Gentlemen,

Last week I got an old (1968) pulse generator out of mothballs and managed
to get it fully functional again. However, before replacing the case, I
(true to form) dropped it on the bench and something on the PCB must have
shorted out against the metal tools it fell on, because it no longer works
properly.
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.

Here's the oscillator output:
https://disk.yandex.com/i/eKAe95xMsiIvNA

I found some weird periodic spikes on the power supply rails in the
oscillator stage. They are actually present on the rail, not just picked
up by the ground lead of the scope out of the ether, as I used a short
ground clip in this instance. I'm not sure if these could cause the
distortion or not.
https://disk.yandex.com/i/eKAe95xMsiIvNA

I'm out of ideas. What could cause such distortion if the PS rail isn't
responsible?

Your pal,

CD.

On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom wrote:

> Gentlemen,
>
> Last week I got an old (1968) pulse generator out of mothballs and
> managed to get it fully functional again. However, before replacing the
> case, I (true to form) dropped it on the bench and something on the PCB
> must have shorted out against the metal tools it fell on, because it no
> longer works properly.
> 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.
>
> Here's the oscillator output:
> https://disk.yandex.com/i/eKAe95xMsiIvNA
>
> I found some weird periodic spikes on the power supply rails in the
> oscillator stage. They are actually present on the rail, not just picked
> up by the ground lead of the scope out of the ether, as I used a short
> ground clip in this instance. I'm not sure if these could cause the
> distortion or not.
> [removed]
>
> I'm out of ideas. What could cause such distortion if the PS rail isn't
> responsible?
>
> Your pal,
>
> CD.

Sorry, the trace of the 'ripple' should have been this link:
https://disk.yandex.com/i/P7AIraCaJybIMw

Cursitor Doom <cd999666@notformail.com> wrote:
> On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom wrote:
>
>> Gentlemen,
>>
>> Last week I got an old (1968) pulse generator out of mothballs and
>> managed to get it fully functional again. However, before replacing the
>> case, I (true to form) dropped it on the bench and something on the PCB
>> must have shorted out against the metal tools it fell on, because it no
>> longer works properly.
>> 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.
>>
>> Here's the oscillator output:
>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>
>> I found some weird periodic spikes on the power supply rails in the
>> oscillator stage. They are actually present on the rail, not just picked
>> up by the ground lead of the scope out of the ether, as I used a short
>> ground clip in this instance. I'm not sure if these could cause the
>> distortion or not.
>> [removed]
>>
>> I'm out of ideas. What could cause such distortion if the PS rail isn't
>> responsible?
>>
>> Your pal,
>>
>> CD.
>
> Sorry, the trace of the 'ripple' should have been this link:
> https://disk.yandex.com/i/P7AIraCaJybIMw
>

Looks like an AC-coupled square wave with a too-short time constant, which
probably means that either the oscillator gain is running wide open, or the
second stage gain is too high, due e.g. to its feedback loop being open.

Is the output amplitude close to the knob setting, or is it way off?

And can you get your hands on a schematic?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

On Sun, 13 Oct 2024 17:58:08 -0000 (UTC), Phil Hobbs wrote:

> Cursitor Doom <cd999666@notformail.com> wrote:
>> On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom wrote:
>>
>>> Gentlemen,
>>>
>>> Last week I got an old (1968) pulse generator out of mothballs and
>>> managed to get it fully functional again. However, before replacing
>>> the case, I (true to form) dropped it on the bench and something on
>>> the PCB must have shorted out against the metal tools it fell on,
>>> because it no longer works properly.
>>> 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.
>>>
>>> Here's the oscillator output:
>>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>>
>>> I found some weird periodic spikes on the power supply rails in the
>>> oscillator stage. They are actually present on the rail, not just
>>> picked up by the ground lead of the scope out of the ether, as I used
>>> a short ground clip in this instance. I'm not sure if these could
>>> cause the distortion or not.
>>> [removed]
>>>
>>> I'm out of ideas. What could cause such distortion if the PS rail
>>> isn't responsible?
>>>
>>> Your pal,
>>>
>>> CD.
>>
>> Sorry, the trace of the 'ripple' should have been this link:
>> https://disk.yandex.com/i/P7AIraCaJybIMw
>>
>>
> Looks like an AC-coupled square wave with a too-short time constant,
> which probably means that either the oscillator gain is running wide
> open, or the second stage gain is too high, due e.g. to its feedback
> loop being open.
>
> Is the output amplitude close to the knob setting, or is it way off?
>
> And can you get your hands on a schematic?
>
> Cheers
>
> Phil Hobbs

I'll see what I can do, Phil...

