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

[Peter Bell]

On the FE-5680A, there are two mosfets on the lamp housing - one of
them is purely a heater, and the other is the drive FET for the RF
lamp exciation signal.  I assume the designers figured it was going to
run pretty hot anyway and they might as well use the heat for
something useful.  In practice, the lamp housing temperature seems to
be fairly uniform - probably becasue the other end of it is attached
in a way (PCB soldered to the heater cup and fibre washers) that gives
it a quite high thermal resistance.  The lamp is attached to the
housing using what appears to be RTV silicone at the base and does not
touch the housing at other points.  See the attached photo.

The cell cavity has a single MOSFET heater on it and is thermally
insulated by a sheet of what looks like FR4 PCB materal at one end and
the support for the C-field coil (which seems to be made of similar
stuff) at the other end.

This sort of level of temperature control seems to be characteristic
of small Rb standards based on what I've seen.

Regards,

Pete


On Fri, Dec 2, 2011 at 4:46 AM, Steve . <iteration69 at gmail.com> wrote:
> Are there any other (mosfet)heaters on the oven cavity? Seems that heating
> from one point is going to create a large temperature gradient which would
> probably effect spatial phase congruency. I believe this problem exists on
> the two mosfet configuration as well. Anyway, my idea is to remove the
> mosfet heaters, wrap my own heater using light gauge nichrome  (aka, fuse
> wire), mica insulator and build a proper heater/oven assembly. Finally, pot
> it all together with some sauereisen ceramic cement.  Nice, clean,
> controlled gradient.
>
> Maybe I'm over thinking the whole thing too. I guess I'll find out when I
> start my experimenting.
>
> Steve
>
> On Thu, Dec 1, 2011 at 9:52 AM, Arthur Dent <golgarfrincham at yahoo.com>wrote:
>
>> The referenced photos are poor quality and taken in tungsten lighting
>> so you really can't see much from them. Here is a photo I just took of
>> a somewhat similar 5650A that may be new so it is still pretty shiny.
>>
>> http://farm8.staticflickr.com/7142/6436441767_26980890bb_b.jpg
>>
>>  -Arthur
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