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.
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:Looks like an AC-coupled square wave with a too-short time constant,
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
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
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?
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:Looks like an AC-coupled square wave with a too-short time constant,
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
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
Cursitor Doom <cd999666@notformail.com> wrote:
On Sun, 13 Oct 2024 17:10:30 -0000 (UTC), Cursitor Doom wrote:Looks like an AC-coupled square wave with a too-short time constant,
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
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?
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: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
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
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.
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 <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 DoomIt would have to have failed short-circuit for the gain to be too high,
<cd999666@notformail.com> wrote:
Gentlemen,Maybe the light bulb broke.
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.
and that’s not the usual failure mode.
Cheers
Phil Hobbs
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?
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.
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:Looks like an AC-coupled square wave with a too-short time constant,
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
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.
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 17:58:08 -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:Looks like an AC-coupled square wave with a too-short time constant,
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
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
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.
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.
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.
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/>
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 16/10/2024 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 byOld_STC_Thermistors.pdf?rlkey=jozht29aj1u6ocxnmw8okedrq&st=ohzxz0mw&raw=1>
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/
piglet
On Thu, 17 Oct 2024 11:33:52 +0100, piglet wrote:
On 16/10/2024 3:20 pm, Cursitor Doom wrote:Old_STC_Thermistors.pdf?rlkey=jozht29aj1u6ocxnmw8okedrq&st=ohzxz0mw&raw=1>
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/
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 Thu, 17 Oct 2024 13:35:14 +0100, piglet wrote:
On 17/10/2024 12:28 pm, Cursitor Doom wrote:rlkey=jozht29aj1u6ocxnmw8okedrq&st=ohzxz0mw&raw=1>
On Thu, 17 Oct 2024 11:33:52 +0100, piglet wrote:
On 16/10/2024 3:20 pm, Cursitor Doom wrote:Old_STC_Thermistors.pdf?
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/
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:rlkey=jozht29aj1u6ocxnmw8okedrq&st=ohzxz0mw&raw=1>
On Thu, 17 Oct 2024 11:33:52 +0100, piglet wrote:
On 16/10/2024 3:20 pm, Cursitor Doom wrote:Old_STC_Thermistors.pdf?
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/
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
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.
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
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