A posistor is a combination of a PTC (positive temperature coefficient)
resistor and another resistor-element to heat it up and keep it hot.
Sometimes, these will go by the name posister or thermistor. The heater
is a disk shaped resistor across the power line and the themister
is a disk shaped device in series with the degauss coil. They are in
clamped together to be in close contact thermally. You can pry off the
lid and see for yourself.
|NotTaR of Television Sets : What is a posistor?
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The most common failure mode is for the part to short across the line.
Its function is to control degauss, so the only thing you lose when you
remove one of these is the degauss function on power-on. When you turn
the TV or monitor on, the PTC resistor is cold and low resistance. When
heated, it becomes very high resistance and turns off the degauss coil
but gradually - the current ramps down to zero rather than being abruptly
Computer Component Source stocks a wide variety, I believe but it may be
cheaper to go direct to the manufacturer if they will sell you one.