On Thu, 17 Oct 2024 13:35:14 +0100, piglet wrote:

> 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

According to the datasheet, this device boasts "a fast thermal response
time" but doesn't quantify that. It could do the trick with a bit of bias
tweaking.

On 17/10/2024 2:35 pm, Cursitor Doom wrote:
> On Thu, 17 Oct 2024 13:35:14 +0100, piglet wrote:
>
>> 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
>
> According to the datasheet, this device boasts "a fast thermal response
> time" but doesn't quantify that. It could do the trick with a bit of bias
> tweaking.

That part is intended for temp measurement rather than self heated
regulation so will be slower than the well isolated original R23 part -
I expect it would work OK but you'd notice a longer amplitude bounce
settling time after each change of frequency.

piglet

On Thu, 17 Oct 2024 15:19:17 +0100, piglet wrote:

> On 17/10/2024 2:35 pm, Cursitor Doom wrote:
>> On Thu, 17 Oct 2024 13:35:14 +0100, piglet wrote:
>>
>>> 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
>>
>> According to the datasheet, this device boasts "a fast thermal response
>> time" but doesn't quantify that. It could do the trick with a bit of
>> bias tweaking.
>
> That part is intended for temp measurement rather than self heated
> regulation so will be slower than the well isolated original R23 part -
> I expect it would work OK but you'd notice a longer amplitude bounce
> settling time after each change of frequency.
>
> piglet

Got it, thanks.

In article <veguu6$ofj1$3@dont-email.me>,
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?

Not entirely unrelated to this subject.
Image a Wien bridge oscillator, stabilized with a lightbulb.
This could be described with differential equations, including
the heating and cooling of the tungsten wire.

I suspect a relation between the cooling time constant and the
periods of an oscillator, and the distortion.
A rule could be
if you need N periods to have the distance to the
the stable wave halved, you have circa 1/N distortion.

Is this known territory?
>
>Your pal,
>
>CD.

Groetjes Albert
>
>
--
Temu exploits Christians: (Disclaimer, only 10 apostles)
Last Supper Acrylic Suncatcher - 15Cm Round Stained Glass- Style Wall
Art For Home, Office And Garden Decor - Perfect For Windows, Bars,
And Gifts For Friends Family And Colleagues.

On Sun, 20 Oct 2024 11:24:06 +0200, albert wrote:

> In article <veguu6$ofj1$3@dont-email.me>,
> 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?
>
> Not entirely unrelated to this subject.
> Image a Wien bridge oscillator, stabilized with a lightbulb. This could
> be described with differential equations, including the heating and
> cooling of the tungsten wire.
>
> I suspect a relation between the cooling time constant and the periods
> of an oscillator, and the distortion.
> A rule could be if you need N periods to have the distance to the the
> stable wave halved, you have circa 1/N distortion.
>
> Is this known territory?
>>
>>Your pal,
>>
>>CD.
>
> Groetjes Albert
>>
>>

You people who explain everything they see in terms of higher mathematics
normally leave me baffled. But I believe I've deduced what you're trying
to say and it's not an issue in this case. Thanks for the suggestion
anyway.

On 2024-10-20 5:24 a.m., albert@spenarnc.xs4all.nl wrote:
> In article <veguu6$ofj1$3@dont-email.me>,
> 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.

The whole idea of making a sine oscillator is positive feedback with an
AC gain  =1  while DC biased with negative feedback and not boosting to
a pulse with high gain. So something must limit the gain smoothly like a
hot bulb with lower impedance driven by a lower voltage.

Below the inverting gain can be  R1/(R2 + (R(Q1)) =2 maximum unless Q2
is slowly turned off. and quickly turned on ;) otherwise known as fast
attack , slow decay.

  The non-inverting side is unity gain for AC signals. So you got an
oscillator and the diode voltage turns off the PFET or Pch JFET. The
output amplitude is controlled by the gate control voltage V(AGC) which
can be attenuated to boost output voltage to 10Vpp with a series R
around 4 Meg to the 1 Meg shunt.  The FET threshold of 0.5V and the
diode voltage with 1Meg is only 0.4V so slightly less than 1Vp is achieved.

 With a -voltage below ground must meet the FET threshold  to control
gain with a ground reference.   This was copied directly from LTSpice
examples > education.  If you understand any of what I said then you
recognize the differences with Hewlett Packard's old design.

Whatever is boosting the gain of your circuit or NOT cutting the gain
with high R must be fixed.

Cheers

Tony Stewart, near Toronto

EE since 1975

Learning how to retire since age 54.

