Thermal Resistors - UK Vintage Radio Repair and Restoration Discussion Forum

Stylo N M · 13th Dec 2013, 3:55 pm

Hi all,

Can anyone tell me please, how do you know when a thermal resistor has reached the end of it's life?.

Thanks, paul.

G6Tanuki · 13th Dec 2013, 4:20 pm

What exactly are you meaning by "thermal resistor"?

I guess it could be a Thermistor, or possibly one of the thermal-overload power-resistors used in TV sets which have a spring-loaded contact usually held closed using low-melting-point solder.

Peter.N. · 13th Dec 2013, 8:02 pm

High power ones usually fall to pieces.

Stylo N M · 13th Dec 2013, 8:48 pm

Hi,

I mean the ones that just look like a piece of carbon rod, with a metal cap at each end with the wire protruding from the end of the connecting caps.

Paul.

matthewhouse · 13th Dec 2013, 10:45 pm

Thermistor? If so they often physically break, I've not seen a faulty one in one piece.

Peter.N. · 14th Dec 2013, 1:11 am

Normally they go on working until they literally fall apart. The early ones didn't have metal caps they had wire wound around the ends and soldered, I have seen them still working quite happily where the solder had all but disappeared and the rod was just sitting there, any movement and they could disintegrate. They were used in most TVs in the '50's and '60s and probably more were in a state of decay than in good condition.

Peter

MotorBikeLes · 14th Dec 2013, 9:01 pm

Thermisters (NTC and PTC), VDRs and varisters all have temperature dependant properties. Some appear to age, some may not.
I remember the VDR rods used in the focus circuits of some CTVs could age. (Some probably got damp, and "shaled off", but others looked physically unchanged. The ones Grundig used on some earlier sets seemed to go low resistance, and then heat up like furnace rods, before melting their top solder and plastic housing. Replacement only option. They all looked the same, but their were various part numbers with description such as "with two black dots" to distinguish them in the workshop.
I supplied a PTC to somebody recently and I think his measured resistance values were quite different to the spec sheet.
Les.

13th Dec 2013, 3:55 pm	#1
Stylo N M Heptode Join Date: Jul 2012 Location: Derby DE1, Derbyshire, UK. Posts: 626	Thermal Resistors Hi all, Can anyone tell me please, how do you know when a thermal resistor has reached the end of it's life?. Thanks, paul.

13th Dec 2013, 4:20 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Thermal Resistors What exactly are you meaning by "thermal resistor"? I guess it could be a Thermistor, or possibly one of the thermal-overload power-resistors used in TV sets which have a spring-loaded contact usually held closed using low-melting-point solder.

13th Dec 2013, 8:02 pm	#3
Peter.N. Dekatron Join Date: Jul 2008 Location: Charmouth, Dorset, UK. Posts: 3,601	Re: Thermal Resistors High power ones usually fall to pieces.

13th Dec 2013, 8:48 pm	#4
Stylo N M Heptode Join Date: Jul 2012 Location: Derby DE1, Derbyshire, UK. Posts: 626	Re: Thermal Resistors Hi, I mean the ones that just look like a piece of carbon rod, with a metal cap at each end with the wire protruding from the end of the connecting caps. Paul.

13th Dec 2013, 10:45 pm	#5
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Re: Thermal Resistors Thermistor? If so they often physically break, I've not seen a faulty one in one piece.

14th Dec 2013, 1:11 am	#6
Peter.N. Dekatron Join Date: Jul 2008 Location: Charmouth, Dorset, UK. Posts: 3,601	Re: Thermal Resistors Normally they go on working until they literally fall apart. The early ones didn't have metal caps they had wire wound around the ends and soldered, I have seen them still working quite happily where the solder had all but disappeared and the rod was just sitting there, any movement and they could disintegrate. They were used in most TVs in the '50's and '60s and probably more were in a state of decay than in good condition. Peter

14th Dec 2013, 9:01 pm	#7
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Thermal Resistors Thermisters (NTC and PTC), VDRs and varisters all have temperature dependant properties. Some appear to age, some may not. I remember the VDR rods used in the focus circuits of some CTVs could age. (Some probably got damp, and "shaled off", but others looked physically unchanged. The ones Grundig used on some earlier sets seemed to go low resistance, and then heat up like furnace rods, before melting their top solder and plastic housing. Replacement only option. They all looked the same, but their were various part numbers with description such as "with two black dots" to distinguish them in the workshop. I supplied a PTC to somebody recently and I think his measured resistance values were quite different to the spec sheet. Les.