[time-nuts] E1938 oven design

Richard (Rick) Karlquist richard at karlquist.com
Thu Jun 8 19:17:20 EDT 2017

Yes, that's right.  The copper oven mass
has two pieces:  a main piece and a lid.
The main piece has a wall around the outside
into which the cover fits, using an O-ring.
In the center, there is a cylindrical cavity
into which the crystal mounts.  It is like
a 10811 crystal, except the height is reduced.
In retrospect, there is no reason why the
10811 crystal shouldn't have been that size.
It is potted into the cavity, therefore, it
doesn't use the famous flat head 4-40 screw
welded to the can as in the 10811.  Thermistors
are potted into holes drilled in the cavity
wall.  The lid has screws that screw into
the cavity wall.

The main piece holds a circular PC board
containing the bridge oscillator circuit.
It has a big hole in the middle for the
crystal cavity.  The production people
immediately named it the "donut board."
And the finished oscillator is the "hockey

Summarizing, the crystal is very well thermally
connected to the copper oven mass.

There are 3 flex circuit heaters.  One for
the lid, one for the other face of the
main piece, and one that goes around the
outside.  The two face heaters are operated
together as one heater.  The ratio of heat
to the faces vs the outside rim is adjusted
for maximum thermal gain.  The rim heater is
the difference between under 1,000 gain and
over 1,000,000 gain.

We were stuck under 1,000 for a long time
using only face heaters.  I still remember the
day that I rigged up the first crude rim header
by winding a piece of magnet wire around the rim
and holding it in place with 5 min epoxy.  I
didn't know what else to try.  This seemed like
a Hail Mary play at the time, until I
measured the gain.  We instantly went to gains
above 20,000.  It seemed high at the time but
it was only the beginning.

The exact insulation is relatively unimportant.
We even tried still air using a knife edge
cradle.  Didn't make much difference.

Rick N6RK

On 6/8/2017 1:27 PM, Attila Kinali wrote:
> On Wed, 7 Jun 2017 14:21:52 -0700
> "Richard (Rick) Karlquist" <richard at karlquist.com> wrote:
>> The crystal case is well connected to the oven mass and gets
>> heated by conduction.  I don't think radiation is a player.
> Do I interpret the papers correctly, that the oven mass is
> a closed can, with the crystal holder "molded" into it?
> Then wrapped around it are the face and the rim heaters,
> and outside those comes the insulation and the outer can?
> 			Attila Kinali

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