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

[Julien Goodwin]

This afternoon I swapped the fixed resistor that should have been a 
trimpot back to a trimpot, and quickly dialed it up to where I was 
getting ~115c on the lamp surface as expected.

As I was in there anyway I also swapped the 20 ohm resistor back to the 
Vishay wirewound part it should have been from the hack I'd made up 
using cheap carbon resistors from the local shop.

No obvious light, but I also still need to see if the excitation 
oscillator is even vaguely in the ballpark (or possibly working, I may 
potentially have messed with things while I was in there today), for 
which I'm still waiting on some safer probe options.

On 15/2/22 6:10 pm, Darren Freeman wrote:
> 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.

I need to replace the main filter cap in the FRT power supply before 
I'll be comfortable leaving it on for an extended duration, but once I 
do (and the replacement is now here, I just need to hook it up) I'll be 
fine leaving it on for a week or so to see.

If it doesn't light after that, I may try just swapping in the lamp from 
my house standard FRK-LN to see if it's that simple, and this has a bad 
lamp (at which point I think I'd give up for now).

> (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!

Fortunately Sydney power is good enough that my UPS' have caused more 
outages than they've helped with.

> 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
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