Symmetricom/Datum FTS-1050A Disciplined Frequency Standard
Thanks Ed - another thing I'll try.
Right now with the 3V cell in the EFC line the osc output freq is
holding within the limits of not illuminating either the Low or Hi freq
LED's and the actual 10MHz measured on an HP 5335A with a GPS locked
Time Base is showing0.03 of a Hz high
Merv
On 2/09/2022 8:58 am, ed breya via time-nuts wrote:
Since it appears there's no nice and easy way to access the OCXO
coarse tuning cap (there almost certainly is one in there, whether
it's accessible or not), a practical fix is a simple circuit mod (add
one resistor) to the DAC circuit.
I looked up the AD1861, and see that it has the typical built in opamp
and feedback resistor for current to voltage conversion. Also,
apparently, depending on hookup and the data formatting, it can be
arranged to go between +/- 3V output. The simplest way to fix the
output range issue, without changing the basic hookup, any software,
or VCO gain factors, is to just add pure DC offset current at the
summing node.
If you need the output voltage to be more plus, then pull some current
from the summing node into the -5V supply through a resistor. The FS
current from the DAC seems to be around 1-2 mA, so a value for R in
the few to tens of k-ohms range can move it significantly. Just figure
out what the scaling actually is, and set it up for enough offset to
keep the range good for a while - you can adjust again as it ages. Two
things that will be different from original is that the external
offset R won't track the internal feedback R thermally, and using the
minus 5V for a reference may not be as clean as you'd want. The data
sheet I found doesn't say much about the internal reference, but I'm
guessing it's a +1.25 or 2.5V bandgap. Or, it may simply be the +5V
supply.
Since it's overall in a closed loop feedback system, the offset R
tracking is probably irrelevant, but the noise on the -5V may be an
issue, depending on the PLL characteristics. First prove the concept
with what you already have, then refine it as needed.
Good luck.
Ed
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Don't forget that you'll likely need a resonating capacitor to be placed
directly across the loopstick. Every WWVB RX unit I've seen has this,
although there may be other schemes nowadays that don't use it.
If you are hand-winding your own antenna, you can adjust the number of
turns to make it resonate with a particular capacitance. The rather
large fixed capacitor will dominate, but you may want to include the
effects of the wiring or cable capacitance if the antenna is fairly
remote from the RX input.
Ed
Can you open up this antenna/RX unit to inspect the guts? Being an
all-in-one, it must have been sitting outside for years, subject to
environment exposure. You could have some moisture leakage/damage. If
you can look inside, you'll get some idea - it may range from pristine
to a corroded mess. If it looks pretty clean with no obvious issues,
take a very close look at the electrical attachment to the ceramic patch
antenna surface element. This can be a weak spot sometimes.
Ed
Jim, your A18 is likely just fine, so don't rip into it without knowing
more about what's going on. I knew nothing about the 5061A except that
it's a Cs standard. I got curious about all this discussion, so found
the manual and did a quick look-through. It's pretty fascinating and
educational.
Others have already given suggestions that cover most of the pieces.
Here's a quick summary as I see it:
-
A Cs tube that's been inactive for a very long time needs to be
ion-pumped down to proper vacuum level, as indicated by the pump
current. The system protects the hot-wire ionizer and electron
multiplier by disabling their activation until the vacuum level is right. -
The built in 3,500 V ion pump supply only provides enough current to
run this function for "normal" conditions like continuous or
"often-enough" (every 6 months) operation. When the pump current is too
high, the voltage drops off, but it still pumps, just at a lower rate.
The fact that it's drawing (excessive) current on the meter shows that
it must be pumping, unless the current is taking another path due to
some failure. If there is true pump current, it should still eventually
get it down far enough, if the tube is viable. -
The procedure in the manual shows the use of an external HV supply to
relatively quickly get the vacuum into the right range. The recommended
maximum current is 5 mA! I found it kind of surprising to be this high,
but that's the deal. It further says that if the current remains 5 mA or
more after a minute, the tube is likely shot. So, to get an assessment
and recovery in a reasonable time, you need an external HV supply. -
I'd recommend looking for a commercial small HV PS in the 1 to 5 mA
range. A 1 mA one will be much easier and cheaper to find, and should
still get the job done, just slower. I strongly advise against rolling
your own with big iron or big power HV stuff like microwave oven guts,
unless you're thoroughly familiar with how to safely handle and work
with HV. This can be fatal if you screw up badly enough.
