Discussion:
Datum PRS-50 ionizer voltage supply
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Dirk Niggemann
2018-11-01 19:56:37 UTC
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Hi,

I acquired a PRS-50 Caesium beam frequency standard in more-or-less working
order back in 2011. The manufacture date was February 2001, so the tube was
probably exhausted by this stage, but it did still achieve lock so may have
been in storage for some time rather than run for the whole of its 10-year
lifespan.

I recovered it from storage a year or so ago to test and it worked,
initially. The power supply was noisy, and the system wouldn't achieve lock
on power-up from cold.

I suspect that that it may have been retired from service originally for
an intermittent PSU failure as it would not find lock on every power up,
rather than an exhausted or failed tube.

Since then the power supply in the 4201A module failed completely. All the
electrolytics in the SMPSU leaked.

I have managed to restore the PSU to the point that the PRS-50 boots and
tries to find a lock, but fails after about 15 minutes with error F3
(ioniser voltage out of spec)

When watching in monitor3, I can see the both the OCXO and the Caesium oven
power and heat up, and i have voltages in spec on all rails, except that
the ioniser voltage remains below 0.1V.

I also see almost no ion pump current. which is consistent with the ioniser
remaining off.

I haven't managed to retrieve the tube constants, but i assume the ioniser
wants about 1V at 1A like most other Caesium beam tubes.

I believe i have also identified the ioniser supply leads from the tube and
these meter out at very low resistance (< 1 ohm), possibly too low (though
how a hot-wire ioniser should fail short escapes me).

Does anybody know how the 4201A module generates the ioniser voltage? It's
likely to be similar to the FTS4065C in that respect. When is the ioniser
meant to turn on in the power-up cycle?

I suspect i'm missing the ioniser supply rail. There's at least 24
unlabelled connections between the SMPSU board and the junction board which
connects both the LV lines for the Cs tube, the main processor board and
the SMPSU. I have no idea which particular connection this could be.

I don't believe Datum/Symmetricom/Microsemi ever published schematics for
this Caesium module so I'm a little at a loss where to go next with
troubleshooting.

I'd like to at least eliminate a failed tube as far as possible, since i
really don't see myself acquiring a replacement tube at list price.

Unfortunately i don't have any test equipment that will work at the
microwave frequencies needed to do a direct test of the tube.

Suggestions? Has anybody ever tried a repair like this before? I'm tempted
to feed an external supply to the ioniser to see what happens, or at least
disconnect the ioniser leads to see if it fails with an ioniser overvoltage
instead.

Regards,

Dirk M0KRD
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Bob kb8tq
2018-11-01 20:10:08 UTC
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Hi

Just as a guess, it would be rare for a “heated wire” type of thing to have much over a
10:1 ratio between hot and cold. Most have ratios that are less. (think of a light bulb …)
If you suspect the ionizer, I’d suggest working out just what it’s resistance is. If it’s down
below 0.1 ohms (and your 1V / 1A guess is correct) then it is shorted. Is it worth trying
something crazy to unshift it? Who knows …..

