The Talking Machine Forum — For All Antique Phonographs & Recordings

I'm putting this here since there's some of us who work on the later electrically amplified phonos. People who work with acoustics can read this and feel lucky.

I'd always figured that once an old electrolytic capacitor is in the "dead short" mode (the kind that can take out the transformer in a puff of smoke if you turn the unit on) it was best to yank them and replace them with new ones. That's what I've always done and, because the replacement caps are apparently of good quality, there hasn't been an issue.

However, on a computer-tech podcast (Steve Gibson's "Security Now," episode #177), the issue was raised because some folks are restoring vintage computers and running into the same electrolytic capacitor issues as old phono or radio guys: their solution, before the capacitor "arcs" inside and becomes junk, is to lift the cap out of the circuit and run a few tenths of a volt through it for a while. This allows the capacitor to replate itself on the inside and, essentially, become a capacitor again instead of a dead short.

I didn't know that was possible. Is this a new thing, or has this practice been around for a while and I'm just hearing about it for the first time?

To hear the podcast in question, go here:

http://www.grc.com/SecurityNow.htm

I've been into radios for a while (about to start on a theremin) and although this sounds plausible...why not just go to Mouser and buy some new ones? I guess it's like recharging a battery: unplating the plated metal from the other electrode onto the original one, I guess. I still wouldn't rely on that and just buy some new ones!

This process of restoring old electrolytics is called "reforming" the capacitor. It has been known and done for decades. It probably started from people being too cheap to replace the tired capacitors. But it has new relevance today as people wish to restore old electronics and retain as much of the original equipment as possible. The basic method is to bring up the voltage on the electrolytic slowly and monitor the current in the circuit to keep it at a low level of a few milliamps or so. Ideally, a good electrolytic shouldn't be leaky at all and there will be almost no current flow thru it at full rated operating voltage. If the capacitor is capable of being reformed, the current thru it will gradually decrease. You then increase the voltage across it and continue to monitor the current. If the capacitor is still capable, it will be possible to eventually place full rated operating voltage across it with very little or no current flow. At this point, you may want to actually measure the capacitance to see if it still has some or if it has merely burned out and become an open circuit.

Personally, I seldom bother with reforming capacitors. The reforming results in a weakened capacitor that is likely to fairly quickly fail again. This is especially true of the older "wet" electrolytics. As the electrolyte dries out, the capacitance is permanently reduced. Some of the truly obsessed restorers out there remove the guts from the old capacitor cans and restuff the can with a smaller, modern capacitor. This restores the functionality and retains the original appearance. Again, that's usually too much bother in my estimation. If the can is mounted above the chassis, I simply leave it in place, disconnect it, and wire in another new capacitor out of view beneath the chassis. This gets the job done without affecting the outward appearance of the chassis. I don't spend my time removing chassis from perfectly working apparatus just to admire the original wiring underneath, so I don't care what that looks like so long as the item works well and appears original to the typical observer. Just my opinion on that.