A small lab type HV PS can give you a good zap, but you'll live to tell
about it, unless you really really really screw up. Again, smaller is
safer, so a 1 mA or so seems good. Small HV PSs are common, often used
inside equipment, for all sorts of applications, or NIM racks and such.
Preferably you'd want the right polarity (many - especially smaller ones
- are reversible either way), and variable from zero to at least your
needed voltage, and with V and I readouts.
If you want to make one yourself, and learn about HV with reasonable
safety, I'd recommend starting with some small resonant-Royer converter
circuits - these are everywhere, in the form of CCFL tube drivers for
LCD backlights. You should be able to find lots of DIY examples and
parts to inspire.
- Back to the measurements, as has been suggested, you simply must use
a high enough dropping resistance for probing the HV, without
overloading the output. In this case though, you don't really need to
worry about it, since you have a probe built in. The 200 megs of R4
drops the output voltage for the regulator circuit. In conjunction with
the resistors in the bottom of the divider, it appears the feedback
signal is normally +4 V. It's complicated a bit by the current sensing,
but close enough to be a good proxy for the output. The trick is to
measure the feedback with a non-loading voltmeter - or at least one with
very high input resistance like 10 megs or more. 10 megs would give a
few percent error, tops, in both measurement error and possible upset to
the circuit operation. Many lab grade DMMs like HP or Fluke have a
native range over +/-10 V, so can easily look at the 4V signal without
loading.
So, if you properly monitor the feedback in the as-is condition, it will
indicate the very low actual output, divided by about 875 times
(3500/4). Now, if you safely unhook the tube load per the manual, the
feedback should come up to around the proper 4V setpoint, and the output
voltage should be right. If not, then there's more trouble.
Anyway, even if there's still a problem, I wouldn't worry about the A18
yet, and first do the external HV recovery. If it seems to work (low
enough ultimate current), then hook it back to normal and see what
happens. If the current stays very high, then the tube is shot, and
A18's condition is moot.
BTW, you may want to study up on ion pumps in general, to get some idea
of what's going on. I see here that there's a huge possible range - well
over 100 to 1 - of ion current involved, so I'm wondering what ions are
being pumped here, exactly. I assume it's mostly Cs ions that are loose
in the beam-line, where they don't belong. At first I thought the whole
deal was some sort of gettering function, to trap out any bad
contamination ions like in most vacuum tubes, but to take up to 5 mA
initially, I'd say it would have be a pretty crappy vacuum to begin
with. So, I think the ions are the good Cs ones, that just need to be
relocated. The manual probably explains it, but I haven't seen it yet.
Also speaking of ion pumps, long ago I acquired a small one for
developing high vacuum. These kinds of pumps need the same sort of deal
as the tiny one in the Cs tube, just scaled up many times. The main
thing is some form of current limiting (ballast), forming a variable
voltage range depending on the current. I forget the exact numbers, but
mine needed up to 5 kV at maybe 15-25 mA maximum (high vacuum, medium
pumping rate - the current gradually drops as particles are cleared),
and maybe 150 mA from 1 kV to dead short. This is where big iron is
ideal. I used a microwave oven transformer and voltage multiplier, and a
big ballast choke and resonator capacitor on the primary. After much
fine tuning of the basic circuits and parts, I got it to nicely match
the desired I-V curve of the commercial unit normally used.
Ed
On further study of the manual and ion pump, I see that the A18 is a
canned module, so you'd risk some damage opening it up and restoring it.
The description also says the 3,500 volts is set by a resistor inside,
controlling the blocking oscillator. So, it's actually not in a closed
regulator loop, just a fixed circuit that makes about the right voltage
when operation is in the normal range. So, the 200 meg R4 is kind of a
load and divider, but not for voltage regulation.
I think it can still be used to monitor the output voltage, but not with
high accuracy, which doesn't seem all that important now.
Anyway, since you'd have to wreck it to work on it, cutting it open
should only be the last resort. Treating it like a simple black box, it
should be fairly easy to isolate it from the system and see what it does
with external measurements.
Ed
Ed,
Thanks for all the info. Indeed, the old HP manuals are really good and
informative. Your comment led me to the A18 schematic - I assumed it
would just be a block in the schematic, but alas, HP does it right and
shows the schematic of a "Non Repairable" hermetically sealed unit!
