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

[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