Bob
Post by Dirk Niggemann
Hi,
I acquired a PRS-50 Caesium beam frequency standard in more-or-less working
order back in 2011. The manufacture date was February 2001, so the tube was
probably exhausted by this stage, but it did still achieve lock so may have
been in storage for some time rather than run for the whole of its 10-year
lifespan.
I recovered it from storage a year or so ago to test and it worked,
initially. The power supply was noisy, and the system wouldn't achieve lock
on power-up from cold.
I suspect that that it may have been retired from service originally for
an intermittent PSU failure as it would not find lock on every power up,
rather than an exhausted or failed tube.
Since then the power supply in the 4201A module failed completely. All the
electrolytics in the SMPSU leaked.
I have managed to restore the PSU to the point that the PRS-50 boots and
tries to find a lock, but fails after about 15 minutes with error F3
(ioniser voltage out of spec)
When watching in monitor3, I can see the both the OCXO and the Caesium oven
power and heat up, and i have voltages in spec on all rails, except that
the ioniser voltage remains below 0.1V.
I also see almost no ion pump current. which is consistent with the ioniser
remaining off.
I haven't managed to retrieve the tube constants, but i assume the ioniser
wants about 1V at 1A like most other Caesium beam tubes.
I believe i have also identified the ioniser supply leads from the tube and
these meter out at very low resistance (< 1 ohm), possibly too low (though
how a hot-wire ioniser should fail short escapes me).
Does anybody know how the 4201A module generates the ioniser voltage? It's
likely to be similar to the FTS4065C in that respect. When is the ioniser
meant to turn on in the power-up cycle?
I suspect i'm missing the ioniser supply rail. There's at least 24
unlabelled connections between the SMPSU board and the junction board which
connects both the LV lines for the Cs tube, the main processor board and
the SMPSU. I have no idea which particular connection this could be.
I don't believe Datum/Symmetricom/Microsemi ever published schematics for
this Caesium module so I'm a little at a loss where to go next with
troubleshooting.
I'd like to at least eliminate a failed tube as far as possible, since i
really don't see myself acquiring a replacement tube at list price.
Unfortunately i don't have any test equipment that will work at the
microwave frequencies needed to do a direct test of the tube.
Suggestions? Has anybody ever tried a repair like this before? I'm tempted
to feed an external supply to the ioniser to see what happens, or at least
disconnect the ioniser leads to see if it fails with an ioniser overvoltage
instead.
Regards,
Dirk M0KRD
_______________________________________________
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paul swed
2018-11-01 21:01:51 UTC
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Dirk
Kind of a good and bad thing. Shame it doesn't work, oh boy now you get to
dig in. Nothing to lose so the fun begins.
I am of the belief ionizers are low ohms. Maybe not that low but low. If
its actually shorted somehow to the case then as Bob suggests you might
float it with an independent supply. Respecting the fact that it might
expect to be at some higher voltage or isolated.
The Ionizers in the HP do essentially float so that a offset voltage can be
introduced.
Others will have far better suggestions and much more knowledge.
anything I ever hear about symetricoms and FTS are that no schematic were
released.
Ah for the oldy but goldy's HPs.
Regards
Paul
WB8TSL
Post by Bob kb8tq
Hi
Just as a guess, it would be rare for a “heated wire” type of thing to have much over a
10:1 ratio between hot and cold. Most have ratios that are less. (think of
a light bulb …)
If you suspect the ionizer, I’d suggest working out just what it’s
resistance is. If it’s down
below 0.1 ohms (and your 1V / 1A guess is correct) then it is shorted. Is it worth trying
something crazy to unshift it? Who knows …..
Bob
Post by Dirk Niggemann
Hi,
I acquired a PRS-50 Caesium beam frequency standard in more-or-less
working
Post by Dirk Niggemann
order back in 2011. The manufacture date was February 2001, so the tube
was
Post by Dirk Niggemann
probably exhausted by this stage, but it did still achieve lock so may
have
Post by Dirk Niggemann
been in storage for some time rather than run for the whole of its
10-year
Post by Dirk Niggemann
lifespan.
I recovered it from storage a year or so ago to test and it worked,
initially. The power supply was noisy, and the system wouldn't achieve
lock
Post by Dirk Niggemann
on power-up from cold.
I suspect that that it may have been retired from service originally for
an intermittent PSU failure as it would not find lock on every power up,
rather than an exhausted or failed tube.
Since then the power supply in the 4201A module failed completely. All
the
Post by Dirk Niggemann
electrolytics in the SMPSU leaked.
I have managed to restore the PSU to the point that the PRS-50 boots and
tries to find a lock, but fails after about 15 minutes with error F3
(ioniser voltage out of spec)
When watching in monitor3, I can see the both the OCXO and the Caesium
oven
Post by Dirk Niggemann
power and heat up, and i have voltages in spec on all rails, except that
the ioniser voltage remains below 0.1V.
I also see almost no ion pump current. which is consistent with the
ioniser
Post by Dirk Niggemann
remaining off.
I haven't managed to retrieve the tube constants, but i assume the
ioniser
Post by Dirk Niggemann
wants about 1V at 1A like most other Caesium beam tubes.
I believe i have also identified the ioniser supply leads from the tube
and
Post by Dirk Niggemann
these meter out at very low resistance (< 1 ohm), possibly too low
(though
Post by Dirk Niggemann
how a hot-wire ioniser should fail short escapes me).
Does anybody know how the 4201A module generates the ioniser voltage?
It's
Post by Dirk Niggemann
likely to be similar to the FTS4065C in that respect. When is the ioniser
meant to turn on in the power-up cycle?
I suspect i'm missing the ioniser supply rail. There's at least 24
unlabelled connections between the SMPSU board and the junction board
which
Post by Dirk Niggemann
connects both the LV lines for the Cs tube, the main processor board and
the SMPSU. I have no idea which particular connection this could be.
I don't believe Datum/Symmetricom/Microsemi ever published schematics for
this Caesium module so I'm a little at a loss where to go next with
troubleshooting.
I'd like to at least eliminate a failed tube as far as possible, since i
really don't see myself acquiring a replacement tube at list price.
Unfortunately i don't have any test equipment that will work at the
microwave frequencies needed to do a direct test of the tube.
Suggestions? Has anybody ever tried a repair like this before? I'm
tempted
Post by Dirk Niggemann
to feed an external supply to the ioniser to see what happens, or at
least
Post by Dirk Niggemann
disconnect the ioniser leads to see if it fails with an ioniser
overvoltage
Post by Dirk Niggemann
instead.
Regards,
Dirk M0KRD
_______________________________________________
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
Post by Dirk Niggemann
and follow the instructions there.
_______________________________________________
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Ed Palmer
2018-11-02 00:07:28 UTC
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AFAIK, the Cesium unit in the PRS-50 is a 5045A module. You mentioned a
'4201A module' so this info might not apply.

