[Matthew D’Asaro] was recently entrusted with an entire classroom fleet of fourteen broken Tektronix TLA5202 logic analyzers — a pile of equipment that once was worth hundreds of thousands of dollars. His task: Fixing them. He fixed them all, and on the way documented a number of common failure points in these old but still great devices.
The pioneering years in the history of capacitors was a time when capacitors were used primarily for gaining an early understanding of electricity, predating the discovery even of the electron. It was also a time for doing parlor demonstrations, such as having a line of people holding hands and discharging a capacitor through them. The modern era of capacitors begins in the late 1800s with the dawning of the age of the practical application of electricity, requiring reliable capacitors with specific properties.
One such practical use was in Marconi’s wireless spark-gap transmitters starting just before 1900 and into the first and second decade. The transmitters built up a high voltage for discharging across a spark gap and so used porcelain capacitors to withstand that voltage. High frequency was also required. These were basically Leyden jars and to get the required capacitances took a lot of space.
In 1909, William Dubilier invented smaller mica capacitors which were then used on the receiving side for the resonant circuits in wireless hardware.
Early mica capacitors were basically layers of mica and copper foils clamped together as what were called “clamped mica capacitors”. These capacitors weren’t very reliable though. Being just mica sheets pressed against metal foils, there were air gaps between the mica and foils. Those gap allowed for oxidation and corrosion, and meant that the distance between plates was subject to change, altering the capacitance.
In the 1920s silver mica capacitors were developed, ones where the mica is coated on both sides with the metal, eliminating the air gaps. With a thin metal coating instead of thicker foils, the capacitors could also be made smaller. These were very reliable. Of course we didn’t stop there. The modern era of capacitors has been marked by one breakthrough after another for a fascinating story. Let’s take a look.
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.
This demonstration fixes the power supply of a DVD player, but the skills transcend this one application. [Alan] walks us through the process of repairing a power supply (translated) on a simple consumer electronics unit.
Obviously this starts by cracking open the dead device and verifying that the culprit is the power supply. [Alan] then removes that board from the chassis and gets down to work with a visual inspection. He’s got several images which illustrate things to look for; blistered electrolytic capacitors, cracked solder joins, scorch marks, etc. In his case there’s obviously a burnt out fuse, but that merely protects the hardware from further damage, it’s not the cause. Next he examines the diodes of the bridge rectifier. These need to be removed from the system to do so, which he accomplishes by clipping one end of each as seen above. He found that two diodes on one side of the bridge had broken down. After replacing them he tries a new fuse which immediately burns out. But a quick swap of the capacitors and he gets the thing back up and running.
We perk up every time we see this type of repair hack. We figure if we can build our own hobby electronics we should be able to fix the cheap devices like this one.