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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 11:28:21 -0000 (UTC)
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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.