[time-nuts] Symmetricom XPRO rubidium

[Matthias Welwarsky]

On Montag, 28. September 2020 04:19:12 CEST Stewart Cobb wrote:
> I have a Symmetricom* XPRO rubidium which appears to be reaching its end of
> life. The very sparse manual says that it sets a "service" flag when the
> lamp voltage reaches 600 mV. When I got it, that parameter was at about
> 540. Several months of continuous runtime later, it's down to about 510.  I
> assume this is a measure of light emitted by the lamp, but its label is
> "lamp voltage".

What is the operational range for that parameter? Are you sure about the 
scaling? 540 could be 5.4 Volts as well. In that case, nothing to worry about.

> I don't want to lose this Rb, because it seems to be the most stable
> reference in my lab (about 2x more stable with temperature than a PRS10, by
> eyeball).
> 
> Questions for the hive mind:
> 
> (1) Why and how is the lamp voltage falling?  What's the wear-out
> mechanism?

>From what I know there isn't really a failure mechanism for the lamp itself. 
It's not using up it's Rb like a Cesium beam standard, the only way for it to 
"loose" its Rubidium would be by embedding Rb ions into the glass.

> (2) Is there any hope of repair? Will the heat gun trick for the LPRO work
> on the XPRO? Could I replace the XPRO lamp bulb with one from a young LPRO?

Replacement: if the LPRO lamp fits mechanically, it's worth a try if the lamp 
is really faulty. But before butchering the lamp assembly, it's better to 
check other failure sources. I'm guessing that the overall design is similar 
to the LPRO, so there is an exciter oscillator that could be going bad (not 
pumping enough energy into the lamp), the lamp heater could be going bad (too 
low temperature) and there are a number of failure modes for both.

There doesn't seem to be much known about the innards of this device, but if 
there are patents mentioned in the manual, it's worth looking them up. The 
patents mentioned in the LPRO integration manual turned out to be very, very 
detailed and insightful. Especially those about the lamp exciter.

Regarding the "rejuvenation trick", it'll probably work as good or as bad as 
for the LPRO, provided you can get the lamp out.

HOWEVER: If you're thinking about doing "preventive maintenance", DON'T! You 
will likely impact the stability of the device. Wait for it to fail, then go 
through the common failure sources known for all Rb standards of that era. 

Regards,
Matthias