I did rip into the supply already, but I've found no smoking gun. All
components are in tolerance. (though I understand it may fail under high
voltage) It opened up easily with some careful propane torch
application. No damage!
The 10M input to the DMM is too low, according to one poster. I built a
182MOhm divider and still no luck.
I don't know much about Ion pumping, but there's a few guys here at NRAO
that have had experience. Back in the 70's, Green Bank had some dewars
with ion pumps, but all that now has been superseded by turbo pumps.
This little project is just a fun distraction from my real job, though
it would be nice to have this in the lab, it's totally not necessary for
local oscillator development. It may help our grad student if we can
get a mating pair of standards. I guess the joke about have one or two
clocks may apply here.
I've been offered an A18 for a good price, which, given the labor I've
invested so far, is a distinct possibility. (Thanks Corby!)
Anyway, I can't stand to see this stuff tossed out. It is a high
precision tube and it has a 90's date code. I wish knew more, but
that's it. They should have had a running time meter on these things!
(coming soon - free 5061 parts!!)
Thanks,
Jim
On 11/15/2022 5:14 PM, ed breya via time-nuts wrote:
Jim, your A18 is likely just fine, so don't rip into it without
knowing more about what's going on. I knew nothing about the 5061A
except that it's a Cs standard. I got curious about all this
discussion, so found the manual and did a quick look-through. It's
pretty fascinating and educational.
Others have already given suggestions that cover most of the pieces.
Here's a quick summary as I see it:
-
A Cs tube that's been inactive for a very long time needs to be
ion-pumped down to proper vacuum level, as indicated by the pump
current. The system protects the hot-wire ionizer and electron
multiplier by disabling their activation until the vacuum level is right. -
The built in 3,500 V ion pump supply only provides enough current
to run this function for "normal" conditions like continuous or
"often-enough" (every 6 months) operation. When the pump current is
too high, the voltage drops off, but it still pumps, just at a lower
rate. The fact that it's drawing (excessive) current on the meter
shows that it must be pumping, unless the current is taking another
path due to some failure. If there is true pump current, it should
still eventually get it down far enough, if the tube is viable. -
The procedure in the manual shows the use of an external HV supply
to relatively quickly get the vacuum into the right range. The
recommended maximum current is 5 mA! I found it kind of surprising to
be this high, but that's the deal. It further says that if the current
remains 5 mA or more after a minute, the tube is likely shot. So, to
get an assessment and recovery in a reasonable time, you need an
external HV supply. -
I'd recommend looking for a commercial small HV PS in the 1 to 5 mA
range. A 1 mA one will be much easier and cheaper to find, and should
still get the job done, just slower. I strongly advise against rolling
your own with big iron or big power HV stuff like microwave oven guts,
unless you're thoroughly familiar with how to safely handle and work
with HV. This can be fatal if you screw up badly enough.
A small lab type HV PS can give you a good zap, but you'll live to
tell about it, unless you really really really screw up. Again,
smaller is safer, so a 1 mA or so seems good. Small HV PSs are common,
often used inside equipment, for all sorts of applications, or NIM
racks and such. Preferably you'd want the right polarity (many -
especially smaller ones - are reversible either way), and variable
from zero to at least your needed voltage, and with V and I readouts.
If you want to make one yourself, and learn about HV with reasonable
safety, I'd recommend starting with some small resonant-Royer
converter circuits - these are everywhere, in the form of CCFL tube
drivers for LCD backlights. You should be able to find lots of DIY
examples and parts to inspire.
- Back to the measurements, as has been suggested, you simply must
use a high enough dropping resistance for probing the HV, without
overloading the output. In this case though, you don't really need to
worry about it, since you have a probe built in. The 200 megs of R4
drops the output voltage for the regulator circuit. In conjunction
with the resistors in the bottom of the divider, it appears the
feedback signal is normally +4 V. It's complicated a bit by the
current sensing, but close enough to be a good proxy for the output.
The trick is to measure the feedback with a non-loading voltmeter - or
at least one with very high input resistance like 10 megs or more. 10
megs would give a few percent error, tops, in both measurement error
and possible upset to the circuit operation. Many lab grade DMMs like
HP or Fluke have a native range over +/-10 V, so can easily look at
the 4V signal without loading.