On Sun, 13 Oct 2024 17:58:08 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>Cursitor Doom <cd999666@notformail.com> wrote:
>> On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom wrote:
>>
>>> Gentlemen,
>>>
>>> Last week I got an old (1968) pulse generator out of mothballs and
>>> managed to get it fully functional again. However, before replacing the
>>> case, I (true to form) dropped it on the bench and something on the PCB
>>> must have shorted out against the metal tools it fell on, because it no
>>> longer works properly.
>>> 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.
>>>
>>> Here's the oscillator output:
>>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>>
>>> I found some weird periodic spikes on the power supply rails in the
>>> oscillator stage. They are actually present on the rail, not just picked
>>> up by the ground lead of the scope out of the ether, as I used a short
>>> ground clip in this instance. I'm not sure if these could cause the
>>> distortion or not.
>>> [removed]
>>>
>>> I'm out of ideas. What could cause such distortion if the PS rail isn't
>>> responsible?
>>>
>>> Your pal,
>>>
>>> CD.
>>
>> Sorry, the trace of the 'ripple' should have been this link:
>> https://disk.yandex.com/i/P7AIraCaJybIMw
>>
>
>Looks like an AC-coupled square wave with a too-short time constant, which
>probably means that either the oscillator gain is running wide open, or the
>second stage gain is too high, due e.g. to its feedback loop being open.
>
>Is the output amplitude close to the knob setting, or is it way off?
>
>And can you get your hands on a schematic?

My guess was that the light bulb filament broke from the shock.

It's probably the founding HP 200A Wien Bridge audio oscillator
circuit from 1939.

..<https://en.wikipedia.org/wiki/HP_200A>

Joe Gwinn

On Sun, 13 Oct 2024 17:58:08 -0000 (UTC), Phil Hobbs wrote:

> Cursitor Doom <cd999666@notformail.com> wrote:
>> On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom wrote:
>>
>>> Gentlemen,
>>>
>>> Last week I got an old (1968) pulse generator out of mothballs and
>>> managed to get it fully functional again. However, before replacing
>>> the case, I (true to form) dropped it on the bench and something on
>>> the PCB must have shorted out against the metal tools it fell on,
>>> because it no longer works properly.
>>> 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.
>>>
>>> Here's the oscillator output:
>>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>>
>>> I found some weird periodic spikes on the power supply rails in the
>>> oscillator stage. They are actually present on the rail, not just
>>> picked up by the ground lead of the scope out of the ether, as I used
>>> a short ground clip in this instance. I'm not sure if these could
>>> cause the distortion or not.
>>> [removed]
>>>
>>> I'm out of ideas. What could cause such distortion if the PS rail
>>> isn't responsible?
>>>
>>> Your pal,
>>>
>>> CD.
>>
>> Sorry, the trace of the 'ripple' should have been this link:
>> https://disk.yandex.com/i/P7AIraCaJybIMw
>>
>>
> Looks like an AC-coupled square wave with a too-short time constant,
> which probably means that either the oscillator gain is running wide
> open, or the second stage gain is too high, due e.g. to its feedback
> loop being open.
>
> Is the output amplitude close to the knob setting, or is it way off?

That's a bit tricky to ascertain at the moment. I might be able to verify
it later. However, I have managed to get the relevant section of the
schematic. The supply is just under 12V. The nomenclature is a bit odd.
The core of the oscillator is comprised of VT10, VT11 and VT12. All the
other stuff beyond those to the right is for shaping and whatnot.

https://disk.yandex.com/i/dbsB6wiQMDKtWw

On Sun, 13 Oct 2024 15:00:15 -0400, Joe Gwinn wrote:

