[time-nuts] Why do crystals go bad?

[Alan Melia]

Hi Bill I have never worked on quartz crystals, but I was attached to a
group that had made many of them for the British Post Office in the 1950s. I
also used, and replaced, a lot in commercial 2-way radios in the 1980s and
90s.

First are you really sure the crystal has changed it is more likely that the
capacitors may have changed in a 40 year old circuit. You really need to
measure the crystal ESR to be sure. I came across cases of "lazy" crystals
in the 1990s usually the cheaper variety that had to be swapped out of
two-way radios. Generally these were probably "off-angle" and of marginal
activity. a slight change in ESR would stop them working. Some makers were
notorious for these problems. I suppose a more vigourous drive circuit might
have activated them, but with less stablity. Good oscillator circuits are
usually designed with only just enough drive to start and maintain
oscillation. Any major change in ESR and they stop, or wont start but will
maintain oscillation if tapped or are mechanically shocked.

Older crystals say before about 1965 would probably be made from natural
quartz, synthetic quartz was very expensive at that time and only used for
expensive top notch standards. Whereas I believe now all balanks are
synthetic (grown) crystals. Older natural crystals have more crystal defects
and more contamination (and thus usually age faster I believe). High levels
of drive could I suppose mill-out crystal dislocations which could provide
traps for contamination. Many crystal drive circuits, particularly in older
gear,  like the Pierce, can drive the crystal very hard. Then you have
possible problems with the plated electrodes due to contamination (usually
only cause frequency changes) and also micro cracks because the
evapourated/sputtered plating is quite thin.

After all they are passive but they are a "moving part" :-))

Alan G3NYK
----- Original Message ----- 
From: "Bill Hawkins" <bill at iaxs.net>
To: "'Discussion of precise time and frequency measurement'"
<time-nuts at febo.com>
Cc: "'Scott Robinson'" <spr at earthlink.net>; "'Roy Morgan'"
<k1lky at earthlink.net>; "'Jim Garland'" <4cx250b at muohio.edu>
Sent: Monday, February 14, 2011 3:26 AM
Subject: [time-nuts] Why do crystals go bad?


> 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"
>
> One of the replies was:
> "Broken families, drugs, drink... the normal, I suppose. John K5MO"
>
> Scott Robinson asked: "Receiver crystals aren't getting beaten up by high
> power,
> but something has killed a lot of them in my R-390A and Drake R-4A.
> Curiously yours, Scott"
>
> And Roy Morgan asked:
> "I have a 1960's frequency standard from a Nike site: the Sulzer
Oscillator
> and would like to find tech into on it."
>
> Any help appreciated.
>
> Bill Hawkins
>
>
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