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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: Oscillator Distortion
Date: Wed, 16 Oct 2024 16:02:53 -0400
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On 16 Oct 2024 15:46:04 GMT, Roger Hayter <roger@hayter.org> wrote:

>On 16 Oct 2024 at 16:29:02 BST, "Cursitor Doom" <cd999666@notformail.com>
>wrote:
>
>> On 16 Oct 2024 14:39:27 GMT, Roger Hayter wrote:
>> 
>>> On 16 Oct 2024 at 15:20:53 BST, "Cursitor Doom"
>>> <cd999666@notformail.com>
>>> wrote:
>>> 
>>>> On Tue, 15 Oct 2024 16:14:21 +0100, Liz Tuddenham wrote:
>>>> 
>>>>> Cursitor Doom <cd999666@notformail.com> wrote:
>>>>> 
>>>>>> On Mon, 14 Oct 2024 11:41:24 +0100, Liz Tuddenham wrote:
>>>>>> 
>>>>>>> Cursitor Doom <cd999666@notformail.com> wrote:
>>>>>>> 
>>>>>>>> On Sun, 13 Oct 2024 17:39:53 -0700, Dave Platt wrote:
>>>>>>>> 
>>>>>>>>> In article <veguu6$ofj1$3@dont-email.me>,
>>>>>>>>> Cursitor Doom  <cd999666@notformail.com> wrote:
>>>>>>>>> 
>>>>>>>>>> I've found an issue with the principal oscillator. It's
>>>>>>>>>> generating distorted sine waves. It's a wien bridge type using
>>>>>>>>>> BJTs as the gain element and fine tungsten filaments as
>>>>>>>>>> thermistors, so should produce near perfect sine waves before
>>>>>>>>>> they're chopped and shaped by subsequent circuitry, but since the
>>>>>>>>>> fall, it's not.
>>>>>>>>> 
>>>>>>>>> Is there a chance that the impact broke one of those fine tungsten
>>>>>>>>> filaments?  Do they read low-Z when cold, as they should?
>>>>>>>> 
>>>>>>>> It's one thing I need to look at, if only for the sake of
>>>>>>>> completeness,
>>>>>>>> next time I have access to it. Since these are so hard to replace,
>>>>>>>> I'd really rather deal with my original suspicion that something
>>>>>>>> got shorted out in the fall.
>>>>>>> 
>>>>>>> Could a pre-set pot have gone open circuit?
>>>>>> 
>>>>>> I don't think so. TH2 on the schematic appears to have gone open
>>>>>> circuit. Those filaments are pretty fragile! Not sure what they've
>>>>>> used - looks like one of those old dashboard bulbs you see in old
>>>>>> cars. I won't be able to find a direct replacement, but I do have
>>>>>> some spare thermistors from other wein-bridge test gear I've
>>>>>> plundered over the years which I dare say could be pressed into
>>>>>> service with a little teak of the biasing. I might even experiment
>>>>>> with some small filament bulbs which are not part of the WB variety
>>>>>> just out of curiosity.
>>>>> 
>>>>> The circuit, is pretty similar to the Venner TSA 625/2, which was sold
>>>>> as a standalone general purpose oscillator - one of which I have in
>>>>> pieces on the desk in front of me.  The thermistor (which is TH1 in
>>>>> this circuit but more likely to be TH2 in yours) looks like a glass
>>>>> tube, about the size of a DM70 valve (for those who remember them),
>>>>> with two flying leads.  It is supported in a plastic clip.
>>>>> 
>>>>> The interior has a pinch with two substantial, longish support wires.
>>>>> Joining the ends of the support wires there is a very fragile wire and
>>>>> suspended by that wire is the thermistor bead.  The idea is that the
>>>>> glass tube is evacuated and there is very little thermal conductivity
>>>>> along the support wires, so the bead is free to self-heat with only a
>>>>> few milliwatts of power.
>>>>> 
>>>>> If yours is like this, you cannot replace it with a light bulb as the
>>>>> characteristics will be completely different.  The resistance drops as
>>>>> it heats up, where the resistance of a light bulb rises.  The good
>>>>> news is that these are still made (or were until very recently) and
>>>>> you may find the type number of the exact part you need in the parts
>>>>> list for the instrument.
>>>>> 
>>>>> My guess is that TH1 in your circuit is a slow-acting thermistor that
>>>>> compensates for thermal effects in the transistor biassing and it may
>>>>> look like a black resistor or a small tablet of carborundum mounted on
>>>>> the board with ordinary component lead-out wires.
>>>> 
>>>> This one's the TSA628. I have the service manual for it, but it only
>>>> quotes Venner part numbers for all devices listed and "R23" for the
>>>> type/ value of this thermistor. I don't think either of those numbers
>>>> would be of much use today. TH1 is "Type R52" so while they appear
>>>> identical, it seems they aren't.
>>>> TH1 and TH2 both *appear* identical: glass encapsulations about an inch
>>>> long by 3/8 wide at a guess. They're juxtaposed together on the board.
>>>> I'm guessing - it is only a guess - that TH2 (the failed one)
>>>> functioned as some sort of AGC to stabilise the amplitude of the
>>>> oscillator and the TH1 was the actual Wein-Bridge element as is
>>>> commonly understood in this type of oscillator. That would account for
>>>> why - as Phil Hobbs observed - the gain has gone up enough to run the
>>>> output into the supply rails and give rise to the distortion I'm
>>>> seeing.
>>> 
>>> I've got an R54 somewhere.  I think they are still available as NOS.
>> 
>> I'm just wondering if a modern bead thermistor would work as well. The
>> ones in this 56 year old piece of kit are large, glass-encapsulated types,
>> but maybe that was just the way them made them back then when everything
>> was bigger. I might try a few bead thermistors in place of the busted one
>> once I've removed it just out of curiosity.
>
>The advantage of the vacuum encapsulated ones is that it requires vastly less
>power to heat them, and thus I doubt an open bead one would easily work at the
>same signal level. 

One can insulate an open bead with some foam.

Glassivated NTC thermistors are still made.  If we have a sample, it's
easy to take some data and figure out what the best modern match is.

..<https://www.ametherm.com/blog/thermistor/glass-encapsulated-thermistors-automotive-and-industrial-applications/>

Joe Gwinn