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

Bob Camp lists at rtty.us
Fri May 17 10:21:13 EDT 2013


The DRAM's (and some cpu's) of that era were not as well specified for clock timing as the parts we have today. It's a matter of degree, but it was much worse back then. I would worry less about frequency than about duty cycle. In this case with a binary divide, the duty cycle on the clock should have held up pretty well. Who knows how well the DRAM chips have held up. 


On May 17, 2013, at 9:06 AM, paul swed <paulswedb at gmail.com> wrote:

> Grant on the purest of speculation. Most likely not. Yes the xtal ages but
> as you say the frequency is not critical. Though that actually may not be a
> true statement. How far is to far for the dram timing? The other thing that
> most likely is not an issue is a variable cap. Most likely it never had
> one. At this age I have seen resistors change value.
> I am confused are the boards really 25 years or are they 1980 at 30 plus
> years.
> My actual concern would be the dram technology at that age simply various
> breakdowns that might be occurring creating unreliable memory.
> Regards
> Paul
> On Fri, May 17, 2013 at 8:07 AM, 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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