PTC thermistor

[David]

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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[Eeyore]

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?

I guess it might do that. Sounds like the power-on degaussing circuit.

Graham

 

[Mr. J D]

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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It could make a fizzing noise from burning compounds in the component.

 

[Franc Zabkar]

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?

Yes. If it is a 3-pin PTC resistor, it will have two discs which can
develop poor contacts, after which they will rattle. This kind of
problem usually takes out the fuse, though.

- Franc Zabkar

 

[ian field]

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.