So, if you properly monitor the feedback in the as-is condition, it
will indicate the very low actual output, divided by about 875 times
(3500/4). Now, if you safely unhook the tube load per the manual, the
feedback should come up to around the proper 4V setpoint, and the
output voltage should be right. If not, then there's more trouble.
Anyway, even if there's still a problem, I wouldn't worry about the
A18 yet, and first do the external HV recovery. If it seems to work
(low enough ultimate current), then hook it back to normal and see
what happens. If the current stays very high, then the tube is shot,
and A18's condition is moot.
BTW, you may want to study up on ion pumps in general, to get some
idea of what's going on. I see here that there's a huge possible range
- well over 100 to 1 - of ion current involved, so I'm wondering what
ions are being pumped here, exactly. I assume it's mostly Cs ions that
are loose in the beam-line, where they don't belong. At first I
thought the whole deal was some sort of gettering function, to trap
out any bad contamination ions like in most vacuum tubes, but to take
up to 5 mA initially, I'd say it would have be a pretty crappy vacuum
to begin with. So, I think the ions are the good Cs ones, that just
need to be relocated. The manual probably explains it, but I haven't
seen it yet.
Also speaking of ion pumps, long ago I acquired a small one for
developing high vacuum. These kinds of pumps need the same sort of
deal as the tiny one in the Cs tube, just scaled up many times. The
main thing is some form of current limiting (ballast), forming a
variable voltage range depending on the current. I forget the exact
numbers, but mine needed up to 5 kV at maybe 15-25 mA maximum (high
vacuum, medium pumping rate - the current gradually drops as particles
are cleared), and maybe 150 mA from 1 kV to dead short. This is where
big iron is ideal. I used a microwave oven transformer and voltage
multiplier, and a big ballast choke and resonator capacitor on the
primary. After much fine tuning of the basic circuits and parts, I got
it to nicely match the desired I-V curve of the commercial unit
normally used.
Ed
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
--
Jim Muehlberg
Senior Engineer
National Radio Astronomy Observatory
ngVLA Local Oscillator Lead
1180 Boxwood Estates Rd B-111
Charlottesville, VA 22903-4602
P 434.296.0270
F 434.296.0324
www.cv.nrao.edu/~jmuehlbe
Jim
Is the unit an option 004 tube? If so they ran the CS oven hotter for a
better signal to noise. That consumed the Cs faster. In my small
experience 004 tubes do seem depleted. Non-004s may have life in them and
the unit I call Frankenstein uses a DC oven and runs at a higher temp.
Boiling off the fumes. It still locks quite well even though I don't see
any beam current. Sort of can by doing unnatural things.
Regards
Paul
WB8TSL
On Tue, Nov 15, 2022 at 8:57 PM Jim Muehlberg via time-nuts <
time-nuts@lists.febo.com> wrote:
Ed,
Thanks for all the info. Indeed, the old HP manuals are really good and
informative. Your comment led me to the A18 schematic - I assumed it
would just be a block in the schematic, but alas, HP does it right and
shows the schematic of a "Non Repairable" hermetically sealed unit!
I did rip into the supply already, but I've found no smoking gun. All
components are in tolerance. (though I understand it may fail under high
voltage) It opened up easily with some careful propane torch
application. No damage!
The 10M input to the DMM is too low, according to one poster. I built a
182MOhm divider and still no luck.
I don't know much about Ion pumping, but there's a few guys here at NRAO
that have had experience. Back in the 70's, Green Bank had some dewars
with ion pumps, but all that now has been superseded by turbo pumps.
This little project is just a fun distraction from my real job, though
it would be nice to have this in the lab, it's totally not necessary for
local oscillator development. It may help our grad student if we can
get a mating pair of standards. I guess the joke about have one or two
clocks may apply here.
I've been offered an A18 for a good price, which, given the labor I've
invested so far, is a distinct possibility. (Thanks Corby!)
Anyway, I can't stand to see this stuff tossed out. It is a high
precision tube and it has a 90's date code. I wish knew more, but
that's it. They should have had a running time meter on these things!
(coming soon - free 5061 parts!!)
Thanks,
Jim
On 11/15/2022 5:14 PM, ed breya via time-nuts wrote:
Jim, your A18 is likely just fine, so don't rip into it without
knowing more about what's going on. I knew nothing about the 5061A
except that it's a Cs standard. I got curious about all this
discussion, so found the manual and did a quick look-through. It's
pretty fascinating and educational.