I forgot about image if not shown with plain text
https://www.sendspace.com/file/wnp7a3
<https://www.sendspace.com/file/wnp7a3>

On 2024-11-17 12:01, chuck wrote:
> On 2024-10-20 5:24 a.m., albert@spenarnc.xs4all.nl wrote:
>> In article <veguu6$ofj1$3@dont-email.me>,
>> 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.
>
>
> The whole idea of making a sine oscillator is positive feedback with an
> AC gain  =1  while DC biased with negative feedback and not boosting to
> a pulse with high gain. So something must limit the gain smoothly like a
> hot bulb with lower impedance driven by a lower voltage.
>
>
> Below the inverting gain can be  R1/(R2 + (R(Q1)) =2 maximum unless Q2
> is slowly turned off. and quickly turned on ;) otherwise known as fast
> attack , slow decay.
>
>   The non-inverting side is unity gain for AC signals. So you got an
> oscillator and the diode voltage turns off the PFET or Pch JFET. The
> output amplitude is controlled by the gate control voltage V(AGC) which
> can be attenuated to boost output voltage to 10Vpp with a series R
> around 4 Meg to the 1 Meg shunt.  The FET threshold of 0.5V and the
> diode voltage with 1Meg is only 0.4V so slightly less than 1Vp is achieved.
>
>  With a -voltage below ground must meet the FET threshold  to control
> gain with a ground reference.   This was copied directly from LTSpice
> examples > education.  If you understand any of what I said then you
> recognize the differences with Hewlett Packard's old design.
>
> Whatever is boosting the gain of your circuit or NOT cutting the gain
> with high R must be fixed.

There's no requirement that the amplitude regulation be continuous. It
can also be done with a comparator that fires occasionally when the
amplitude gets too large.

One approach is to have the oscillator's tail current controlled by an
integrator, with a resistor causing the current to increase slowly, and
a comparator causing it to kick it down by a fixed amount whenever the
amplitude threshold is crossed.

Adjusting the phase of the signal at the comparator so that the kick
arrives at the peak of the collector current waveform reduces the
resulting small phase jitter.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

Like any PLL or ILL or AGC , you only have samples to correction error
and never "continuous" correction unless integrated to a DC level. It is
a discontinuous error correction with a filter to hold between inputs.

There are many ways to correct error by limiting the gain > 1 yet
ensuring it can reach 1 to oscillate. S/H, non-linear soft limiters,
filters, peak detectors/comparators. It all depends on your specs for
lock-in time and THD/IMD specs of the sine wave.

> > There's no requirement that the amplitude regulation be continuous.
>
> Phil Hobbs
>

--
Cheers,
Tony Stewart N. Of Toronto
EE since 1975 practising retirement since 2004

On 2024-11-18 08:30, chuck wrote:
> Like any PLL or ILL or AGC , you only have samples to correction error
> and never "continuous" correction unless integrated to a DC level. It is
> a discontinuous error correction with a filter to hold between inputs.
>
> There are many ways to correct error by limiting the gain > 1 yet
> ensuring it can reach 1 to oscillate.  S/H, non-linear soft limiters,
> filters, peak detectors/comparators. It all depends on your specs for
> lock-in time and THD/IMD specs of the sine wave.
>
> >  > There's no requirement that the amplitude regulation be continuous.

Any oscillator with a nonlinear or bilinear gain control element that
has to respond during a cycle has to deal with the distortion caused by
that element. OTAs, JFET variable resistors, PIN diode attenuators,
Vactrols, light bulbs, and so on, all have that problem. Tail current
sources can avoid it, because you can make them as stiff as you like by
cascoding, and filter the control voltage as well as you like. (I often
use two- or three-pole capacitance multipliers on the supply rails of
discrete circuitry, which is a similar idea.)

Once the gain is close to 1.000, the amplitude growth or decay happens
over many cycles, so the update rate can be correspondingly low--just
enough to compensate for temperature and power supply variations.

Something like a baby-scale loop can help with that, to allow the
amplitude to stabilize rapidly before slowing down the time constant on
the control voltage.

If the oscillator needs to be tunable, you get into slightly different
tradeoffs, but you still don't have to deal with nonlinear gain control
elements.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

The FET regulator is a filtered servo with fast cutoff and very slow
build up over thousands of cycles. The same is done with other
non-linear limiters using exponentially orders of magnitude higher
resistance when regulating so the decay rate is slow and low distortion.

Phil I think you had different assumptions for your au contraire opinion
on tail currents vs quasi-linear low decay currents when regulated
effectively. I don't see the difference except a non-linear device
starts up faster.

On 2024-11-18 3:14 p.m., Phil Hobbs wrote:

>
> Any oscillator with a nonlinear or bilinear gain control element that
> has to respond during a cycle has to deal with the distortion caused by
> that element.  OTAs, JFET variable resistors, PIN diode attenuators,
> Vactrols, light bulbs, and so on, all have that problem.  Tail current
> sources can avoid it, because you can make them as stiff as you like by
> cascoding, and filter the control voltage as well as you like.  (I often
> use two- or three-pole capacitance multipliers on the supply rails of
> discrete circuitry, which is a similar idea.)

