[NLRS] DB6NT PTC crystal heater

[Doug Reed]

At 10:58 AM 2/12/2003 -0600, you wrote:
>I've used PTC thermistors for stabilizing crystal oscillators for
>decades. Generally I've used 40=B0 C rating presuming that Iowa weather
>during operating periods would be cooler than that. That may not apply
>to the Gulf coast. In Mouser I see a few PTC thermistors rated 110 or
>120=B0F.=20

Just make sure you're looking at degrees F rather than degrees C when you
order. There are many more PTC thermistors rated for temps 80 degrees C and
above than there are at 40 degrees C. I think Jerry is correct in choosing
the 40 degree C thermistor rather than the cooler 30 degree C version. 30C
=3D 86F. 40C =3D 104F. 50C =3D 122F.

>I take a 1 inch or so cube of foam insulation like Dow blue board
>and carve a hole to just accept the crystal and thermistor. I've never
>bothered to solder the thermistor to the crystal can. It would improve
>heat transfer, but since there's only a couple thin steel wires and some
>gas to transfer heat to the quartz inside the can, a little more
>impedance outside isn't that much of a deal, and the enclosing foam gets
>the heat where its needed.=20

There certainly isn't much room around the crystal in the DB6NT. I tend to
use the white Styrofoam since I can always find scraps laying around. The
blue board would be much better to work with. I still prefer the idea of
heat sinking the thermistor to the can for best heat transfer but the
bottom line is that anything you can do to remove temperature variation is
going to reduce frequency drift.=20

>I don't like the idea of heating a crystal up
>to soldering temperature. I don't like cutting leads of a crystal with
>ordinary diagonal cutters because I don't want to feed the crystal that
>shock. Its best to hold the lead between the crystal and the wire cutter
>with sturdy pliers to absorb most of the shock of cutting. Or I use
>flush cutting dikes with the flush side towards the crystal so the shock
>all goes to the free end of the wire.
>73, Jerry, K0CQ

I mentioned soldering to the can because that was the original magazine
suggestion. But I've always considered it a silly idea since I over heated
some crystals in the past causing the base to blow off the can or venting
the can by melting the solder plug on the can. Using a metal clip to hold
the thermistor is the method used by the big companies. I'd prefer to add
heat sink compound since I have it. I've never had any trouble with cutting
the leads on the crystal, either before or after it was soldered in the
circuit. But I expect if I was paying $100 for a high precision crystal I'd
take more precautions.....=20

73, Doug Reed, N0NAS.