I've been having some conversations with the masses (I do not suggest you do this) about old components in devices, specifically 70+ year old silver-mica capacitors. Specifically, the ones in the image, which were called "domino" or "postage stamp" style devices because of their size, dots, and multi-color areas denoting value. In particular here, I have a device on the bench that I bought a kit of parts for because it has some oddball values in the circuit, and I didn't feel like buying 10 parts just because I needed one. It was a bit more expensive if I account for each part, but far less than me acquiring the parts on my own. The only thing this kit did not have is a replacment for a mica postage stamp in the measurement circuit. Almost every one of the old-school capacitor eyetube checkers used a 200pF capacitor in their measurement circuits, and the device I have on the bench right now, an EICO 950A, is no exception. It was usually mica, although sometimes an adjustable trimmer was used. Construction was not important here, value and voltage rating were. This part, however, is 70 or more years old. Sure, mica parts were rock-stable (being rock) but it's still 70. The conversations from some of the people are "You are replacing a good part. This never goes bad. I've never had one go bad!" Well, good for you. Here's a gold star. It has 6 points. The problem with this argument is twofold. First, You don't really know what the construction of these old postage stamp capacitors really is. Some manufacturers would package plain paper parts in this case instead of mica. Second, is that you're running up against age. These were used because the both offered the stability of mica, and a nice small high-voltage sealed package. After 70 years of heating and cooling and simple age, the resin has started to crack and allow water ingress, or the resin itself has absorbed water and collected dirt. Wet + high potential + silver = silver ions dissolving in the water and migrating towards the opposite potential in a dendritic manner. This both changes the capacitor's value and eventually will cause leakage and shorts. The latter is going to take longer, but it was known in the 1990s that this was starting to be a problem. Those all contribute to why this type of part should be replaced when you're doing a major rebuild - but in reality, you're already replacing all the passives in a device. Don't skimp on a couple of $4 parts just because "they never go bad!"