Re: Oscillator Distortion

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.