Ancient OCXO in scope calibrator.
I'm having some problems with an ancient Bulova OCXO in a Tektronix 184
'scope calibrator.
This is an crystal mounted on an octal valve base with a heater winding
wrapped round it, and a common or garden variety thermostat which is
supposed to switch at 75 celsius. I've not measured the switching
temperature, as I don't have the means but the outer case does get fairly
warm (40 plus degrees?). The oven is turning on and off.
It is used in a circuit with a 7587 Nuvistor tetrode (yes it's a valve/tube
circuit). The crystal is connected cathode to G1 with a trimmer capacitor
of 3-12pF. The signal at the cathode is supposed to be about 70V p-p.
I'm measuring the frequency of the signal after the transformer stage which
couples it to the first stage of a countdown board.
For the first minute or so after turn on from cold, it sits below 10MHz and
is fairly stable and climbing as the oven warms up, and I can adjust the
frequency up towards 10MHz with the capacitor (but not all the way), then
suddenly, at the stage where it is starting to look as if it will soon
stabilise at about the right frequency, it jumps to way over 10MHz and the
lowest I can get it down to with the capacitor is about 10.0003xxx MHz where
xxx is not very stable at all - in fact it can vary up to to 10.0005xxx and
down to 10.0002xxx.
I've tried freeze spray on most of the components round there to no effect.
If I try to probe the signal at the cathode of the nuvistor even with a high
impedance active probe with a P6201 with a 100x attenuator (about 1pF
loading IIRC) the oscillation just drops dead.
Now for calibrating 'scopes, it doesn't need to be any more accurate than it
is (30ppm) - but ...
Do any of the collected mavens have an explanation for the behaviour, and
recommendations for fixing things?
Cheers
Dave
Oops I think its about 300ppm there...
D.
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of David C. Partridge
Sent: 07 September 2008 14:56
To: 'Discussion of precise time and frequency measurement'
Subject: [time-nuts] Ancient OCXO in scope calibrator.
I'm having some problems with an ancient Bulova OCXO in a Tektronix 184
'scope calibrator.
This is an crystal mounted on an octal valve base with a heater winding
wrapped round it, and a common or garden variety thermostat which is
supposed to switch at 75 celsius. I've not measured the switching
temperature, as I don't have the means but the outer case does get fairly
warm (40 plus degrees?). The oven is turning on and off.
It is used in a circuit with a 7587 Nuvistor tetrode (yes it's a valve/tube
circuit). The crystal is connected cathode to G1 with a trimmer capacitor
of 3-12pF. The signal at the cathode is supposed to be about 70V p-p.
I'm measuring the frequency of the signal after the transformer stage which
couples it to the first stage of a countdown board.
For the first minute or so after turn on from cold, it sits below 10MHz and
is fairly stable and climbing as the oven warms up, and I can adjust the
frequency up towards 10MHz with the capacitor (but not all the way), then
suddenly, at the stage where it is starting to look as if it will soon
stabilise at about the right frequency, it jumps to way over 10MHz and the
lowest I can get it down to with the capacitor is about 10.0003xxx MHz where
xxx is not very stable at all - in fact it can vary up to to 10.0005xxx and
down to 10.0002xxx.
I've tried freeze spray on most of the components round there to no effect.
If I try to probe the signal at the cathode of the nuvistor even with a high
impedance active probe with a P6201 with a 100x attenuator (about 1pF
loading IIRC) the oscillation just drops dead.
Now for calibrating 'scopes, it doesn't need to be any more accurate than it
is (30ppm) - but ...
Do any of the collected mavens have an explanation for the behaviour, and
recommendations for fixing things?
Cheers
Dave
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
David C. Partridge wrote:
I'm having some problems with an ancient Bulova OCXO in a Tektronix 184
'scope calibrator.
This is an crystal mounted on an octal valve base with a heater winding
wrapped round it, and a common or garden variety thermostat which is
supposed to switch at 75 celsius. I've not measured the switching
temperature, as I don't have the means but the outer case does get fairly
warm (40 plus degrees?). The oven is turning on and off.
It is used in a circuit with a 7587 Nuvistor tetrode (yes it's a valve/tube
circuit). The crystal is connected cathode to G1 with a trimmer capacitor
of 3-12pF. The signal at the cathode is supposed to be about 70V p-p.
