De-Gauss coil thermistors in TVs

Isn't the second one across the back side of the first, rather than directly across the mains? Sort of a two thermistor series / shunt arrangement. I seemed to recall from my long-ago theory days, that they were the inverse of each other, and that the one that's across is initially high resistance, and gets lower as it heats up, to shunt the degausing coils, mitigating their effect, and keeping the series thermistor hot - and hence in the high resistance condition - by continuing to draw just enough current through it to do that. Could be wrong - it's been a long time ago - but it 'feels' right, and seems to work, in my head at least ... !!

Mind you, I'm going back to when they were two (definitely) different individual components. The series one was a granular grey, and the shunt was a little smaller diameter and a little thicker, with a mainly blue (painted) body, with a yellow stripe at the top. I've probably still got some somewhere. But perhaps they've come up with some other way of doing it since they've encapsulated the two elements together.

Arfa

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This is the basic circuit from the schematic and values from an actual TV. The 16 ohm one is PTC but the 470 one across the mains is NTC heating each in turn with a soldering iron. The schematic states PTC for both so more mystery On closer inspection the two discs are in good thermal contact to the central pin extended to a dividing metal disc but good thermal contact, the spring plate connections are on the outer contacts.

ASCII , view in Courier or perhaps Properties/ Message Source

|------------- \.-. | |\| C| 16 | |\ C| degauss coil '-' C| | | L ------------| | \.-. | |\| | 240V 470| |\ | '-' | | | N ------------------------- (created by AACircuit v1.28.6 beta 04/19/05

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Geez, there's a program to draw mediocre ASCII schematics? :)

I believe your discarded explanation is correct. The second one is there to keep the series thermistor hot after the initial surge through the degauss coil.

--- sam | Sci.Electronics.Repair FAQ:

Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.

Based on that then, you'd have to say that it was just a physical contact thing. I'll have to see if I've still got any of my old TV schematics about, to see what the configuration was before they put both together in a single package.

Arfa

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I found a similar one and tested that and NTC again for the "heater" section. But that can't be the case, presumably it must initially be NTC and then go positive above 150 degrees or something. I will try heating longer and see what happens

I assumed the one feeding the coil would reach some stable elevated resistance and stay in that stable state at some elevated temp until switched off.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Yes the heater one dropped 50 ohms or so and then rattled rapidly up to more than 150K with continued heating from a soldering iron.

So I've learnt something else today.

Now just to discover the source of manure smells off kit. Currently orking on a Fender Pro 185 from 1989 and the workshop has a noticeable smell of manure. On poking my nose directly over caps, transformer, cabinet (for fish glue or something) etc I cannot localise it. I looked at another of this exact same model a few months ago and no such smell. A year ago I repaired a JBL domestic amp from the 1960s and that had a very strong , even stronger, manure smell, but again could not localise the smell. With reservations about breaking the "special relationship" one common factor is they were both made in the USA.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

On Mon, 03 Sep 2007 08:30:01 GMT, "Arfa Daily" put finger to keyboard and composed:

I think that's correct. Whenever I suspected a 3-pin thermistor as the cause of a blown fuse, I used to shake it and listen for rattling.

- Franc Zabkar