Others have already given suggestions that cover most of the pieces.
Here's a quick summary as I see it:
-
A Cs tube that's been inactive for a very long time needs to be
ion-pumped down to proper vacuum level, as indicated by the pump
current. The system protects the hot-wire ionizer and electron
multiplier by disabling their activation until the vacuum level is right. -
The built in 3,500 V ion pump supply only provides enough current
to run this function for "normal" conditions like continuous or
"often-enough" (every 6 months) operation. When the pump current is
too high, the voltage drops off, but it still pumps, just at a lower
rate. The fact that it's drawing (excessive) current on the meter
shows that it must be pumping, unless the current is taking another
path due to some failure. If there is true pump current, it should
still eventually get it down far enough, if the tube is viable. -
The procedure in the manual shows the use of an external HV supply
to relatively quickly get the vacuum into the right range. The
recommended maximum current is 5 mA! I found it kind of surprising to
be this high, but that's the deal. It further says that if the current
remains 5 mA or more after a minute, the tube is likely shot. So, to
get an assessment and recovery in a reasonable time, you need an
external HV supply. -
I'd recommend looking for a commercial small HV PS in the 1 to 5 mA
range. A 1 mA one will be much easier and cheaper to find, and should
still get the job done, just slower. I strongly advise against rolling
your own with big iron or big power HV stuff like microwave oven guts,
unless you're thoroughly familiar with how to safely handle and work
with HV. This can be fatal if you screw up badly enough.
A small lab type HV PS can give you a good zap, but you'll live to
tell about it, unless you really really really screw up. Again,
smaller is safer, so a 1 mA or so seems good. Small HV PSs are common,
often used inside equipment, for all sorts of applications, or NIM
racks and such. Preferably you'd want the right polarity (many -
especially smaller ones - are reversible either way), and variable
from zero to at least your needed voltage, and with V and I readouts.
If you want to make one yourself, and learn about HV with reasonable
safety, I'd recommend starting with some small resonant-Royer
converter circuits - these are everywhere, in the form of CCFL tube
drivers for LCD backlights. You should be able to find lots of DIY
examples and parts to inspire.
- Back to the measurements, as has been suggested, you simply must
use a high enough dropping resistance for probing the HV, without
overloading the output. In this case though, you don't really need to
worry about it, since you have a probe built in. The 200 megs of R4
drops the output voltage for the regulator circuit. In conjunction
with the resistors in the bottom of the divider, it appears the
feedback signal is normally +4 V. It's complicated a bit by the
current sensing, but close enough to be a good proxy for the output.
The trick is to measure the feedback with a non-loading voltmeter - or
at least one with very high input resistance like 10 megs or more. 10
megs would give a few percent error, tops, in both measurement error
and possible upset to the circuit operation. Many lab grade DMMs like
HP or Fluke have a native range over +/-10 V, so can easily look at
the 4V signal without loading.
So, if you properly monitor the feedback in the as-is condition, it
will indicate the very low actual output, divided by about 875 times
(3500/4). Now, if you safely unhook the tube load per the manual, the
feedback should come up to around the proper 4V setpoint, and the
output voltage should be right. If not, then there's more trouble.
Anyway, even if there's still a problem, I wouldn't worry about the
A18 yet, and first do the external HV recovery. If it seems to work
(low enough ultimate current), then hook it back to normal and see
what happens. If the current stays very high, then the tube is shot,
and A18's condition is moot.
BTW, you may want to study up on ion pumps in general, to get some
idea of what's going on. I see here that there's a huge possible range
- well over 100 to 1 - of ion current involved, so I'm wondering what
ions are being pumped here, exactly. I assume it's mostly Cs ions that
are loose in the beam-line, where they don't belong. At first I
thought the whole deal was some sort of gettering function, to trap
out any bad contamination ions like in most vacuum tubes, but to take
up to 5 mA initially, I'd say it would have be a pretty crappy vacuum
to begin with. So, I think the ions are the good Cs ones, that just
need to be relocated. The manual probably explains it, but I haven't
seen it yet.
