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

[Ed Palmer]

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

On 11/1/2018 4:53 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