>
> Cheers
>
> Phil Hobbs
>

--
Cheers,
Tony Stewart N. Of Toronto
EE since 1975 practising retirement since 2004

chuck <donnyduck@gmail.com> wrote:
On 2024-11-18 3:14 p.m., Phil Hobbs wrote:
>
>>
>> Any oscillator with a nonlinear or bilinear gain control element that
>> has to respond during a cycle has to deal with the distortion caused by
>> that element.  OTAs, JFET variable resistors, PIN diode attenuators,
>> Vactrols, light bulbs, and so on, all have that problem.  Tail current
>> sources can avoid it, because you can make them as stiff as you like by
>> cascoding, and filter the control voltage as well as you like.  (I often
>> use two- or three-pole capacitance multipliers on the supply rails of
>> discrete circuitry, which is a similar idea.)
>
>>
>> Cheers
>>
>> Phil Hobbs
>>
>

> The FET regulator is a filtered servo with fast cutoff and very slow
> build up over thousands of cycles. The same is done with other
> non-linear limiters using exponentially orders of magnitude higher
> resistance when regulating so the decay rate is slow and low distortion.
>
> Phil I think you had different assumptions for your au contraire opinion
> on tail currents vs quasi-linear low decay currents when regulated
> effectively. I don't see the difference except a non-linear device
> starts up faster.

You’re apparently missing the distinction between a control element that
has to respond within a cycle, such as a JFET variable resistor used as a
feedback element, and one that just sits at a very slowly varying
operating point, such as a cascoded BJT tail current source with a big
emitter resistor and lots of bypassing on the base.

The first kind gets run through its (inevitably somewhat nonlinear) I-V
curve on every half cycle, regardless of the bandwidth of the control loop.
This contributes an amount of distortion that isn’t improved by narrowing
the loop BW. The second kind’s distortion can be reduced to any desired
degree by careful design.

The active element’s nonlinearity is of the first kind, of course, but
that’s just amplifier design.

Cheers

Phil Hobbs

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

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

[...]
> Any oscillator with a nonlinear or bilinear gain control element that
> has to respond during a cycle has to deal with the distortion caused by
> that element. OTAs, JFET variable resistors, PIN diode attenuators,
> Vactrols, light bulbs, and so on, all have that problem.

Light bulbs and thermistors can have a controlling DC superimposed on a
miniscule signal current, so that the distortion caused by the latter is
negligible.

Another alternative is an indirectly-heated thermistor with a very small
signal current in a large thermistor which is primarily heated by a
separate resistive element. It would be slow to respond, but at 1 Kc/s
and -90 dB distortion, a long response time is essential to avoid
distortion from the amplitude-settling transient.

--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
> [...]
>> Any oscillator with a nonlinear or bilinear gain control element that
>> has to respond during a cycle has to deal with the distortion caused by
>> that element. OTAs, JFET variable resistors, PIN diode attenuators,
>> Vactrols, light bulbs, and so on, all have that problem.
>
> Light bulbs and thermistors can have a controlling DC superimposed on a
> miniscule signal current, so that the distortion caused by the latter is
> negligible.
>
> Another alternative is an indirectly-heated thermistor with a very small
> signal current in a large thermistor which is primarily heated by a
> separate resistive element. It would be slow to respond, but at 1 Kc/s
> and -90 dB distortion, a long response time is essential to avoid
> distortion from the amplitude-settling transient.
>
>

Depending on omega*tau_th, sure. The HP 200 exhibits increasing
second-order distortion at lower frequencies.

Down at -90 dBc, depending on the signal level you might have to worry
about deviations from Ohm’s law in an oxide thermistor. (Metals are pretty
linear, but the carrier density in an oxide is going to be much much
lower.)

Eventually it’s bound to be a tradeoff between distortion and noise.

Cheers

Phil Hobbs

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

On Tue, 19 Nov 2024 18:49:33 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>
>> [...]
>>> Any oscillator with a nonlinear or bilinear gain control element that
>>> has to respond during a cycle has to deal with the distortion caused by
>>> that element. OTAs, JFET variable resistors, PIN diode attenuators,
>>> Vactrols, light bulbs, and so on, all have that problem.
>>
>> Light bulbs and thermistors can have a controlling DC superimposed on a
>> miniscule signal current, so that the distortion caused by the latter is
>> negligible.
>>
>> Another alternative is an indirectly-heated thermistor with a very small
>> signal current in a large thermistor which is primarily heated by a
>> separate resistive element. It would be slow to respond, but at 1 Kc/s
>> and -90 dB distortion, a long response time is essential to avoid
>> distortion from the amplitude-settling transient.
>>
>>
>
>Depending on omega*tau_th, sure. The HP 200 exhibits increasing
>second-order distortion at lower frequencies.
>
>Down at -90 dBc, depending on the signal level you might have to worry
>about deviations from Ohm’s law in an oxide thermistor. (Metals are pretty
>linear, but the carrier density in an oxide is going to be much much
>lower.)
>
>Eventually it’s bound to be a tradeoff between distortion and noise.
>
>Cheers
>
>Phil Hobbs

Use the opamp dual-integrator sort of oscillator with a loop gain of
1.01, and give the (always nonlinear) variable-gain element 2%
influence.

