A friend gave me a TV that would not switch on. The standby light comes on and there is a kind of rattling noise coming from the circuit board near the power switch. I noticed a PTC thermistor was very hot to the touch and was showing signs of burning underneath on the circuit board. Is it normal for a failed thermistor to make a noise?
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There are 2 basic types of 3-pin degauss posistor and some care is required to obtain the correct part! The less common type actually contains 1x PTC &
1x NTC, the NTC element is wired in series with the mains in just as you would expect an inrush limiting NTC to be - the danger arises when this type is mistakenly used to replace the more common 2x PTC, as the NTC will then be directly across the mains in and will go off with a loud bang!!!
In both cases the thermistor disc not directly associated with the degauss coils is one way or another fed directly with mains current to heat the degauss posistor above its knee curve so no residual current flows in the coil and causes picture wobble. The type with an NTC disc is heated by passing the current drawn by the set and the PTC only type one of the 2 PTC elements is directly across the mains so the power this consumes keeps the degauss PTC heated.
The less common PTC & NTC combined type can go O/C on the NTC element and cut off the supply to the set - or if its fizzing it may be passing too little or irregular current for the set to start - this is the only cause I can think of for STDBY light on but not starting other than another fault elsewhere in the set.
With either type its a simple matter to unsolder the degauss unit and shake it next to your ear - if it rattles fit a replacement.