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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.repair,sci.electronics.design
Subject: Re: Oscillator Distortion
Date: Wed, 16 Oct 2024 14:20:53 -0000 (UTC)
Organization: A noiseless patient Spider
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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.