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

Dr. David Kirkby david.kirkby at onetel.net
Mon Dec 28 23:58:08 UTC 2009


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.

Thanks for the idea of the ferrite rod.

Dave

> --- 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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