"gregz" wrote in message
...
"N_Cook" wrote:
Bill Proms wrote in message
...
I have 3 Intelli-Time LED alarm clocks around the house, just like the
one
he


http://www.acurite.com/clock/alarm-c...alarm-clock-13
027a2.html

I initially bought these due to them keeping time when the power goes
off
and auto resetting for DST. There is a problem, however. Each of the
clocks becomes inaccurate over time. If I set them all manually to the
same
time, within a few months, each one will be off by 3-5 minutes.

So I ask, what is the problem and is there any way to repair it?

Thanks in advance,
Bill



I've always put this down to hash on the mains being interpreted as extra
cycles by the clock monitoring input. The supply companies contractually
have to correct the mains frequency so an exact number of cycles per day
(50/60)x60x60x24, but at any instant can be above or below the nominal
frequency.

I doubt if any use power line for sync. Most have battery backup. Crystals
jump frequency from time to time.


Even one I bought recently uses mains sync, but it has a battery & crystal
divider backup to cover outages.

The mains frequency varies depending on peak demand/off peak, but long term
its average has less drift than the cheap crystal oscillator they're going
to put in a radio alarm clock.