Hi,
They are controlled by a clock crystal.
In many cases, not enough care is taken with the calibration of the clock 
crystal.
If your clock gains or looses more than say 30 seconds a month, then as far as 
I am concerned, they need to be adjusted.
There is a trim pot across the crystal for this purpose.
And a frequency counter is used to calibrate the crystal.
Unfortunately, crystals are temperature sensitive and change frequency slightly 
according to change of temperature, that's why they are not entirely accurate.
Regards.
Gary Pearse
Ph: +61 7 3855 1082
Mobile 0414 39 3937
WEB SITE: http://www.users.on.net/gpearse/


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