Re: Oscillator Distortion

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