Capacitors

A capacitor is, in its most basic essence, a short term energy storage device. The ways capacitors are designed and be can be used vary wildly though for such a simple premise. When it comes to repairs understanding how capacitors function in a circuit can help with diagnosing issues and finding replacements.

Types
Capacitors come in many different shapes, sizes, chemical make-ups, brands, and utilities and knowing the differences between them is one of the first things you should focus on when learning about them. The ones you need to worry about the most are Electrolytic/Aluminum/Wet which will dry or leak over time. These are usually attached to the circuit board as either a Through Hole or SMD package shaped like cylinders sticking out of the PCB. Tantalum capacitors can have some issues and especially older Through Hole examples that are dipped in a blue or yellow coating. Newer ones are made as a Rectangular SMD part and are more reliable. Polymer capacitors are a hybrid part that can come in pretty much any shape. They can fail and in some cases have shorter life spans but are more reliable and predictable than pure electrolytic. Ceramic capacitors are another very common type that used to be shaped like discs on legs as Through Hole but quickly became a Rectangular SMD part that is a tan little block soldered down on each end. These rarely fail and are very unlikely to be a cause of issues.

Specifications
When looking at replacing a capacitor you need to determine the unique properties of it and try to get a replacement that is either as close as possible or better in some ways. The actual capacitance of the capacitor is the most important thing that you should look at first. You should try to get an exact match for that which should be possible most of the time, if not, err for a higher value than original. You usually won't have much of an issue with a slightly different capacitance because it has a tolerance rating that is frequently as high as +/- 20%. This means a capacitor rated at 100uF could be as low as 80uF or as high as 120uF. The tolerance isn't typically printed on the part, so going lower is usually the safer bet. There will be higher cost for this which is more related to the tolerance being a binning process rather than quality. The application of the part may also require a higher precision which you can read more about in the full section on tolerance. The voltage is a maximum rating that it can be used with, it is only acceptable to use a higher voltage part if an exact match cannot be found. But it is not preferable to get higher values as it effects the dielectric properties of the capacitor which can cause other changes over time. The ESR value can be thought of as a heat generation rating which matters mostly for AC voltage applications or rapidly changing DC voltages. The lower the ESR the less heat will be generated when the voltage level of the capacitor changes, so generally less is better. The only way to know what the ESR is for a given capacitor is to identify it's series and look up the data sheet or use a dedicated tool to measure it.

The shape and method of being attached to the PCB are part of the footprint of the capacitor. These have dimensions that will need to be measured, typically in millimeters, to find similarly sized parts.

Measuring
Measuring the properties of a capacitor are not very straight forward and dedicated tools exist for this task. The principals behind the measurements are not complicated or proprietary and cheaper tools can work well for hobbyists. Some multimeters may have the capability to measure capacitance but the accuracy of these, even in higher end models, is not always good.

Measurements are made by charging and discharging a capacitor at different rates and timing how long it takes to reach a target voltage. The dedicated tools to do this automate the process at different frequencies which capacitor manufactures will rate the ESR of their capacitors at.

Reforming
Reforming capacitors is a process that is only needed on parts that have not been used in a long time. It is intended to rebuild the dielectric layer in electrolytic capacitors by slowly rising the voltage up to the rated maximum by limiting the current. This process cannot "repair" capacitors, but it can prevent them from being damaged by shorting when powered if they have sat and degraded over time.

Brands
There are a large number of capacitor manufacturers and not all of them are as well regarded. Rifa is one brand that has maligned themselves with the vintage computer community as their paper line filter capacitors have a tendency to explode.