> On Sun, 13 Oct 2024 17:58:08 -0000 (UTC), Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>>Cursitor Doom <cd999666@notformail.com> wrote:
>>> On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom wrote:
>>>
>>>> Gentlemen,
>>>>
>>>> Last week I got an old (1968) pulse generator out of mothballs and
>>>> managed to get it fully functional again. However, before replacing
>>>> the case, I (true to form) dropped it on the bench and something on
>>>> the PCB must have shorted out against the metal tools it fell on,
>>>> because it no longer works properly.
>>>> 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.
>>>>
>>>> Here's the oscillator output:
>>>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>>>
>>>> I found some weird periodic spikes on the power supply rails in the
>>>> oscillator stage. They are actually present on the rail, not just
>>>> picked up by the ground lead of the scope out of the ether, as I used
>>>> a short ground clip in this instance. I'm not sure if these could
>>>> cause the distortion or not.
>>>> [removed]
>>>>
>>>> I'm out of ideas. What could cause such distortion if the PS rail
>>>> isn't responsible?
>>>>
>>>> Your pal,
>>>>
>>>> CD.
>>>
>>> Sorry, the trace of the 'ripple' should have been this link:
>>> https://disk.yandex.com/i/P7AIraCaJybIMw
>>>
>>>
>>Looks like an AC-coupled square wave with a too-short time constant,
>>which probably means that either the oscillator gain is running wide
>>open, or the second stage gain is too high, due e.g. to its feedback
>>loop being open.
>>
>>Is the output amplitude close to the knob setting, or is it way off?
>>
>>And can you get your hands on a schematic?
>
> My guess was that the light bulb filament broke from the shock.
>
> It's probably the founding HP 200A Wien Bridge audio oscillator circuit
> from 1939.
>
> .<https://en.wikipedia.org/wiki/HP_200A>
>
> Joe Gwinn

Somewhat unusually for me, this is not an HP instrument. It was made by
some company called Venner in London. It did come with a very helpful
manual which gives expected waveforms at various key points in the
circuit. As you might expect, it shows sine waves for the TPs in the osc
section.

On Sun, 13 Oct 2024 19:31:14 -0000 (UTC), Cursitor Doom
<cd999666@notformail.com> wrote:

>On Sun, 13 Oct 2024 15:00:15 -0400, Joe Gwinn wrote:
>
>> On Sun, 13 Oct 2024 17:58:08 -0000 (UTC), Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>
>>>Cursitor Doom <cd999666@notformail.com> wrote:
>>>> On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom wrote:
>>>>
>>>>> Gentlemen,
>>>>>
>>>>> Last week I got an old (1968) pulse generator out of mothballs and
>>>>> managed to get it fully functional again. However, before replacing
>>>>> the case, I (true to form) dropped it on the bench and something on
>>>>> the PCB must have shorted out against the metal tools it fell on,
>>>>> because it no longer works properly.
>>>>> 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.
>>>>>
>>>>> Here's the oscillator output:
>>>>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>>>>
>>>>> I found some weird periodic spikes on the power supply rails in the
>>>>> oscillator stage. They are actually present on the rail, not just
>>>>> picked up by the ground lead of the scope out of the ether, as I used
>>>>> a short ground clip in this instance. I'm not sure if these could
>>>>> cause the distortion or not.
>>>>> [removed]
>>>>>
>>>>> I'm out of ideas. What could cause such distortion if the PS rail
>>>>> isn't responsible?
>>>>>
>>>>> Your pal,
>>>>>
>>>>> CD.
>>>>
>>>> Sorry, the trace of the 'ripple' should have been this link:
>>>> https://disk.yandex.com/i/P7AIraCaJybIMw
>>>>
>>>>
>>>Looks like an AC-coupled square wave with a too-short time constant,
>>>which probably means that either the oscillator gain is running wide
>>>open, or the second stage gain is too high, due e.g. to its feedback
>>>loop being open.
>>>
>>>Is the output amplitude close to the knob setting, or is it way off?
>>>
>>>And can you get your hands on a schematic?
>>
>> My guess was that the light bulb filament broke from the shock.
>>
>> It's probably the founding HP 200A Wien Bridge audio oscillator circuit
>> from 1939.
>>
>> .<https://en.wikipedia.org/wiki/HP_200A>
>>
>> Joe Gwinn
>
>Somewhat unusually for me, this is not an HP instrument. It was made by
>some company called Venner in London. It did come with a very helpful
>manual which gives expected waveforms at various key points in the
>circuit. As you might expect, it shows sine waves for the TPs in the osc
>section.

HP's patent ran out a very long time ago. The bit about tungsten
filaments was the tell. Read the HP patent - it's short.

