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About Posistor (Positive Temperature Coeeficient-PTC)
If you have been in the CRT Monitor and TV repair, i’m sure that you must have come across the square black plastic with two or three legs at the input AC area just after the EMI/RFI Filter circuit. That component is posistor (PTC) and the function is to control the degauss. When you switch On the CRT Monitor or Television, 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 cut off.
Usually when this posistor have problem, the degauss function would not work and many times it would blow up the main fuse (but the bridge rectifier was okay). It is quite hard to test the posistor and sometimes when it has problem you could hear some loosen parts in it when you shake the posistor. Since this part is easily available from electronic stores and from junk Monitor or TV, i would usually make a direct replacement and retest the set again.
Tips: If you can’t demagnetize the CRT tube that have color patches even with the help of a degausser (demagnetizer) then chances is high the tube may have problem. If the color turned back to normal after using the degausser, then look for dry joints in the degaussing circuit and replace the posistor and test it out again.