[time-nuts] Re: Efratom FRK with "inoperative lamp"

[Darren Freeman]

On Sat, 2022-02-05 at 20:48 +1100, Julien Goodwin wrote:
> I /think/ it's using more power than it did before (right around 20W
> at 
> total cold, dipping to 10W after a while), although silly me didn't
> make 
> a note of it, but even after 30 minutes it's not locking.

I don't have any experience with this brand/model, but I have
resurrected some "dead" rubidium standards. I'm mostly going from
experience with gas lasers, but I found the same approach worked on the
rubidium units that I have here. 

It's well worth being patient with the lamp, if it's been off for many
years, it may have a gas pressure/mix that's far from optimal. If you
let it sit with the heater and exciter running, it may just strike
after a while.

(You should measure the current going into the exciter, it should jump
up suddenly when the lamp strikes. Measure the unlit condition with the
lamp cold and the exciter on. On mine, the current roughly doubles when
it is lit.)

If it does strike, keep it running for a few days, don't power it off.
It can get even harder to light, before it gets easier again. You might
consider using a UPS for this procedure, as Murphy could strike just
after the lamp does!

On one of my units, I had to manually run the lamp heater to a higher
than normal temperature, before it would light. (With a thermocouple,
it's best to use some heatsink grease, otherwise the readings can be a
long way off.)

The needed temperature gradually came down, until the unit could be
reassembled and used normally. On another, I just waited for several
hours at the usual temperature, and that was enough. Although again, it
took a couple of days of operation before it would light without
hesitation.

On yet another unit, the lamp would light normally, but the photodiode
signal was too weak to achieve a lock. (The firmware said nope.) So I
just left it running overnight, and then after a power cycle, it locked
immediately!

So I wanted to point out that if the lamp has everything it needs, and
still won't light, you might just need to leave it powered for a while.
And you have not much to lose, if you need to run it at a higher
temperature for a while.

Have fun,
Darren