A couple of years ago, I helped a guy on eevblog fix a Datum 4040A which
used the same 5045A module as my 4065A Cesium. It turned out that most
of the capacitors in his 5045A were fried. I posted a 'map' to the
capacitors and the voltages I measured on mine. If yours is the same,
it might give you some clues. Unfortunately, the pictures he posted are
gone, but mine are still there.

https://www.eevblog.com/forum/metrology/no-luck-on-my-first-attempt-at-acquiring-a-cesium-frequency-standard/

Ed
Post by Dirk Niggemann
Hi,
I acquired a PRS-50 Caesium beam frequency standard in more-or-less working
order back in 2011. The manufacture date was February 2001, so the tube was
probably exhausted by this stage, but it did still achieve lock so may have
been in storage for some time rather than run for the whole of its 10-year
lifespan.
I recovered it from storage a year or so ago to test and it worked,
initially. The power supply was noisy, and the system wouldn't achieve lock
on power-up from cold.
I suspect that that it may have been retired from service originally for
an intermittent PSU failure as it would not find lock on every power up,
rather than an exhausted or failed tube.
Since then the power supply in the 4201A module failed completely. All the
electrolytics in the SMPSU leaked.
I have managed to restore the PSU to the point that the PRS-50 boots and
tries to find a lock, but fails after about 15 minutes with error F3
(ioniser voltage out of spec)
When watching in monitor3, I can see the both the OCXO and the Caesium oven
power and heat up, and i have voltages in spec on all rails, except that
the ioniser voltage remains below 0.1V.
I also see almost no ion pump current. which is consistent with the ioniser
remaining off.
I haven't managed to retrieve the tube constants, but i assume the ioniser
wants about 1V at 1A like most other Caesium beam tubes.
I believe i have also identified the ioniser supply leads from the tube and
these meter out at very low resistance (< 1 ohm), possibly too low (though
how a hot-wire ioniser should fail short escapes me).
Does anybody know how the 4201A module generates the ioniser voltage? It's
likely to be similar to the FTS4065C in that respect. When is the ioniser
meant to turn on in the power-up cycle?
I suspect i'm missing the ioniser supply rail. There's at least 24
unlabelled connections between the SMPSU board and the junction board which
connects both the LV lines for the Cs tube, the main processor board and
the SMPSU. I have no idea which particular connection this could be.
I don't believe Datum/Symmetricom/Microsemi ever published schematics for
this Caesium module so I'm a little at a loss where to go next with
troubleshooting.
I'd like to at least eliminate a failed tube as far as possible, since i
really don't see myself acquiring a replacement tube at list price.
Unfortunately i don't have any test equipment that will work at the
microwave frequencies needed to do a direct test of the tube.
Suggestions? Has anybody ever tried a repair like this before? I'm tempted
to feed an external supply to the ioniser to see what happens, or at least
disconnect the ioniser leads to see if it fails with an ioniser overvoltage
instead.
Regards,
Dirk M0KRD
_______________________________________________
time-nuts mailing list -- time-***@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.

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