[time-nuts] Why do crystals go bad?

[Magnus Danielson]

On 14/02/11 04:26, Bill Hawkins wrote:
> Group,
>
> Jim Garland on the boatanchors at theporch.com list asked about crystals:
> "A 22.5MHz crystal (HC-5 case) in my homebrew receiver, built about forty
> years ago, no longer oscillates. It seems to be purely an age-related
> problem.
> It is in a standard solid state circuit which bandswitches six crystals, and
> the other five work just fine.  I wonder what causes a crystal to stop
> working, and whether it is possible to repair them?  I've "repaired" dead
> 100kHz calibrator crystals, and hamband crystals in FT-243 cases, by
> cleaning off the brass pressure plates, but am not sure if one can do this
> on thin high crystals. As I recall, the metal electrodes are evaporated onto
> the sides of the element. 73, Jim W8ZR"
>
> Any help appreciated.

On the any help level:

Aging usually makes crystals go adrift in frequency. Other parameters 
may also shift, especially if the DC connectivity is lost due to 
oxidation or so. I think it is likely that this is what happend and all 
of a sudden the loop conditions does not support oscillation (i.e. gain 
 > 1 when phase shift is 0 degrees modulus 360 degrees). There might be 
hacks to be added to get the loop oscillating again and there might be 
hacks to the crystal assembly, but swapping the crystal is probably a 
good remedy in this case.

Cheers,
Magnus