Joe Gwinn

On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom
<cd999666@notformail.com> wrote:

>Gentlemen,
>
>Last week I got an old (1968) pulse generator out of mothballs and managed
>to get it fully functional again. However, before replacing the case, I
>(true to form) dropped it on the bench and something on the PCB must have
>shorted out against the metal tools it fell on, because it no longer works
>properly.
>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.
>
>Here's the oscillator output:
>https://disk.yandex.com/i/eKAe95xMsiIvNA
>
>I found some weird periodic spikes on the power supply rails in the
>oscillator stage. They are actually present on the rail, not just picked
>up by the ground lead of the scope out of the ether, as I used a short
>ground clip in this instance. I'm not sure if these could cause the
>distortion or not.
>https://disk.yandex.com/i/eKAe95xMsiIvNA
>
>I'm out of ideas. What could cause such distortion if the PS rail isn't
>responsible?
>
>Your pal,
>
>CD.
>

Maybe the light bulb broke.

john larkin <JL@gct.com> wrote:
> On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom
> <cd999666@notformail.com> wrote:
>
>> Gentlemen,
>>
>> Last week I got an old (1968) pulse generator out of mothballs and managed
>> to get it fully functional again. However, before replacing the case, I
>> (true to form) dropped it on the bench and something on the PCB must have
>> shorted out against the metal tools it fell on, because it no longer works
>> properly.
>> 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.
>>
>> Here's the oscillator output:
>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>
>> I found some weird periodic spikes on the power supply rails in the
>> oscillator stage. They are actually present on the rail, not just picked
>> up by the ground lead of the scope out of the ether, as I used a short
>> ground clip in this instance. I'm not sure if these could cause the
>> distortion or not.
>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>
>> I'm out of ideas. What could cause such distortion if the PS rail isn't
>> responsible?
>>
>> Your pal,
>>
>> CD.
>>
>
> Maybe the light bulb broke.
>
>

It would have to have failed short-circuit for the gain to be too high, and
that’s not the usual failure mode.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

On Sun, 13 Oct 2024 20:56:15 -0000 (UTC), Phil Hobbs wrote:

> john larkin <JL@gct.com> wrote:
>> On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom
>> <cd999666@notformail.com> wrote:
>>
>>> Gentlemen,
>>>
>>> Last week I got an old (1968) pulse generator out of mothballs and
>>> managed to get it fully functional again. However, before replacing
>>> the case, I (true to form) dropped it on the bench and something on
>>> the PCB must have shorted out against the metal tools it fell on,
>>> because it no longer works properly.
>>> 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.
>>>
>>> Here's the oscillator output:
>>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>>
>>> I found some weird periodic spikes on the power supply rails in the
>>> oscillator stage. They are actually present on the rail, not just
>>> picked up by the ground lead of the scope out of the ether, as I used
>>> a short ground clip in this instance. I'm not sure if these could
>>> cause the distortion or not.
>>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>>
>>> I'm out of ideas. What could cause such distortion if the PS rail
>>> isn't responsible?
>>>
>>> Your pal,
>>>
>>> CD.
>>>
>>>
>> Maybe the light bulb broke.
>>
>>
>>
> It would have to have failed short-circuit for the gain to be too high,
> and that’s not the usual failure mode.
>
> Cheers
>
> Phil Hobbs

Anyway, it looks like the gain's too high, then. I don't have access to
the generator again for a day or two, but I will focus on that aspect and
hope to identify the precise issue through that.
Thanks, Phil; you're a pal. ;-)

CD

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?

On Sun, 13 Oct 2024 17:39:53 -0700, dplatt@coop.radagast.org (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?

The time reponse of the filaments, as compared to the Q of the basic
oscillator, must be interesting. There are no doubt papers somewhere.

