[time-nuts] aging/failure of un-powered xtal oscillators?

[Azelio Boriani]

Usually aged crystals are reliable.

On Fri, May 17, 2013 at 2:07 PM, Grant Hodgson
<grant at ghengineering.co.uk> wrote:
> A client company has sourced a quantity of 'New in Box' iSBC series memory
> modules manufactured by Intel in the 1980s for a MULTIBUS based computer
> system. These are still in their original, sealed packaging and have been
> stored (for 25 years) in controlled conditions. These cards are required as
> part of a refresh programme for a mission-critical application (electricity
> generation), which are currently using original Intel cards from the same
> era.
>
> The memory cards use a 64.1kHz oscillator module as the refresh clock for
> the DRAM.  (I suspect that the oscillator module uses either a 2.5MHz or
> more likely a 5MHz crystal whose output is divided down within the module by
> 39 or 78 to 64.1kHz).
>
> As this oscillator is used only for DRAM refresh timing, the accuracy and
> drift are not particularly important.  However, reliability is important -
> i.e. the oscillator must function, even if it is a few ppm off frequency.
>
> AS a matter of precaustion, all the tantalum and electrolytic capacitors on
> the NIB cards are to be replaced as a matter of course, and any socketed ICs
> will probably be removed, cleaned and replaced (to be decided).   There are
> several dozen cards to be replaced.
>
> There is an option to replace the crystal oscillator module, but it is
> uncertain as to whether it makes sense to to the extra work.  The
> replacement oscisllator module will be from a different supplier to the one
> Intel used, and would need some form of qualification.
>
> Is there a known failure mechanism whereby a cystal oscillator module that
> has been sat on a shelf (as part of a complete card) in a benign environment
> develops one or more characteristics that could impact it's reliability?
>
> regards
> Grant
>
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