This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
Almost all volume controls are variable resistors. This goes
for rotary controls and slide controls. Variable resistors
consist of a resistive material like carbon in a strip and a
conductive metal spring wiper which moves across the strip as
the control is adjusted. The position of the wiper determines
the amount of signal coming out of the volume control.
Volume controls are quiet from the factory, but will get noisier
as they get older. This is in part due to wear and in part due
to dirt or fragments of resistive material on the resistive
strip. Volume control noise comes as a scratch when the control
is turned. This scratch is rarely serious, and most often just
an annoyance. However, as the problem gets worse, the sound of
your system will degrade. Also, as the problem gets worse, the
scratching noise will get louder. The scratching noise has a
large high-frequency component, so in the extreme, this noise
could potentially damage tweeters, although I have never seen
a documented case of tweeter damage due to control noise.
Some controls are sealed at the factory, so there is no
practical way to get inside and clean out the dirt. Others have
access through slots or holes in the case. These open controls
are more subject to dirt, but also are cleanable. You can clean
an open volume control with a VERY QUICK squirt of lubricating
contact cleaner, such as Radio Shack 64-2315. Even better is a
non-lubricating cleaner, such as Radio Shack 64-2322. With any
cleaner, less is better. Too much will wash the lubricant out
of the bearings and gunk up the resistive element.
You can also clean some controls by twisting them back and forth
vigorously ten times. This technique pushes the dirt out of the
way, but is often just a short term fix. This technique is also
likely to cause more wear if it is done too often. Try to do it
with the power applied, but the speaker disconnected, so that
there is some signal on the control.
Sealed and worn controls should be replaced rather than cleaned.
Critical listeners claim that some controls, such as those made
by "Alps" and by "Penny and Giles" sound better than common
controls. Regardless of the brand, however, it is essential
that whatever control you buy have the same charcteristics as
the one you are replacing. For most volume controls, this
means that they must have AUDIO TAPER, meaning that they are
designed as an audio volume control, and will change the level
by a constant number of dB for each degree of rotation.
Badly designed circuits will wear out volume controls very
quickly. Specifically, no volume control is able to work for
a long time if there is significant DC current (or bias current)
in the wiper. If the output of the control goes to the input of
an amplifier, the amplifier should be AC coupled through a
capacitor. If there is a capacitor there, it might be leaky,
causing undesirable DC current through the volume control.
If you have a circuit with no blocking capacitor or a bad
blocking capacitor, you can add/replace the capacitor when
you replace the control. However, get some expert advise
before modifying. If you add a capacitor to a device which
doesn't have one, you will have to make other modifications
to insure that the amplifier has a source for its bias current.