[time-nuts] Datum PRS-50 ionizer voltage supply

[paul swed]

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

On Thu, Nov 1, 2018 at 4:11 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> 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
>
> > On Nov 1, 2018, at 3:56 PM, Dirk Niggemann <dirk.niggemann at gmail.com>
> wrote:
> >
> > 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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