I'm measuring the frequency of the signal after the transformer stage which
couples it to the first stage of a countdown board.
For the first minute or so after turn on from cold, it sits below 10MHz and
is fairly stable and climbing as the oven warms up, and I can adjust the
frequency up towards 10MHz with the capacitor (but not all the way), then
suddenly, at the stage where it is starting to look as if it will soon
stabilise at about the right frequency, it jumps to way over 10MHz and the
lowest I can get it down to with the capacitor is about 10.0003xxx MHz where
xxx is not very stable at all - in fact it can vary up to to 10.0005xxx and
down to 10.0002xxx.
I've tried freeze spray on most of the components round there to no effect.
If I try to probe the signal at the cathode of the nuvistor even with a high
impedance active probe with a P6201 with a 100x attenuator (about 1pF
loading IIRC) the oscillation just drops dead.
Now for calibrating 'scopes, it doesn't need to be any more accurate than it
is (30ppm) - but ...
Do any of the collected mavens have an explanation for the behaviour, and
recommendations for fixing things?
Cheers
Dave
Dave
Need a few more circuit details.
The crystal current wont be very high unless their are other components
connected to G1.
Bruce
Dave
Need a few more circuit details.
The crystal current wont be very high unless their are other components
connected to G1.
Bruce
The anode G1 capacitance is about 10fF so with 25V rms at the anode, the
crystal current would be around 16uA if the circuit relies on feedback
via the anode to grid 1 capacitance.
This seems to be a little low to be useful.
You could try substituting a newer 7587 there are several currently
available on ebay and elsewhere.
Bruce
Dave, Bruce,
Need a few more circuit details.
The crystal current wont be very high unless their are other components
connected to G1.
As expected, BAMA can assist:
http://bama.edebris.com/manuals/tek/184/
I am thinking about undamped overtones/other modes of the crystal and
similar.
Cheers,
Magnus
Magnus Danielson wrote:
Dave, Bruce,
Need a few more circuit details.
The crystal current wont be very high unless their are other components
connected to G1.
As expected, BAMA can assist:
http://bama.edebris.com/manuals/tek/184/
I am thinking about undamped overtones/other modes of the crystal and
similar.
Cheers,
Magnus
The crystal oscillator is in effect a miller oscillator using the screen
grid as the oscillator anode/plate see:
http://www.ieee-uffc.org/freqcontrol/tutorials/s6310508.pdf
The oscillator plate/anode (screen grid is grounded) in this circuit.
The tube plate/anode forms a well isolated auxiliary output tuned to the
5th harmonic of the crystal.
If the tube emission has dropped the gm may have fallen rather too close
to the critical value required for oscillation making the circuit
sensitive to loading at the grid.
Both the trimmer and the cathode tank slug affect the frequency.
Bruce
Nuvistor is NOS, and I've tried swapping them.
Circuit has anode (plate) connected to a 50MHz tuned tank.
G1 is connected to ground via a 100K resistor, and to the cathode via the
crystal and trimmer cap.
Cathode connected to ground via a ferrite tunable slug inductor (6-10uH)
close to a similar one also 6-10uH. The second one has 100K in parallel
with 1nF to ground, and 15pF across the inductor. The second one has a
pick off coil of a couple of turns which sends the signal to the countdown
board.
The full schematic is in the Tektronix 184 manual on BAMA which is in djvu
format.
Cheers
Dave
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Bruce Griffiths
Sent: 07 September 2008 23:38
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Ancient OCXO in scope calibrator.
Dave
Need a few more circuit details.
The crystal current wont be very high unless their are other
components connected to G1.
Bruce
The anode G1 capacitance is about 10fF so with 25V rms at the anode, the
crystal current would be around 16uA if the circuit relies on feedback via
the anode to grid 1 capacitance.
This seems to be a little low to be useful.
You could try substituting a newer 7587 there are several currently
available on ebay and elsewhere.
Bruce
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
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Hi Dave:
Do you have a photo?
It's been my experience that older electronic equipment fails because of poor
connections rather than bad components. For many examples see:
http://www.prc68.com/I/HaT.shtml
Have Fun,
Brooke Clarke
http://www.prc68.com/P/Prod.html Products I make and sell
http://www.prc68.com/Alpha.shtml All my web pages listed based on html name
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Web Cam
David C. Partridge wrote:
Nuvistor is NOS, and I've tried swapping them.
