assembly is connected to the existing leads. Instrument wire, with a rating of 6KV, is used. The center wire is ground (earth).
The assembly is wrapped in high voltage insulating cambric cloth to assure it doesn't arc to the can.
Finally, a glue gun is used to apply hot glue to the fabric and the two pieces of the can. After cleaning the can is ready to be re-installed.
Cylindrical Soldered Can Style
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First, remove the solder from both ends
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Use a 1/4 inch drill to drill through the top so that the ring can be removed
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Drill further to remove the fiber insulator
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Elongate the hole in the can to remove the metal piece and the old capacitor
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Cut off the small portion of the metal piece that was removed |
Solder the new capacitor to the ring and the small portion of the metal piece
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Slide the assembly into the can
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Use superglue to attach the wafer to the can, then clamp it until the glue dries. Then cut off the excess lead length and solder the ends.
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