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From: Roger Hayter <roger@hayter.org>
Newsgroups: sci.electronics.design,sci.electronics.repair
Subject: Re: Oscillator Distortion
Date: 16 Oct 2024 15:46:04 GMT
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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. 

-- 

Roger Hayter