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

Bob kb8tq kb8tq at n1k.org
Thu Nov 1 16:10:08 EDT 2018


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 …..


> 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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