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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 14:39:27 GMT
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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.

-- 

Roger Hayter