[time-nuts] Heated crystal? & Rb tube corrosion (FE-5680A)

Steve . iteration69 at gmail.com
Thu Dec 1 15:22:27 EST 2011


On Thu, Dec 1, 2011 at 12:10 PM, Bob Camp <lists at rtty.us> wrote:

> Hi


> The amount of Rb in the cell is quite small. Cell leakage would stop the
> unit from working. There's pretty much no way that the cell can / would
> cause corrosion and the unit keep working.
>

This is exactly what I was getting at with my original post.  /If/ The cell
can leak it will corrode the neighboring material, then signs of corrosion
would be a tale-tale sign that a DOA module is not easily repairable
without additional parts.

For what it's worth, I'll share some repair/ lab experiences.

Per my experience with heated ovens operating between 30c and 100c, in non
carrier atmosphere, an alkali leak of 10 parts per billion, at a rate of
two weeks, will cause corrosion in nearby low grade metallic parts in 6-8
months. Notice the strange units ppb/2wk, and wide range of conditions (30c
- 100c) these are the natural terms of the problem, which is all i have to
work with. These numbers were determined empirically by sampling a nearby
junction and sending the sample to the metals lab for analysis. 10ppb/two
weeks is a slow leak, but a leak none the less. 10ppm? It's amazing to see
what alkalines can do to components, solder joints, vias, and burried
traces on multiple layer boards. To be fair, the oven temperature,
operating voltage, and chopping frequency are major contributors to the
damage I have seen as well. But it all starts with a leak.

This one time an instrument had failed and I eventually figured out which
circuit was at fault, upon visual inspection of the board i noted several
vias which were dull, almost as if they were oxidized in some manor. But it
was only a handful if vias, all localized. Under a 300x microscope I pushed
the micro browser tip in to the via, the dull solder(or what was once
solder) came oozing out like putty. Keep in mind this was at 300x, and room
temperature. Sure, leaded solder appears somewhat plyable at 300x when
browsing with micro probes, but this was different. Very different. Being
very strange to me, I poked around all the damaged vias for awhile. I noted
that they had very high electrical resistance and was more so a capacitor
than a resistor or junction (but it was suppose to be a via connection..)
and it pitted my gold plated browser tip. I was not happy to destroy a good
HP micro browser, but at least i did find the problem with circuit. I also
noted that the plated through-holes, which made up the vias as well, was no
longer plated and the plating and nearby circuit appeared to have undergone
electrolysis. I cleaned the board with de ionized water, dried it,  and
manually jumped around the vias with light gauge wire.  Problem fixed.
Unfortunately I did not have the solder analyzed. I'm still curious as to
what had happened to the solder.


Steve




>  The cryst.l is heated to reduce the pull range of the VCXO. Since it's
> locked to the Rb, improving it's temperature performance does not have a
> significant impact on the unit's performance.
>
> Bob
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Peter Bell
> Sent: Thursday, December 01, 2011 4:42 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Heated crystal? & Rb tube corrosion (FE-5680A)
>
> Hi, Steve
>
> Both the lamp and the absorption cell are made out of glass (or
> possibly quartz - see attached image).  The unit mounted to the
> crystal is a PTC thermistor wired directly across one of the power
> supplies.  I suspect it's just there to heat up the xtal to a
> temperature close to it's knee point - absolute accuracy is not that
> important since the xtal is being run in a VCXO config and is locked
> to the Rb cell.
>
> None of the units I've looked at appear to have any corrosion on them
> - the discoloration you can see on that photo is just a side effect of
> that fact that the lamp housing (intentionally) runs very hot - about
> 100c.  The only negative effect I've seen from this is that the fiber
> washers that hold the lamp housing in position get somewhat cooked and
> the used ones are easily damaged if you tighten up the screws too
> much.
>
> Regards,
>
> Pete
>
> On Thu, Dec 1, 2011 at 1:05 PM, Steve . <iteration69 at gmail.com> wrote:
> > I've been paying particular attention to the discussions involving the
> > FE-5680A frequency standards of recent attention. I do not have a
> FE-5680A
> > yet, but rather I am studying what is shared from the others prior to
> > buying. At the very least I want to know what I'm up against should I get
> a
> > DOA module.
> >
> > It appears that these units use a heated crystal.(..i sure hope it's
> heater
> > and not an acoustic resonator). Has anyone performed sub 1degree c drift
> > testing against a known stable source? What are the performance gains by
> > using tighter temperature control? Also It appears that quite a few of
> > these have corroded Rb tube interfaces. My guess is the corrosion is a
> > tale-tale sign of small amounts of rb gas leakage in combination with the
> > raised temperatures of the tube oven?  If this is the case I suppose a
> > visual check of the tube interface for corrosion would yield a fair
> > approximation of the tube condition?
> >
> > Lastly, is the Rb tube a quartz tube or is it a metal(silver
> > lined?)canister sealed with polymer tape?
> >
> >
> > Thanks,
> > Steve
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>
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