Also speaking of ion pumps, long ago I acquired a small one for
developing high vacuum. These kinds of pumps need the same sort of
deal as the tiny one in the Cs tube, just scaled up many times. The
main thing is some form of current limiting (ballast), forming a
variable voltage range depending on the current. I forget the exact
numbers, but mine needed up to 5 kV at maybe 15-25 mA maximum (high
vacuum, medium pumping rate - the current gradually drops as particles
are cleared), and maybe 150 mA from 1 kV to dead short. This is where
big iron is ideal. I used a microwave oven transformer and voltage
multiplier, and a big ballast choke and resonator capacitor on the
primary. After much fine tuning of the basic circuits and parts, I got
it to nicely match the desired I-V curve of the commercial unit
normally used.
Ed
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
--
Jim Muehlberg
Senior Engineer
National Radio Astronomy Observatory
ngVLA Local Oscillator Lead
1180 Boxwood Estates Rd B-111
Charlottesville, VA 22903-4602
P 434.296.0270
F 434.296.0324
www.cv.nrao.edu/~jmuehlbe
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Hi
The high performance tubes have a life of about 6.0 +/- 1.0 years when run
24/7/365. We went through several running them that way at work. You pay
a run time “price” for the higher performance.
With a date code from the 90’s, there’s been 20 to 30 years passed by to use
up those 6 years of run time.
The standard tubes have a much longer life.
Bob
On Nov 15, 2022, at 8:53 PM, Jim Muehlberg via time-nuts time-nuts@lists.febo.com wrote:
Ed,
Thanks for all the info. Indeed, the old HP manuals are really good and informative. Your comment led me to the A18 schematic - I assumed it would just be a block in the schematic, but alas, HP does it right and shows the schematic of a "Non Repairable" hermetically sealed unit!
I did rip into the supply already, but I've found no smoking gun. All components are in tolerance. (though I understand it may fail under high voltage) It opened up easily with some careful propane torch application. No damage!
The 10M input to the DMM is too low, according to one poster. I built a 182MOhm divider and still no luck.
I don't know much about Ion pumping, but there's a few guys here at NRAO that have had experience. Back in the 70's, Green Bank had some dewars with ion pumps, but all that now has been superseded by turbo pumps.
This little project is just a fun distraction from my real job, though it would be nice to have this in the lab, it's totally not necessary for local oscillator development. It may help our grad student if we can get a mating pair of standards. I guess the joke about have one or two clocks may apply here.
I've been offered an A18 for a good price, which, given the labor I've invested so far, is a distinct possibility. (Thanks Corby!)
Anyway, I can't stand to see this stuff tossed out. It is a high precision tube and it has a 90's date code. I wish knew more, but that's it. They should have had a running time meter on these things!
(coming soon - free 5061 parts!!)
Thanks,
Jim
On 11/15/2022 5:14 PM, ed breya via time-nuts wrote:
Jim, your A18 is likely just fine, so don't rip into it without knowing more about what's going on. I knew nothing about the 5061A except that it's a Cs standard. I got curious about all this discussion, so found the manual and did a quick look-through. It's pretty fascinating and educational.
Others have already given suggestions that cover most of the pieces. Here's a quick summary as I see it:
-
A Cs tube that's been inactive for a very long time needs to be ion-pumped down to proper vacuum level, as indicated by the pump current. The system protects the hot-wire ionizer and electron multiplier by disabling their activation until the vacuum level is right.
-
The built in 3,500 V ion pump supply only provides enough current to run this function for "normal" conditions like continuous or "often-enough" (every 6 months) operation. When the pump current is too high, the voltage drops off, but it still pumps, just at a lower rate. The fact that it's drawing (excessive) current on the meter shows that it must be pumping, unless the current is taking another path due to some failure. If there is true pump current, it should still eventually get it down far enough, if the tube is viable.
-
The procedure in the manual shows the use of an external HV supply to relatively quickly get the vacuum into the right range. The recommended maximum current is 5 mA! I found it kind of surprising to be this high, but that's the deal. It further says that if the current remains 5 mA or more after a minute, the tube is likely shot. So, to get an assessment and recovery in a reasonable time, you need an external HV supply.
-
I'd recommend looking for a commercial small HV PS in the 1 to 5 mA range. A 1 mA one will be much easier and cheaper to find, and should still get the job done, just slower. I strongly advise against rolling your own with big iron or big power HV stuff like microwave oven guts, unless you're thoroughly familiar with how to safely handle and work with HV. This can be fatal if you screw up badly enough.
