You turn a volume knob and you're greeted with an awful crunching sound. An older TV has a picture that "jumps" or otherwise changes unpredictably when its control knobs are adjusted, touched, or set to certain positions. It boggles the mind to think about how widespread and costly the problem of "scratchy" potentiometers (i.e., variable resistors) is. It has been widely claimed that dust is the main culprit, but this is not always, or in my opinion, even usually, the true cause. The problem is mainly caused by oxidation or other surface degradation of the resistive carbon film and/or wiper contacts. The common, cheap, carbon film potentiometers are the most likely to become scratchy. More expensive designs such as cermet (ceramic metal), which are commonly used in the military and aerospace industries, are far less prone to this problem. I strongly recommend that cermet units be used in any homemade circuits (if it's worth the cost of your labor, it's worth being built to last).
Note: In what follows, it is assumed that the person doing the repair has a basic understanding of electronics and safe electrical practices. The example shown is typical, but may not be representative of every appliance. Always ensure that the device is unplugged before opening the case. The procedures discussed here will almost certainly void the product warranty, may not be effective, and can result in property damage, and if done improperly, even injury or death. This information is offered as-is without any warranty. User assumes all risk.
That said, what is a person to do about existing equipment, especially those in which the proper replacement potentiometer is not easy to obtain? You should always try conventional cleaning and chemical treatments first. If the potentiometer is not sealed, blow out any dust with compressed air, then use a chemical cleaning product designed for potentiometers. DeOxIT from CAIG Laboratories is widely regarded as the best potentiometer deoxidizer and cleaner. (TV mechanical tuner and switch cleaners may also work.) After cleaning, lubricate the potentiometer with some type of oil to protect the resistive film and wiper contacts from the air to prevent future oxidation. CAIG also carries a product called DeoxIT FaderLube which is designed for this purpose. I've used plain WD-40 as a lubricant with good results (others argue against the use of WD-40 for this purpose), but I would hesitate to use grease (e.g., petroleum jelly) because such high viscosity lubes could cause the wipers to loose contact with the resistive surface (it might be OK though). Some people object to the use of any such lubricants because the liquid residue can attract dust. This may be true, but a dust buildup problem is far easier to correct (e.g., it is easy enough to wash away with another cleaning spray) than oxidation problems and, in my opinion, the latter is far more common than the former.
However, I didn't write this article just to suggest using a cleaning spray, but rather to explain the procedure of last resort. When all else has failed and you are ready to give up, you can try a manual disassembly and cleaning. Be forewarned that this may not work and could totally ruin the potentiometer. Figure 1 shows a typical potentiometer on an older TV. Note the little tangs that hold the unit together. These will have to be pried and bent away from the cover/bracket to dismantle it. The metal the tangs are made of typically fatigues quickly, so bend them the minimum you need to get it apart and understand that they will typically break off after just a couple bend cycles. So, don't plan on repeating this disassembly. You can usually use the unit if as many as half of the four tangs break, as long as the two that remain are on opposite sides. If you loose too many tangs, you'll have to resort to techniques such as epoxy glue to hold it together.
Figure 1: A potentiometer before opening.
Figure 2 shows the disassembled potentiometer. Note the heavy oxidation on the rotary wiper on the right. Now, take a "before" measurement of the total resistance of the unit (between the left and right terminals) which should match the value stated on the cover. The circuit it is connected to may affect the readings, so you may want to disconnect (unsolder) either the left or right terminal before taking the measurement.
Figure 2: Potentiometer internals before cleaning.
One trick to repair really bad potentiometers with noticeable wear tracks on the resistive film, is to bend the rotary wiper contacts radially inward or outward (i.e., apart for two-pronged contacts) so as to run on a fresh area of the resistive film. Be sure to keep the contact points parallel to the resistive surface. You can also bend the stationary (Y-shaped one shown on the left) and rotary (shown on the right) contacts upward/outward to increase their contact pressure, but this can result in increased wear. (Still, I usually do this a little.) For cleaning the contacts, I use a wire brush on a Dremel tool. I know this is a barbaric method with which it is easy to snag and ruin a contact, especially if one is not paying attention to the direction of brush rotation, but if it's bad enough for me to have to manually clean, it's bad enough to go mid-evil on it. Take care not to touch the resistive film with the wire brush as the slightest contact will ruin it. The result is shown in figure 3.
Figure 3: Potentiometer internals after cleaning.
Note the how clean and shiny the rotary wiper is. I even removed some of the plating on it, but I really don't care. However, I would hesitate to do this if it had gold plating which is oxidation proof. Now here's the part that makes strong men tremble. I gently sand the resistive film. No joke. If you do this a little too aggressively, you'll ruin it. You have to use very fine sandpaper in the 1000-2000 grit range. You can obtain this at automotive and furniture refinishing supply stores. Don't try to use anything coarser than 600 grit. Make 3 to 5 light, even passes ensuring that the entire film track surface is evenly "freshened". Monitor the total resistance while you do this. Expect about a 25% increase in resistance as some of the film surface is removed. Most circuits are not bothered by this change. Some can tolerate a lot more than a 25% increase. If you over do it and sand through the film, continuity will be lost and the potentiometer will be ruined.
Before reassembling, blow out any stray grit with compressed air. Gently wipe down the resistive film with a lint-free cloth wetted with some control lubricate (or WD-40) and then blow it out again. Flush out the two parts with spray cleaner (or WD-40) and then apply some control lubricant (or WD-40). Ensure that the rotary wiper contact is oriented to rest in the middle of the resistive film (and thus is at a valid position) and then reassemble the two halves, securing them by crimping the tangs with a needle-nosed pliers. Ensure the potentiometer rotates smoothly and spray in some additional lubrication if you wish. Check how resistance changes with rotation (meter across the middle and either the left or right terminals). The resistance reading should not be jumpy or loose continuity as the shaft is rotated, jiggled and/or wiggled. You may want to do this test while holding the halves together before you crimp the tangs to avoid having to rebend the tangs to regain access for a "touch up".
You can now reassemble the appliance and test it out. Good luck.