Circuit has anode (plate) connected to a 50MHz tuned tank.
G1 is connected to ground via a 100K resistor, and to the cathode via the
crystal and trimmer cap.
Cathode connected to ground via a ferrite tunable slug inductor (6-10uH)
close to a similar one also 6-10uH. The second one has 100K in parallel
with 1nF to ground, and 15pF across the inductor. The second one has a
pick off coil of a couple of turns which sends the signal to the countdown
board.
The full schematic is in the Tektronix 184 manual on BAMA which is in djvu
format.
Cheers
Dave
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Bruce Griffiths
Sent: 07 September 2008 23:38
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Ancient OCXO in scope calibrator.
Dave
Need a few more circuit details.
The crystal current wont be very high unless their are other
components connected to G1.
Bruce
The anode G1 capacitance is about 10fF so with 25V rms at the anode, the
crystal current would be around 16uA if the circuit relies on feedback via
the anode to grid 1 capacitance.
This seems to be a little low to be useful.
You could try substituting a newer 7587 there are several currently
available on ebay and elsewhere.
Bruce
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and follow the instructions there.
Agree that the tank slug can also affect the frequency slightly.
I've tried a number of 7587s including what I was fairly sure to be NOS
parts - at least they came in the correct box.
I suspect I can pull the frequency a bit by paralleling another 5 or 6pF
across the trimmer (why has the parts box never got what you are looking
for? - this feels like a corollary to Murphy).
The main issue is how unstable it is - maybe I just stop chasing this and
write it down to a very old crystal that's aged out of spec, and just
replace it with a regular 20ppm room temperature part.
Cheers
Dave
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Bruce Griffiths
Sent: 08 September 2008 08:41
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Ancient OCXO in scope calibrator.
Magnus Danielson wrote:
Dave, Bruce,
Need a few more circuit details.
The crystal current wont be very high unless their are other
components connected to G1.
As expected, BAMA can assist:
http://bama.edebris.com/manuals/tek/184/
I am thinking about undamped overtones/other modes of the crystal and
similar.
Cheers,
Magnus
The crystal oscillator is in effect a miller oscillator using the screen
grid as the oscillator anode/plate see:
http://www.ieee-uffc.org/freqcontrol/tutorials/s6310508.pdf
The oscillator plate/anode (screen grid is grounded) in this circuit.
The tube plate/anode forms a well isolated auxiliary output tuned to the 5th
harmonic of the crystal.
If the tube emission has dropped the gm may have fallen rather too close to
the critical value required for oscillation making the circuit sensitive to
loading at the grid.
Both the trimmer and the cathode tank slug affect the frequency.
Bruce
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Brooke Clarke wrote:
Hi Dave:
Do you have a photo?
It's been my experience that older electronic equipment fails because of poor
connections rather than bad components. For many examples see:
http://www.prc68.com/I/HaT.shtml
Have Fun,
Brooke Clarke
Brook,
A little off topic from the original thread, but on your referenced
page, in the "Dead CRT Computer Monitor" section, the blown device is
probably a TVS (Transient Voltage Suppressor). My first thought was that
it was a MOV (Metal Oxide Varistor) but the symbol was different. With a
bit of Google, I found this page:
http://books.google.com/books?id=4SkqFozu80MC&pg=PA122&lpg=PA122&dq=schematic+symbol+MOV&source=web&ots=VydlPTFGNA&sig=A8fKQfYQ3Agb0OU8iEMwM9Zhdsc&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPA122,M1
(Wow! What a URL. Good chance it will wrap in the email. If you can't
copy that, it is page 122 in the google online book "Embedded Systems
Design Using the Rabbit 3000 Microprocessor". Seems like a pretty good
description of voltage suppressors if you scroll back and forth a few
pages.)
So I just thought I'd try to help with the device, as your page still
has it as an open question. Still unknown is the voltage rating and what
caused it to blow. Since you mention it was on the AC mains, there is a
good chance it was just doing what it was supposed to do, and the
voltage should be something a bit greater than the AC input voltage.
-Rex
Rex wrote:
Brooke Clarke wrote:
Hi Dave:
Do you have a photo?
