Vintage high-spec electrolytic capacitors can be used as replacements in less critical applications, such as the linear power supply in an Echorec, provided they’ve been stored properly. High voltage, small value electrolytic caps have a longer shelf-life than low voltage, high value electrolytic caps because their dielectric is thicker. But, although NOS (New Old Stock) multi-section capacitors were common in the 50s and 60s, today they’re rare as hen’s teeth, and they don’t come cheap, if you can even get hold of them. If you can’t, then the only option is to roll your own, a.k.a. “restuffing”.
Restuffing involves removing the internals of an old capacitor and fitting modern electrolytic capacitors into its outer casing. This ensures the inside of the Echorec remains nice and tidy and that the machine retains its authenticity. I chose two stubby little 47μF 350V capacitors for this rebuild; these guys are small enough to fit inside the can when stacked on top of each other.
There’s no easy way to get into the can. The plastic lid is firmly crimped in position along its entire circumference—an impressively neat bit of engineering. Use a small file to file away the top lip if the can. Another method of getting in there is to saw through the end of the can with a fine jewellers saw—be careful not to bend the can and distort its shape though.
Once you’ve filed the aluminium lip away you should be able to remove the plastic lid by gently wiggling and pulling it. The rubber O-ring seal can then be pried out to reveal guts of the capacitor. This gungy, acidic mess resembling a lump of wet cotton wool is the working part of the capacitor—actually the aluminium foil electrodes and dielectric. Pull out the guts. It’s advised that protective gloves are worn when undertaking this work.
Clean the inside of the can with isopropyl alcohol. It’s now ready to take the new capacitors. Connect the negative pin of each capacitor together. These need to be brought out through the lid and connected to the can to make an earth connection. And that’s all there is to it. The rebuilt multi-section capacitor is now ready for installation in your Echorec.
Properly constructed electrolytic capacitors seldom show any signs of corrosion or deterioration whilst kept in storage or when operated within their specified limits. So, the take home message here is only replace an old electrolytic power supply capacitor unless it absolutely needs to be done. Doing so will save you a great deal of time, expense and frustration. That’s all folks!
1. If you’d like to know more about the history of Sprague then I can recommend reading Sprague Electric: An Electronics Giant’s Rise, Fall, and Life after Death by John L. Sprague
2. Since writing this article suitable modern dual-section 350V 47uF + 47uF capacitors have become available from F & T Capacitors in Germany.
3. Suggested further reading: The Electrolytic Capacitor – Alexander M. Georgiev
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