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

[Bob kb8tq]

Hi

The oscillator is a “works or it doesn’t” sort of thing. Frequency and output 
level usually are close enough to do what is needed. 

Bob

> On Feb 25, 2022, at 3:10 AM, Julien Goodwin <time-nuts at studio442.com.au> wrote:
> 
> 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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