It's been my experience that older electronic equipment fails because of poor
connections rather than bad components. For many examples see:
http://www.prc68.com/I/HaT.shtml
Have Fun,
Brooke Clarke
Brook,
A little off topic from the original thread, but on your referenced
page, in the "Dead CRT Computer Monitor" section, the blown device is
probably a TVS (Transient Voltage Suppressor). My first thought was that
it was a MOV (Metal Oxide Varistor) but the symbol was different. With a
bit of Google, I found this page:
http://books.google.com/books?id=4SkqFozu80MC&pg=PA122&lpg=PA122&dq=schematic+symbol+MOV&source=web&ots=VydlPTFGNA&sig=A8fKQfYQ3Agb0OU8iEMwM9Zhdsc&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPA122,M1
(Wow! What a URL. Good chance it will wrap in the email. If you can't
copy that, it is page 122 in the google online book "Embedded Systems
Design Using the Rabbit 3000 Microprocessor". Seems like a pretty good
description of voltage suppressors if you scroll back and forth a few
pages.)
So I just thought I'd try to help with the device, as your page still
has it as an open question. Still unknown is the voltage rating and what
caused it to blow. Since you mention it was on the AC mains, there is a
good chance it was just doing what it was supposed to do, and the
voltage should be something a bit greater than the AC input voltage.
-Rex
Dang! Glancing again at the page just after I sent, the symbol IS an
alternative form for a MOV, not a TVS. All the rest still applies.
-Rex
Following the good advice I received about increasing the value of the
trimmer capacitor, I replaced the trimmer which was originally 3-12pF, with
another one which was 2-22pF.
This has given the additional adjustment range to allow me to pull the
crystal to meet the specification.
It's not mega-stable, varying between 9.9999619 MHz to 10.0000232 MHz over
the course of a minute or so as the thermostat cycles, but certainly within
the 10ppm specification and varying up and down by about 6-7ppm. This is
before the full 2 hour warm up period is complete, but I don't things will
change much over the next 45 minutes or so. The stability seems to be
improving as the warm up continues which is good.
I may be able to tweak it so the variation is more evenly balanced around
10.0000000 MHz, but the trimmer adjustment is pretty sensitive.
Still don't understand why probing with a 10Meg, 1pF load kills the
oscillation stone dead until the next power cycle.
Thanks to all
Cheers
Dave
Following the good advice I received about increasing the value of the
trimmer capacitor, I replaced the trimmer which was originally
3-12pF, with
another one which was 2-22pF.
This has given the additional adjustment range to allow me to pull the
crystal to meet the specification.
It's not mega-stable, varying between 9.9999619 MHz to 10.0000232 MHz over
the course of a minute or so as the thermostat cycles, but
certainly within
the 10ppm specification and varying up and down by about 6-7ppm. This is
before the full 2 hour warm up period is complete, but I don't things will
change much over the next 45 minutes or so. The stability seems to be
improving as the warm up continues which is good.
I may be able to tweak it so the variation is more evenly balanced around
10.0000000 MHz, but the trimmer adjustment is pretty sensitive.
Still don't understand why probing with a 10Meg, 1pF load kills the
oscillation stone dead until the next power cycle.
You might have an intermittent solder joint or component lead that's being
flexed when you touch various points with the probe. Does a plastic
alignment tool cause the same behavior?
Or, more likely, there's so much loss in the circuit due to aging/component
failure that it won't restart oscillating on its own without going through a
power-up cycle.
-- john, KE5FX
I checked all the solder joints in the area (to the extent of remaking
them). Maybe there's a high resistance joint inside the mess of oxidised
shellac etc inside the OCXO.
Nuvistor is NOS, and I subbed another from my reserves just to be sure
(still in sealed plastic bag with perforated tear off to open) with no
impact on behaviour.
A ceramic alignment tool doesn't cause the same behaviour, but a probe sure
does.
D.
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of John Miles
Sent: 09 September 2008 21:08
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Ancient OCXO in scope calibrator.
You might have an intermittent solder joint or component lead that's being
flexed when you touch various points with the probe. Does a plastic
alignment tool cause the same behavior?
Or, more likely, there's so much loss in the circuit due to aging/component
failure that it won't restart oscillating on its own without going through a
power-up cycle.
-- john, KE5FX
A ceramic alignment tool doesn't cause the same behaviour, but a
probe sure
does.
Something is whispering "grounding".
I'm not sure why, but that is where I would start looking...
Tom Frank, KA2CDK
Thomas A. Frank wrote:
A ceramic alignment tool doesn't cause the same behaviour, but a
probe sure
does.
