Hey, there's a purple spot on my TV (or monitor)
Well, first we'll assume you mean a purple spot on the image you're viewing. :-) (Second, we'll assume you're not watching Barney!).
A color picture tube (CRT) works by mixing three colors of light (Red, Green, and Blue) in different proportions to produce the desired color on the screen.
These colors are produced by phosphor (no, not phosphorus, but phosphor) dots or strips on the inside face of the screen. These phosphors are "excited" by high (relatively, make your own pun) energy electron beams. When the electron shells in the phosphors drop back down in energy level, they emit light at the desired wavelength (color).
There are many stripes/dots on the screen, but there are only three electron beams. Those of you who are thinking "Trinatron" right now really don't need to be reading this, right? Well, in any case, these electron beams are aimed very accurately through a "shadow mask" at the screen. The shadow mask is a curved piece of metal with holes or slots that allows only the proper beam to hit dots of certain colors. Trinatron comment applicable here as well...
Anything that allows an electron beam to strike the wrong dots will result in a color "purity" problem. This most often results in a colored splotch on the image being viewed.
If the unit was previously working well, and only recently developed the spot, there are two likely causes, physical damage to the shadow mask (unlikely unless the unit was dropped) and changes in the local magnetic field.
Splotches due to changes in magnetic fields are often due to some comedian playing around with a permanent magnet. Sometime moving the unit can cause this as well.
Most TV sets have a built-in degaussing circuit that works every time the set is turned on. To try this circuit, turn off the unit and let it cool down for a while. One hour would not be too long. The thermistor that controls the degaussing coil in some TV sets must be dead cold to get the full degaussing effect. Other units aren't so particular. After the set is cool, simply turn it on to degauss.
Most computer monitors have a degauss button. This can be either on the front or the back of the unit. Often it is marked with an icon that looks like a little horseshoe magnet with a circle around it and a line through it. To use this type of degaussing circuit, just push the button and hold it down until the rainbow patterns on the screen subside.
Sometimes these internal circuits just don't have what it takes to demagnetize the tube and its surrounding parts. This is especially likely when someone has been playing around with a magnet.
In these cases, what you need is a hand-held degaussing coil. These may be purchased at radio/tv supply places. A shop that sells to TV service shops is your best bet.
I'll let you in on a little secret though. Most TV technicians don't use these. To make your own, find a big spool of magnet wire, about 22 gauge. Preferably, this is a "found" spool of wire, paying for it defeats the purpose. Then you flip over your stool. The magnet wire is then wound (loosely, so that it can be removed) around the stool legs until the bundle is a bit bigger around than your thumb. Wrap a few tabs of electrical tape around to keep the coil together, then pull it off the legs. Strip the ends of the magnet wire and solder them to a line cord. Insulate the connections with heat-shrink tubing. Wrap the whole coil tightly with electrical tape. Install one of those in-line power switches in the cord at a point that is easy to reach with one hand while you hold the coil on your other hand. You now have a degaussing coil.
To use this coil, turn on the unit to be degaussed and display a picture. This is not really necessary, but if you degauss the unit without watching the picture, you're missing out on quite a show. Hold the coil right in front of the screen, then turn it on. You'll see a rather spectacular "rainbow" effect in the image. Slowly withdraw the coil from the screen while moving it around to cover the entire area of the screen. When you can't see the rainbow effect any more (about 3 to 4 feet away), turn the coil sideways and cut the power. None of this is too critical, just remember that if you cut the power while you can still see the "rainbow" you have about a 50% (depending on what part of the cycle the switch opens at) chance of leaving the unit magnetized.
While you use the coil, notice that it gets warm to the touch. This is normal, but remember that this device is not meant to be left on for long periods of time. 10 seconds or so is the usual maximum. If you leave a coil like this plugged in and leave it, a fire might result. That's a disclaimer people. Consider yourself warned. Be careful! This is not a toy! Keep it away from children. If I was going to make a commercial version of this device, I'd embed a thermal cut-out in the windings.
Another warning note. Keep this device away from any magnetic recording media. This includes floppy disks. It also includes the hard drive in your computer. It takes more to erase any of the above than you might think, but be cautious anyway. It's much better to be safe than sorry!