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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.repair,sci.electronics.design
Subject: Re: Oscillator Distortion
Date: Thu, 17 Oct 2024 13:35:24 -0000 (UTC)
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On Thu, 17 Oct 2024 13:35:14 +0100, piglet wrote:

> On 17/10/2024 12:28 pm, Cursitor Doom wrote:
>> On Thu, 17 Oct 2024 11:33:52 +0100, piglet wrote:
>> 
>>> On 16/10/2024 3:20 pm, Cursitor Doom 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.
>>>
>>>
>>> My guess is type R23 is 2kohm at room temp (the R53/RA53 beloved by
>>> hobbyists in the 1960s/70s was 5k). eBay probably has some close
>>> enough (1.5k to 3.3k at room temp?) replacements.
>>>
>>> This link might help you:
>>>
>>> <https://www.dropbox.com/scl/fi/2gqn85dagr60qo0k8fm25/
>> Old_STC_Thermistors.pdf?
rlkey=jozht29aj1u6ocxnmw8okedrq&st=ohzxz0mw&raw=1>
>>>
>>> piglet
>> 
>> Thanks for the info, Erich; much useful info in that datasheet.
>> Nothing showing up on Ebay at the moment, but I'm making enquiries of
>> vintage parts sellers which hopefully might bear some fruit. Failing
>> that I'll just have to build a new oscillator stage from scratch using
>> a spare vacuum thermistor from my parts bin.
> 
> I saw a seller with Littelfuse GL202F9J which might do electrically if
> maybe a bit slow thermally? That part is also available from Mouser.
> 
> piglet

According to the datasheet, this device boasts "a fast thermal response 
time" but doesn't quantify that. It could do the trick with a bit of bias 
tweaking.