Something is whispering "grounding".
I'm not sure why, but that is where I would start looking...
Tom Frank, KA2CDK
This particular variant of the miller oscillator uses the screen grid to
grid1 capacitance for feedback.
This capacitance is at most a few pF.
Thus even a shunt capacitance as small as 1pF to ground at the grid will
change the effective the feedback capacitance significantly.
Both the crystal and the cathode load impedance are inductive at the
oscillation frequency.
Bruce
On Tue, 9 Sep 2008 19:19:39 +0100, "David C. Partridge"
david.partridge@dsl.pipex.com wrote:
Following the good advice I received about increasing the value of the
trimmer capacitor, I replaced the trimmer which was originally 3-12pF, with
another one which was 2-22pF.
I would leave the same trimmer in there and shunt it with a fixed 10pF cap.
That way the adjustment won't be so touchy.
Still don't understand why probing with a 10Meg, 1pF load kills the
oscillation stone dead until the next power cycle.
That says that the circuit had just enough gain and positive feed back to
oscillate but just barely. I'd have to see the circuit to recommend changes.
The root cause is probably lost gain in the tube but that can be compensated
for.
I was just sitting here thinking about the oven problem. If you replace the
thermostat with the proper power PTC thermister, you could make the oven
linearly regulate at a single temperature.
John
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
Multitasking: Reading in the bathroom!
Still don't understand why probing with a 10Meg, 1pF load kills the
oscillation stone dead until the next power cycle.
That says that the circuit had just enough gain and positive feed back to
oscillate but just barely. I'd have to see the circuit to
recommend changes.
The root cause is probably lost gain in the tube but that can be
compensated
for.
How's the filament voltage at the 7587 socket, by the way? A shorted turn
in your power transformer might account for some lack of enthusiasm on the
Nuvistor's part.
-- john, KE5FX
A completely unused (had to open the perforations on the plastic bag inside
the box) NOS nuvistor substitution made no difference to the effect of
probing.
D.
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Neon John
Sent: 10 September 2008 01:20
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Ancient OCXO in scope calibrator.
On Tue, 9 Sep 2008 19:19:39 +0100, "David C. Partridge"
david.partridge@dsl.pipex.com wrote:
Following the good advice I received about increasing the value of the
trimmer capacitor, I replaced the trimmer which was originally 3-12pF,
with another one which was 2-22pF.
I would leave the same trimmer in there and shunt it with a fixed 10pF cap.
That way the adjustment won't be so touchy.
Still don't understand why probing with a 10Meg, 1pF load kills the
oscillation stone dead until the next power cycle.
That says that the circuit had just enough gain and positive feed back to
oscillate but just barely. I'd have to see the circuit to recommend
changes.
The root cause is probably lost gain in the tube but that can be compensated
for.
I was just sitting here thinking about the oven problem. If you replace the
thermostat with the proper power PTC thermister, you could make the oven
linearly regulate at a single temperature.
John
John De Armond
See my website for my current email address http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
Multitasking: Reading in the bathroom!
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
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and follow the instructions there.
Now that's an interesting thought.
How should one go about specifying such a part?
Do you have any experience of doing this? We'd be looking at (I guess) a
turnover from almost short circuit to darn near open circuit over a range of
a few degrees centred on 75 celsius. Do such beasties exist?
If it's relevant there are two heater windings, one for 230V, one for 115V
though both do go through the thermostat.
This rather surprised me as I would have expected two windings and only use
one for 115V, and switch the second in series for 230V, but if I'm reading
the circuit diagram right (not g'teed) this isn't what happens (it's on the
last schematic page which is the power supply for those who've donwloaded
the manual from BAMA).
There's also something connected between the two windings which looks a bit
like two cup-hooks - what this is I'm not sure.
I don't recognise the symbol at all, it could just represent the bimetal
strip part of the thermostat.
I also don't quite understand the point of the capacitor at the top end of
the 230V winding.
Cheers
Dave
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Neon John
Sent: 10 September 2008 01:20
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Ancient OCXO in scope calibrator.
I was just sitting here thinking about the oven problem. If you replace the
thermostat with the proper power PTC thermister, you could make the oven
linearly regulate at a single temperature.
John
John De Armond
See my website for my current email address http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
Multitasking: Reading in the bathroom!
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.