[time-nuts] How I got my FE-5680A to lock in Sydney, Australia

ewkehren ewkehren at aol.com
Fri Feb 7 20:39:47 EST 2014

Could be the C coil

Sent from Samsung tabletEB4APL <eb4apl at cembreros.jazztel.es> wrote:Hi,

I second the idea about a mechanical issue.  When I opened my FRS-C I 
found that one of the connector pins had never been soldered, it only 
made mechanical contact with the PBC. Perhaps it showed an intermittent 
failure and was retired with low hours.

Ignacio EB4APL

On 07/02/2014 3:54, Ed Palmer wrote:
> Hi Jim,
> On 2/6/2014 3:32 PM, Jamieson (Jim) Rowe wrote:
>> Hi again folks,
>> You may (or may not) recall that a month or so ago, I asked for any 
>> information that might be available regarding how to fix a ‘used’ 
>> FE-5680A rubidium module from China (via ebay) which was tested by 
>> the supplier in China as working OK, but would not seem to lock up to 
>> rubidium here in Sydney. There wasn’t a great deal of info available, 
>> it seems, so I kept on checking ideas myself – mostly with no luck. 
>> The module would never lock, but kept cycling back and forth between 
>> about 9.999770MHz and 10.000036MHz – ‘searching’ for a lock, but 
>> never finding it.
>> Anyway, a couple of days ago I was reading more about the operation 
>> of rubidium vapour oscillators, and noticed that the ‘filter cell’ is 
>> very sensitive to magnetic fields – hence the mu-metal shielding 
>> case, and also for the ‘C-tuning’ coil. And I wondered if the main 
>> reason why the FE-5680A  had apparently worked in China, but wouldn’t 
>> lock up in Sydney (Australia) might be caused by the fact that 
>> Quangzhou (China) is in the northern hemisphere while I’m ‘down 
>> under’ in the southern hemisphere – where the earth’s field is 
>> presumably somewhat different, in terms of both strength and direction.
>> So I decided to test this in a crude way, by inverting the FE-5680A 
>> and seeing what happened. And – lo and behold – it locked up within 
>> 2.5 minutes, and stayed locked until I turned off the power and let 
>> it go cold again. The next morning I applied power again, and within 
>> 3 minutes it locked up again with no problems. And it’s been locked 
>> up now for over 48 hours...
> My first thought was to make a typical 'down-under' joke and suggest 
> you run the 5680A upside down, but you beat me to it! :)
>> So it seems that the different magnetic field here may have been the 
>> problem – either that, or it may have received a ‘jolt’ in transit, 
>> which prevented in from locking unless it was inverted.
>> But how do I tell which of these explanations is right, without 
>> ‘opening her up’ again and looking for some kind of subtle physical 
>> fault?
>> Another idea: perhaps the mu-metal shield case had acquired a small 
>> dose of magnetisation in transit (via a physical shock, or from a 
>> strong field metal detector). I guess in this case that I would have 
>> to remove the two halves of the case, and bake them in a furnace to 
>> demagnetise them again.
> My very limited knowledge regarding mu-metal is that it is so 
> magnetically 'soft' that it can't be magnetized.  If it was somehow 
> magnetized, inverting the unit wouldn't make any difference, would it?
>> Or should I just run the FE-5680A upside down permanently – the 
>> simple but ‘crude’ answer?
> I don't believe for a second (pun intended) that the earth's magnetic 
> field has any effect on the locking of your 5680A.  It's just not 
> strong enough.  The same applies to the C-field which can only nudge 
> the frequency one way or another by a small amount.
> Since flipping the unit DID make a difference, my money would be on a 
> trivial, boring mechanical issue inside the unit.  Could be a bad 
> solder joint, broken wire, floating piece of debris, or something like 
> that.  Worst case might be a broken glue joint somewhere in the 
> physics package.  That could be ugly.  I would definitely open it up 
> and see if anything falls out.
>> I’m not sure if this FE-5680A has the ‘C-tuning’ gizmo fitted, or 
>> wired up. Am I right in thinking that another approach might be to 
>> try varying the tuning via the RS-232C serial port? Does this work 
>> via the C-tuning coil anyway, or by tweaking the DDS?
> The RS-232 commands affect the DDS - assuming your unit does have 
> one.  It will have no effect on the locking, only on the output 
> frequency.
> A few years ago I bought a dead Datum SLCR Rb standard.  It's a cousin 
> to the LPRO.  I thought I'd learn some things by trying to fix it.  
> The problem was intermittent.  I tore it apart and found that one of 
> the legs of the crystal had never been soldered! Never overlook the 
> obvious.
>> I hope a much more experienced time nut can provide a few answers, 
>> please.
> Maybe the blind leading the blind is a closer description.
> Ed
>> Jim Rowe
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