[Milsurplus] crystal has lost activity?

[Bob Camp]

Hi

Fun and games with crystals.

Obscure things that can mess up a crystal:

1) Crystal orientation comes in left and right hand twist. You want  
your entire crystal to be all one or the other. If it's part one and  
part the other you have a problem. Heat above about 400C will allow  
it to re-orient. You can get half left and half right. That equals a  
non working crystal.

2) Micro cracks can occur in the crystal. You can't see them without  
a very fancy microscope. The crack keeps the audio waves from  
propagating through the entire crystal. No waves = no crystal.

3) Particles and a big issue. They pretty much shut down the entire  
FT-243 industry in the middle of WWII. At least they can be fixed  
with a quick dip in ammonium bifluoride.

4) If it's a modern sealed crystal and below about 10 MHz it may be  
vacuum packed. Air damps the crystal and a leak will drive up the  
resistance. High resistance is what we really are seeing when the  
crystal looses "activity".

5) On plated crystals plating can come un-stuck over time. The chunks  
of plating move around. The loose pieces damp the crystal and you  
have problems.

6) Plating can oxidize. Thin films of oxide can have high resistance.

7) On a plated crystal the joint between the clip and the electrode  
is made with conductive cement. Over time the cement can crack.  
Cracked cement = high resistance.

8) The magic conductive cement can pull back from the electrode over  
time (the stuff can shrink). Once it pulls back you have no contact.

There are a bunch of other reasons. Time to head down to work ....

Bob
KB8TQ

On May 14, 2007, at 12:24 AM, john p hutchins wrote:

> Group -
>
> Now educate me please.  A crystal is a rock,. that vibrates when  
> electrical stress is applied through metal plates at particular  
> angles to the crystalline grain?
>
> I have seen many of these kinds of statements:
> "After years it is possible that the  crystal has lost activity."
>
> To stop a crystal form working would be corrosion forming on the  
> metal plates or loss of conduction to the crystal surface.
> Any of combinations of these conditions could be rectified by  
> cleaning, adjusting contact pressure on crystal surface, repairing  
> conductive path?   So with the correct tools any crystal could be  
> "fixed." to operate?
>
> Am I off base?
>
> Thanks
> Hutch
>
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