[HBR] crystal oven

Wed Jul 1 07:11:22 EDT 2009

Thank you for a great reply and invaluable information. I will be building 
the oven with closed loop control. When I get it built and calibrated, I 
will forward the schematic and or pictures when completed. .

Walt Hutchens <waltah at earthlink.net> 
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07/01/2009 06:00 AM
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Re: [HBR] crystal oven

One issue to keep in mind is that vintage military crystal ovens were
intended to keep the crystal at a ROUGHLY fixed temperature over an
extreme range of environmental conditions -- say from 'unheated
shelter north of the Arctic Circle, winter conditions' (or 'unheated
bomber at 24,000 ft.') to 'noonday direct sun, Libyan desert.' If the
crystal harmonics were within a kc or two over the entire range, that
was okay, even if you could hear the frequency going up and down as
the oven cycled on and off.

That won't work for SSB and it would make some of us crazy. The
simplest possibility is the always-on resistor heating source: Our
ambient temp extremes are likely to be more like 60-80 degrees F, with
most of the time being 68-73 or so, so a fairly quick warm up to any
temp above normal internal set temps will work fine. You just choose a
resistor that gives you about the right final temp. 1/4 watt
dissipation might be the right ballpark for the small octal-base
plug-in military ovens.

A better answer (much faster stabilization) is proportional control of
the oven heating so that the heater power decreases gradually as the
oven reaches the desired temperature. This is easy to do using a power
FET and a thermistor and the result will easily fit any of the usual
ovens.

Because they must be hotter than any possible internal equipment temp,
the set temps of military ovens are high enough to gradually destroy
the usual oven materials (Bakelite, etc.) but for ham use 120-140
degrees is generally high enough. If you bring out the thermistor
connections to the oven base and calibrate it accurately around the
desired temp before assembling then you can measure the oven internal
temp to confirm that it's working.

The usual thermistor is a negative temp coefficient resistor. You put
it in a voltage divider so that it reduces the forward gate bias on
the FET as it warms up. The other arm of the divider is chosen so the
FET will be biased 'on' at room temp. Put a small power resistor with
a value chosen to dissipate 1/2 to 1 watt in the drain circuit. It may
be necessary to play with arrangements to get the loop gain right:
thermistor mounted in free air vs. clamped or glued to the crystal and
heater, etc., but I remember these circuits as being almost NO trouble
to get working.

Both the FET and the power resistor in the drain are heaters.  Clamp
them to the case or chassis of the oven -- typically a piece of copper
with the crystal clamped on the other side.

The heater resistor must be stable at full wattage over years.
Wirewound or (better) metal oxide resistors are suitable. Must be
sure the thermistor doesn't dissipate enough heat at the applied
voltage to substantially affect its own temp.

Obviously this is a DC circuit but poorly filtered or even pulsating
DC should be fine. Very little power is required -- a watt, probably,
at most, during early warm-up, in the usual small plug-in oven.

Walt Hutchens
KJ4KV

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