In the old HP Wein bridge oscillators, the light bulb had a huge
influence on gain.

john larkin <jl@glen--canyon.com> wrote:
> On Tue, 19 Nov 2024 18:49:33 -0000 (UTC), Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
>>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>
>>> [...]
>>>> Any oscillator with a nonlinear or bilinear gain control element that
>>>> has to respond during a cycle has to deal with the distortion caused by
>>>> that element. OTAs, JFET variable resistors, PIN diode attenuators,
>>>> Vactrols, light bulbs, and so on, all have that problem.
>>>
>>> Light bulbs and thermistors can have a controlling DC superimposed on a
>>> miniscule signal current, so that the distortion caused by the latter is
>>> negligible.
>>>
>>> Another alternative is an indirectly-heated thermistor with a very small
>>> signal current in a large thermistor which is primarily heated by a
>>> separate resistive element. It would be slow to respond, but at 1 Kc/s
>>> and -90 dB distortion, a long response time is essential to avoid
>>> distortion from the amplitude-settling transient.
>>>
>>>
>>
>> Depending on omega*tau_th, sure. The HP 200 exhibits increasing
>> second-order distortion at lower frequencies.
>>
>> Down at -90 dBc, depending on the signal level you might have to worry
>> about deviations from Ohm’s law in an oxide thermistor. (Metals are pretty
>> linear, but the carrier density in an oxide is going to be much much
>> lower.)
>>
>> Eventually it’s bound to be a tradeoff between distortion and noise.
>>
>> Cheers
>>
>> Phil Hobbs
>
> Use the opamp dual-integrator sort of oscillator with a loop gain of
> 1.01, and give the (always nonlinear) variable-gain element 2%
> influence.
>
> In the old HP Wein bridge oscillators, the light bulb had a huge
> influence on gain.

Yup.

Vernier adjustments to a nearly perfect basic platform are Good Medicine,
very generally.

Cheers

Phil Hobbs

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

Phil with respect, again you make false assumptions and conclusions.
There is no distortion above -40dB with 5mV of gate modulation and less
than -60 dB with 0.5mV of gate ripple with a 1s gate time constant.

The JET is very linear over a small voltage range of 0.5 mV and current
range when the Vin+ node is ground referenced so the JFET modulates the
inverting gain limited by a series R.

Get it?

Tony

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On Fri, 22 Nov 2024 02:20:04 -0500, chuck <donnyduck@gmail.com> wrote:

>Phil with respect, again you make false assumptions and conclusions.
>There is no distortion above -40dB with 5mV of gate modulation and less
>than -60 dB with 0.5mV of gate ripple with a 1s gate time constant.
>
>The JET is very linear over a small voltage range of 0.5 mV and current
>range when the Vin+ node is ground referenced so the JFET modulates the
>inverting gain limited by a series R.
>
>Get it?
>
>Tony
>

Yeah, we all know what a doofus Phil is.

https://www.amazon.com/Building-Electro-Optical-Systems-Making-Applied/dp/1119438977

>
>I’ve linked 1 file to this email:
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> Size: 251 KB
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Can't see that. It wants me to sign up or something.

Download limit? Expiry date?

On 11/22/2024 2:20 AM, chuck wrote:
> Phil  with respect, again you make false assumptions and conclusions.

Wait. What? Do you disagree with Phil's statement, which was:
"Vernier adjustments to a nearly perfect basic platform are Good
Medicine, very generally." ???

Or (based on the rest of your post) did you mean to reply to
a different post?

Ed

> There is no distortion above -40dB with 5mV of gate modulation and less
> than -60 dB with 0.5mV of gate ripple with a 1s gate time constant.
>
> The JET is very linear over a small voltage range of 0.5 mV and current
> range when the Vin+ node is ground referenced so the JFET modulates the
> inverting gain limited by a series R.
>
> Get it?
>
> Tony
>
>
> I’ve linked 1 file to this email:
>
>   * wien.jpg
>     Size: 251 KB
>     Expiry Date: 2024-11-23, 02:18 a.m. EST
>     Download Limit: 5
>     Link:
>     https://send.vis.ee/download/0fd1faaa1717c456/#kvtmn8H8J8AEuI9rw3OjPw
>
>