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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 29th Aug 2019, 8:56 am   #1
Ted Kendall
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Default NTC preset?

I'm resurrecting a Woelke ME101 wow and flutter meter which has seen better days. Damp has done its work, and among other things one of the preset wipers has corroded and snapped off. The circuit diagram shows it as 100K NTC. The requirement is plausible, given that it is involved in the setup of the oscillator, but I don't think I've ever heard of such a thing, never mind seen one. Anybody know different? It looks the same as the other presets in the unit - skeleton Paxolin - which have no special indication.
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Old 29th Aug 2019, 9:29 am   #2
Jon_G4MDC
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Default Re: NTC preset?

Any chance that they mean this preset adjusts negative temp comp for the circuit rather than being a property of the part?

Just thinking out loud. Never heard of this and besides it wouldn't mean anything without a slope. e.g. N750 on Ceramic Caps.
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Old 29th Aug 2019, 9:45 am   #3
paulsherwin
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Default Re: NTC preset?

I've certainly never heard of a NTC preset pot. I can't imagine how such a technology would work.
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Old 29th Aug 2019, 10:41 am   #4
barrymagrec
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Default Re: NTC preset?

It doesn`t seem likely but you could hang a meter across it and see if the value changed if you heated it up or cooled it down.

Do you have a means of calibrating the Woelke when you have got it fixed up?
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Old 29th Aug 2019, 10:43 am   #5
Ted Kendall
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Default Re: NTC preset?

Well, in theory one could make a brew for the track with a negative temp coefficient, but I'm pleased that my mystification is well founded. Given that the diagram is otherwise labelled in German, it could be that it's a supply voltage adjustment.
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Old 29th Aug 2019, 10:47 am   #6
Ted Kendall
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Default Re: NTC preset?

Quote:
Originally Posted by barrymagrec View Post
It doesn`t seem likely but you could hang a meter across it and see if the value changed if you heated it up or cooled it down.

Do you have a means of calibrating the Woelke when you have got it fixed up?
Most of it can be set up with a frequency counter, as it has some self-test capability. There is an Audacity app available, courtesy of another forum member, if memory serves, which generates a controllable amount of wow. Ironically, I have software to take wow out, but not as yet to put it in...
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Old 29th Aug 2019, 10:57 am   #7
barrymagrec
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Default Re: NTC preset?

The test for the Gaumont Kalee 1740 involved two oscillators and a resistive mixing pad - when I tried this (more than 45 years ago) it gave a 25% lower reading than I expected and was also out of kilter with the inbuilt calibration on a model 564 so I gave up and relied on the 564 readings which also seemed to tie in with the BBC`s 1741.

The Woelke also has the DIN weighting, which will complicate matters of course.
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Old 29th Aug 2019, 11:37 pm   #8
Ted Kendall
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Default Re: NTC preset?

OK, problem solved. I misread the circuit symbol - the element is an NTC 100K resistor. There is a 100K preset in there as well, and I missed it on first reading of the diagram...
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Old 29th Aug 2019, 11:50 pm   #9
Jon_G4MDC
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Default Re: NTC preset?

Good news - but doesn't it still need a slope value of ppm/degC - or?
Maybe the preset compensates for it so therefore it becomes non-critical.
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Old 10th Sep 2019, 6:18 am   #10
Chris55000
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Default Re: NTC preset?

Hi!

NTC Thermistors sold these days are characterised by:–

1) Nominal room–temperature resistance at 25C;

2) The B value – this is basically the exponent of the power law that relates the temperature variation with resistance. If you buy new ones from a reputable maker like Epcos or Vishay, there'll be a datasheet which has the approximate value of B given – by the way B I think is dimensionless, i.e., just a number without a unit specified.

Now to the OP's post – it was very common German and Continental practice to denote thermistors with an oblong box and a double–headed arrow through it, hence the OP mistaking it for a potentiometer! This is why, once again, the traditional resistor with a black spot next to it used by BSI originally was far preferable, and is the one I use myself!

Regarding replacement of these in exsisting equipment, if the maker provided a type no. in his parts list or diagram, you might be able to find out what the B value was and select a modern replacement, but if no type number was quoted, then one of the same cold resistance and approximately the same size should be OK.

3) Power rating in milliwatts – again, because an NTC varies with temperature, it's difficult to predict the exact dissipation in many circuits, but since most small NTC thermistors are used for low–current bias stabilisation or temp. compensation I don't think the rated power dissipation would be something to worry about – some datasheets do specify maximum initial current curves for each size and type but it's not universal practice!

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It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!
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