If you’ve ever worked on old gear, you probably know that electrolytic capacitors are prone to failure. [Dexter] undertook a repair of some four-decade-old capacitors in a power supply. He didn’t replace them. He fixed the actual capacitors.
The reason these units are prone to fail is the flip side of what people like about electrolytics: high capacitance in a small package. In a classic parallel plate capacitor, the capacity goes up as the distance between the plates shrinks. In an electrolytic, one plate is a rolled up spiral. The other plate is conductive fluid. The insulator between (the dielectric) is a very thin layer of oxide that forms on the spiral. Over time, the oxide degrades, but this degradation repairs itself when using the capacitor. If the capacitor isn’t powered up for an extended period, the oxide will degrade beyond the point of self-repair.
The manufacturer forms the oxide layer by careful application of a forming voltage into the freshly-made capacitor. This not only creates the dielectric layer but also sets the expected current direction. It is possible, at least sometimes, to use the same technique to regrow the oxide and bring a capacitor back to life. That’s what [Dexter] did, using a current-limiting power supply to prevent damaging the capacitor during the regrowth.
We have covered homebrew capacitor construction a few times before. [Dexter] isn’t sure he trusts the forty-something capacitors over the long haul, but they did seem to repair adequately. The video below shows a different reformation project, this one fixing a video camera from the 1960s.