Cursitor Doom <cd999666@notformail.com> wrote:
> On Sun, 13 Oct 2024 15:00:15 -0400, Joe Gwinn wrote:
>
>> On Sun, 13 Oct 2024 17:58:08 -0000 (UTC), Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>
>>> Cursitor Doom <cd999666@notformail.com> wrote:
>>>> On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom wrote:
>>>>
>>>>> Gentlemen,
>>>>>
>>>>> Last week I got an old (1968) pulse generator out of mothballs and
>>>>> managed to get it fully functional again. However, before replacing
>>>>> the case, I (true to form) dropped it on the bench and something on
>>>>> the PCB must have shorted out against the metal tools it fell on,
>>>>> because it no longer works properly.
>>>>> 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.
>>>>>
>>>>> Here's the oscillator output:
>>>>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>>>>
>>>>> I found some weird periodic spikes on the power supply rails in the
>>>>> oscillator stage. They are actually present on the rail, not just
>>>>> picked up by the ground lead of the scope out of the ether, as I used
>>>>> a short ground clip in this instance. I'm not sure if these could
>>>>> cause the distortion or not.
>>>>> [removed]
>>>>>
>>>>> I'm out of ideas. What could cause such distortion if the PS rail
>>>>> isn't responsible?
>>>>>
>>>>> Your pal,
>>>>>
>>>>> CD.
>>>>
>>>> Sorry, the trace of the 'ripple' should have been this link:
>>>> https://disk.yandex.com/i/P7AIraCaJybIMw
>>>>
>>>>
>>> Looks like an AC-coupled square wave with a too-short time constant,
>>> which probably means that either the oscillator gain is running wide
>>> open, or the second stage gain is too high, due e.g. to its feedback
>>> loop being open.
>>>
>>> Is the output amplitude close to the knob setting, or is it way off?
>>>
>>> And can you get your hands on a schematic?
>>
>> My guess was that the light bulb filament broke from the shock.
>>
>> It's probably the founding HP 200A Wien Bridge audio oscillator circuit
>> from 1939.
>>
>> .<https://en.wikipedia.org/wiki/HP_200A>
>>
>> Joe Gwinn
>
> Somewhat unusually for me, this is not an HP instrument. It was made by
> some company called Venner in London. It did come with a very helpful
> manual which gives expected waveforms at various key points in the
> circuit. As you might expect, it shows sine waves for the TPs in the osc
> section.
>

Venner were famous for their time switches made since forever and used in
street lights before photocells.

--
piglet

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.

On Sun, 13 Oct 2024 15:00:15 -0400, Joe Gwinn wrote:

> On Sun, 13 Oct 2024 17:58:08 -0000 (UTC), Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>>Cursitor Doom <cd999666@notformail.com> wrote:
>>> On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom wrote:
>>>
>>>> Gentlemen,
>>>>
>>>> Last week I got an old (1968) pulse generator out of mothballs and
>>>> managed to get it fully functional again. However, before replacing
>>>> the case, I (true to form) dropped it on the bench and something on
>>>> the PCB must have shorted out against the metal tools it fell on,
>>>> because it no longer works properly.
>>>> 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.
>>>>
>>>> Here's the oscillator output:
>>>> https://disk.yandex.com/i/eKAe95xMsiIvNA
>>>>
>>>> I found some weird periodic spikes on the power supply rails in the
>>>> oscillator stage. They are actually present on the rail, not just
>>>> picked up by the ground lead of the scope out of the ether, as I used
>>>> a short ground clip in this instance. I'm not sure if these could
>>>> cause the distortion or not.
>>>> [removed]
>>>>
>>>> I'm out of ideas. What could cause such distortion if the PS rail
>>>> isn't responsible?
>>>>
>>>> Your pal,
>>>>
>>>> CD.
>>>
>>> Sorry, the trace of the 'ripple' should have been this link:
>>> https://disk.yandex.com/i/P7AIraCaJybIMw
>>>
>>>
>>Looks like an AC-coupled square wave with a too-short time constant,
>>which probably means that either the oscillator gain is running wide
>>open, or the second stage gain is too high, due e.g. to its feedback
>>loop being open.
>>
>>Is the output amplitude close to the knob setting, or is it way off?
>>
>>And can you get your hands on a schematic?
>
> My guess was that the light bulb filament broke from the shock.
>
> It's probably the founding HP 200A Wien Bridge audio oscillator circuit
> from 1939.
>
> .<https://en.wikipedia.org/wiki/HP_200A>
>
> Joe Gwinn

Yes, the TH2 thermistor on the schematic has gone open-circuit. Now I do
have some spare oscillator thermistors which I rifled from old test gear I
broke up for spares and always made sure I got the thermistors out of
wein-bridge circuits. Unfortunately, the ones I have are unlikely to have
the same characteristics as the failed one. But I'm reasonably optimistic
I can correct for that by going through the re-alignment procedure in the
manual.