A small lab type HV PS can give you a good zap, but you'll live to tell about it, unless you really really really screw up. Again, smaller is safer, so a 1 mA or so seems good. Small HV PSs are common, often used inside equipment, for all sorts of applications, or NIM racks and such. Preferably you'd want the right polarity (many - especially smaller ones - are reversible either way), and variable from zero to at least your needed voltage, and with V and I readouts.
If you want to make one yourself, and learn about HV with reasonable safety, I'd recommend starting with some small resonant-Royer converter circuits - these are everywhere, in the form of CCFL tube drivers for LCD backlights. You should be able to find lots of DIY examples and parts to inspire.
- Back to the measurements, as has been suggested, you simply must use a high enough dropping resistance for probing the HV, without overloading the output. In this case though, you don't really need to worry about it, since you have a probe built in. The 200 megs of R4 drops the output voltage for the regulator circuit. In conjunction with the resistors in the bottom of the divider, it appears the feedback signal is normally +4 V. It's complicated a bit by the current sensing, but close enough to be a good proxy for the output. The trick is to measure the feedback with a non-loading voltmeter - or at least one with very high input resistance like 10 megs or more. 10 megs would give a few percent error, tops, in both measurement error and possible upset to the circuit operation. Many lab grade DMMs like HP or Fluke have a native range over +/-10 V, so can easily look at the 4V signal without loading.
So, if you properly monitor the feedback in the as-is condition, it will indicate the very low actual output, divided by about 875 times (3500/4). Now, if you safely unhook the tube load per the manual, the feedback should come up to around the proper 4V setpoint, and the output voltage should be right. If not, then there's more trouble.
Anyway, even if there's still a problem, I wouldn't worry about the A18 yet, and first do the external HV recovery. If it seems to work (low enough ultimate current), then hook it back to normal and see what happens. If the current stays very high, then the tube is shot, and A18's condition is moot.
BTW, you may want to study up on ion pumps in general, to get some idea of what's going on. I see here that there's a huge possible range - well over 100 to 1 - of ion current involved, so I'm wondering what ions are being pumped here, exactly. I assume it's mostly Cs ions that are loose in the beam-line, where they don't belong. At first I thought the whole deal was some sort of gettering function, to trap out any bad contamination ions like in most vacuum tubes, but to take up to 5 mA initially, I'd say it would have be a pretty crappy vacuum to begin with. So, I think the ions are the good Cs ones, that just need to be relocated. The manual probably explains it, but I haven't seen it yet.
Also speaking of ion pumps, long ago I acquired a small one for developing high vacuum. These kinds of pumps need the same sort of deal as the tiny one in the Cs tube, just scaled up many times. The main thing is some form of current limiting (ballast), forming a variable voltage range depending on the current. I forget the exact numbers, but mine needed up to 5 kV at maybe 15-25 mA maximum (high vacuum, medium pumping rate - the current gradually drops as particles are cleared), and maybe 150 mA from 1 kV to dead short. This is where big iron is ideal. I used a microwave oven transformer and voltage multiplier, and a big ballast choke and resonator capacitor on the primary. After much fine tuning of the basic circuits and parts, I got it to nicely match the desired I-V curve of the commercial unit normally used.
Ed
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Great progress. If the pin 4 goes to near zero when the tube is active,
then the ion current is high enough to cancel the test point voltage.
Since you have the A18 can open, if you can now measure the voltage from
pin 4 to the bottom of R5 (instead of ground), you should get an idea of
the actual loaded HV out in this condition. Neglecting the tolerances of
(especially) R4 and R5, the true HV out should be about 906 times the
pin 4 voltage, if measured with a non-loading meter, or 926 times, using
a 10 meg input R meter.
If you measure from the bottom of R5 to ground, you'll see the ion
current signal. This negative voltage should be roughly the current
times the sum of 75 k ohms (R6) and the resistance of the 10 k ohm (R7)
parallel the monitoring circuit (where the "ION PUMP I" line goes). You
can get an even better estimate of the HV out by adding (subtract the
negative) this voltage, but just the simple look should be plenty good
enough.
Ed
Oops - forgot about the other load on pin 4, which is A15 Q10's base
current. It should be fairly small, in the uA region, but still a
relatively big effect here, causing the HV estimate to appear higher
than actual.
Ed