"Ian Field" wrote in
:


"gregz" wrote in message
-september
.org...
"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...igital-alarm-c
lock-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.




radio alarm clock probably won't even use an xtal;like Jeff L. says,they
may use a cheap RC osc.

I note my MW oven clock that derives it's clock from line freq. has better
stability than other "digital" clocks.(like my PC clock....)

(of course,I use an internet program to keep the PC clock fairly close. I
used to use a program(Atomic Clock) that direct-dialed the Naval
Observatory,but the long distance calls cost too much.)

--
Jim Yanik
jyanik
at
localnet
dot com