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.

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

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.

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

On 16 Oct 2024 15:46:04 GMT, Roger Hayter <roger@hayter.org> wrote:

>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.

One can insulate an open bead with some foam.

Glassivated NTC thermistors are still made. If we have a sample, it's
easy to take some data and figure out what the best modern match is.

..<https://www.ametherm.com/blog/thermistor/glass-encapsulated-thermistors-automotive-and-industrial-applications/>

Joe Gwinn

On 17/10/2024 7:02 am, Joe Gwinn wrote:
> On 16 Oct 2024 15:46:04 GMT, Roger Hayter <roger@hayter.org> wrote:
>
>> 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.
>
> One can insulate an open bead with some foam.
>
> Glassivated NTC thermistors are still made. If we have a sample, it's
> easy to take some data and figure out what the best modern match is.
>
> .<https://www.ametherm.com/blog/thermistor/glass-encapsulated-thermistors-automotive-and-industrial-applications/>

Yellow Springs Instruments were the pioneers in glass-encapsulated
thermistors. There are plenty of other manufacturers who make them
today. I've used Betatherm parts.

What Cursitor Doom seems to be missing is that you need a particular
value of resistance to stabilise a given Wein Bridge circuit. You can
buy thermistors which have a wide range of resistances at room
temperature, and he going to need one that has less resistance than the
other resistors in the bridge, but more than enough that the reduction
in resistance as it gets warm drops the gain around the loop to less
than one at just above the desired working amplitude.

It would be simple enough to work out if you knew what you were doing.

--
Bill Sloman, Sydney

On 16/10/2024 3:20 pm, Cursitor Doom 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.

My guess is type R23 is 2kohm at room temp (the R53/RA53 beloved by
hobbyists in the 1960s/70s was 5k). eBay probably has some close enough
(1.5k to 3.3k at room temp?) replacements.

This link might help you:

<https://www.dropbox.com/scl/fi/2gqn85dagr60qo0k8fm25/Old_STC_Thermistors.pdf?rlkey=jozht29aj1u6ocxnmw8okedrq&st=ohzxz0mw&raw=1>

piglet

On Thu, 17 Oct 2024 11:33:52 +0100, piglet wrote:

> On 16/10/2024 3:20 pm, Cursitor Doom 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.
>
>
> My guess is type R23 is 2kohm at room temp (the R53/RA53 beloved by
> hobbyists in the 1960s/70s was 5k). eBay probably has some close enough
> (1.5k to 3.3k at room temp?) replacements.
>
> This link might help you:
>
> <https://www.dropbox.com/scl/fi/2gqn85dagr60qo0k8fm25/
Old_STC_Thermistors.pdf?rlkey=jozht29aj1u6ocxnmw8okedrq&st=ohzxz0mw&raw=1>
>
> piglet

Thanks for the info, Erich; much useful info in that datasheet.
Nothing showing up on Ebay at the moment, but I'm making enquiries of
vintage parts sellers which hopefully might bear some fruit. Failing that
I'll just have to build a new oscillator stage from scratch using a spare
vacuum thermistor from my parts bin.

On 17/10/2024 12:28 pm, Cursitor Doom wrote:
> On Thu, 17 Oct 2024 11:33:52 +0100, piglet wrote:
>
>> On 16/10/2024 3:20 pm, Cursitor Doom 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.
>>
>>
>> My guess is type R23 is 2kohm at room temp (the R53/RA53 beloved by
>> hobbyists in the 1960s/70s was 5k). eBay probably has some close enough
>> (1.5k to 3.3k at room temp?) replacements.
>>
>> This link might help you:
>>
>> <https://www.dropbox.com/scl/fi/2gqn85dagr60qo0k8fm25/
> Old_STC_Thermistors.pdf?rlkey=jozht29aj1u6ocxnmw8okedrq&st=ohzxz0mw&raw=1>
>>
>> piglet
>
> Thanks for the info, Erich; much useful info in that datasheet.
> Nothing showing up on Ebay at the moment, but I'm making enquiries of
> vintage parts sellers which hopefully might bear some fruit. Failing that
> I'll just have to build a new oscillator stage from scratch using a spare
> vacuum thermistor from my parts bin.

I saw a seller with Littelfuse GL202F9J which might do electrically if
maybe a bit slow thermally? That part is also available from Mouser.

piglet