[time-nuts] Can a quartz crystal go off by 2% ?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Dec 29 00:10:55 UTC 2009


Dr. David Kirkby wrote:
> ALAN MELIA wrote:
>> It is possible the crystal has succumbed to a mechanical fatigue. To 
>> check I used an old relay coil with a ferrite rod stuffed through it 
>> and tuned to 32.6khz. If you have a sensitive enough counter you may 
>> be able to measure this without an amp in line. (2% is a long way out 
>> !) It could also be the trimmer capacitor that has failed. I doubt 
>> there is much more mechanical, other than a dodgy solder joint. 
>
>
>> In all probability it will be the bane of all lovers of old 
>> electronics....the power supply electrolytic capacitors......remember 
>> battery quartz clocks run slow or fast as the batt runs out.
>
>> Alan G3NYK
>
> It could be the battery is low. There clearly is a battery backup, and 
> there is something on the clock which implies the holdover period is 
> about 4 days. (I forget the exact wording). Clearly if there is a 
> power failure, the clock must still rotate to keep accurate.
>
> I assume the battery is constantly charged by the incoming supply. 
> Given the age of the battery (> 17 years), it is unlikely to be in 
> good condition! But it should be charged all the time. But perhaps 
> even when charged, its voltage is very different to what it should be.
>
> I would not have thought a trimmer cap going open-circuit could have 
> induced a 2% change. That seems an awful lot.
>
If the trimmer in in series with the crystal and not shunted by another 
capacitor then the crystal will no longer control the oscillator frequency.
0.1pF or 0.01pF in series with a tuning fork crystal instead of the 
nominal value (20pf?) will make a singnificant difference.
> Thanks for the idea of the ferrite rod.
>
> Dave
>
Bruce
>> --- On Mon, 28/12/09, Dr. David Kirkby <david.kirkby at onetel.net> wrote:
>>
>>> From: Dr. David Kirkby <david.kirkby at onetel.net>
>>> Subject: [time-nuts] Can a quartz crystal go off by 2% ?
>>> To: "Discussion of precise time and frequency measurement" 
>>> <time-nuts at febo.com>
>>> Date: Monday, 28 December, 2009, 23:22
>>> I'm on the so-called 'Economy 7'
>>> electric in the UK, where I'm supposed to get cheap electric
>>> from 0030 to 0730 - i.e. a 7 hour period when electricity
>>> demand is low. I'm no longer heating by electric, but do run
>>> some computers 24/7. It's not totally clear whether this
>>> saves me money or costs me money, as I pay a higher price
>>> per unit during the expensive period, to compensate for the
>>> fact I get it cheap for 7 hours. But I run some computers
>>> 24/7. I guess I should do the maths and work it out. Apart
>>> from some heaters in the garage, which are very rarely used,
>>> I no longer heat with it.
>>>
>>> The time when the electric is cheap is set by a clock,
>>> which rotates once/day. It says on it "quartz" somewhere, so
>>> it must be regulated by a crystal and not from the 50 Hz
>>> supply, which would be pretty useless, as the clock would go
>>> wrong if there was ever a power failure. The clock has not
>>> been changed in the 17 years I've lived at my house, though
>>> the meter has on a couple of occasions.
>>>
>>> The clock used to keep accurate, but now it looses time
>>> about 30 minutes/day. I wrote a computer program to predict
>>> when the electric is cheap, so we can schedule when things
>>> like the washing machine, dishwasher, Hoover etc are used.
>>> Even cooking to a certain extent, if it's convenient, though
>>> our life does not revolve around the cheap electric.
>>>
>>> I'm wondering if this is a mechanical fault in the clock,
>>> or whether the crystal has developed a fault. It's clearly
>>> well outside any tolerance or aging process of any crystal -
>>> even the cheapest ones.
>>>
>>> I've not done any very extensive tests, but the error does
>>> not appear to be constant. Hence every month or so I need to
>>> produce a new table, as my predictions get less accurate
>>> with time. Since one can only read the clock to an accuracy
>>> of about 15 minutes, it's not easy to know how far it is
>>> out. Sometimes we hear the contactor go over, as this is
>>> supposed to then power the storage heaters, which we no long
>>> use.
>>>
>>> Dave
>>>
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>>
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>
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