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Old 11-22-2013, 09:54 AM
kayT kayT is offline
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Power outage/electric clock oddity

Our electricity was out last night for about four hours. The clock on our stove this morning showed a time which was about four hours slow (behind the current time), which would be correct.

However, we have two electric alarm clocks, both of which have battery backups and thus both of which should show the correct time this morning. However, both of the clocks are 17 minutes FAST this morning. And I know for a fact that both had the correct time yesterday. So I am gobsmacked. How is this possible? Anyone have a clue?
  #2  
Old 11-22-2013, 10:28 AM
Habeed Habeed is offline
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Originally Posted by kayT View Post
Our electricity was out last night for about four hours. The clock on our stove this morning showed a time which was about four hours slow (behind the current time), which would be correct.

However, we have two electric alarm clocks, both of which have battery backups and thus both of which should show the correct time this morning. However, both of the clocks are 17 minutes FAST this morning. And I know for a fact that both had the correct time yesterday. So I am gobsmacked. How is this possible? Anyone have a clue?
Are both electric alarm clocks the exact same make and model? This would be more difficult to think of an explanation if they don't have identical circuits.
  #3  
Old 11-22-2013, 10:35 AM
KneadToKnow KneadToKnow is offline
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For clarification, analog or digital?

Last edited by KneadToKnow; 11-22-2013 at 10:36 AM.
  #4  
Old 11-22-2013, 10:54 AM
ZenBeam ZenBeam is offline
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The clocks all usually use the line electricity for keeping time. Your clocks with battery backups have to switch to an internal oscillator to keep track of time when the power goes out, and apparently they aren't very accurate. But it's better to be 17 minutes early than to have no alarm at all, and miss that big meeting.

For stoves, holding the time constant is a good feature. If you're away while baking something, and come back to find there was a power failure, you can compare the time, and determine if the food has just been baked a few minutes longer, or if it sat in the cooling oven, breeding bacteria, and needs to be tossed.
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Old 11-22-2013, 11:05 AM
JerrySTL JerrySTL is offline
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I've also noticed that my clocks with battery backups run fast. I assume that the 9V battery can't do as good of a job as 110 VAC at 60 Hrtz.

Plus I'd rather have the alarm go off a few minutes early as opposed to late. If the storm was bad enough, it might take longer to get to work or things might need to be done around the house deal with thawing food in the freezer if power isn't back on yet.
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Old 11-22-2013, 01:02 PM
kayT kayT is offline
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Both clocks are digital; one is GE and one GTX (?).
I figured it was the difference between battery and 110 but I would have thought the clocks would be slow as first of all a time lapse while changing over and second battery not, as JerrySTL said, doing as good a job. Why would not that add up to clocks being slow rather than fast? If someone could explain that to me I'd be delighted but bear in mind that I don't totally "get" electricity, if you know what I mean. (I'm a poet not an electrician.) Thanks.

ETA the oven clock didn't hold the time; it just picked up where it left off and continued from there. Not sure what you meant by "hold the time constant", ZenBeam. Do you mean, stay where it was? As opposed to the microwave which turned to a blinking "set clock" display and didn't resume being a clock until I did set it?

Last edited by kayT; 11-22-2013 at 01:05 PM.
  #7  
Old 11-22-2013, 01:17 PM
Boyo Jim Boyo Jim is offline
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My theory, unsupported by specific knowledge, is that when the power ran out both clocks reset to midnight, and then a moment later switched over to internal power and started running again. If the power went out at 11:43 PM, that would be supporting evidence for my theory.

If not, my theory is pretty much out the window.

Last edited by Boyo Jim; 11-22-2013 at 01:17 PM.
  #8  
Old 11-22-2013, 01:28 PM
johnpost johnpost is offline
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the clocks may not start keeping time with the backup battery upon power failure. the battery might start running its clock once the time is set.

if clocks were set to run slightly fast on backup, then four minutes an hour is overkill.
  #9  
Old 11-22-2013, 02:13 PM
ZenBeam ZenBeam is offline
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ETA the oven clock didn't hold the time; it just picked up where it left off and continued from there. Not sure what you meant by "hold the time constant", ZenBeam. Do you mean, stay where it was? As opposed to the microwave which turned to a blinking "set clock" display and didn't resume being a clock until I did set it?
By "hold the time constant", I meant it remembered the time, but didn't advance it until power was restored. If the power went out at 3:30, whenever the power came back on, the clock would show 3:30 again, and would start running. (Ours also shows a "PF" for power failure, so you know to check.)

So if the power was out until 4:00, and you looked at the oven clock at 4:45, it would show 4:15, and you'd know it was off for a half hour.

If the power was only off for a minute, when you looked at the oven clock at 4:45, it would show 4:44, and you'd know the time off wouldn't matter for whatever you were baking.
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Old 11-22-2013, 02:21 PM
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Originally Posted by ZenBeam View Post
The clocks all usually use the line electricity for keeping time. Your clocks with battery backups have to switch to an internal oscillator to keep track of time when the power goes out, and apparently they aren't very accurate. But it's better to be 17 minutes early than to have no alarm at all, and miss that big meeting.

For stoves, holding the time constant is a good feature. If you're away while baking something, and come back to find there was a power failure, you can compare the time, and determine if the food has just been baked a few minutes longer, or if it sat in the cooling oven, breeding bacteria, and needs to be tossed.
This is not true: Most clocks use a quarts crystal and its oscillations to keep time. Very few analog clocks and AFAIK no digital clocks use the electrical frequency to gauge time . When it comes to a backup method however, and the child laborer in China, and the last time she suckled is the determining factor on accuracy.

Last edited by kanicbird; 11-22-2013 at 02:22 PM.
  #11  
Old 11-22-2013, 02:56 PM
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no digital clocks use the electrical frequency to gauge time.
Really? I was always under the impression that most (all?) digital clocks that run on AC use the line frequency as a time base.
  #12  
Old 11-22-2013, 03:16 PM
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Really? I was always under the impression that most (all?) digital clocks that run on AC use the line frequency as a time base.
This is correct.

But, that said, I’d love to see one that uses a “quarts” crystal...
  #13  
Old 11-22-2013, 03:20 PM
Fluke Starbucker Fluke Starbucker is offline
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I'd have to experiment

1) Set both of the clocks to the proper time
2) unplug one of them
3) see what happens

Of course, this might not explain why whatever happens happens, but it may give a clue. Boyo Jim has a strong hypothesis.
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Old 11-22-2013, 03:26 PM
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Originally Posted by Boyo Jim View Post
My theory, unsupported by specific knowledge, is that when the power ran out both clocks reset to midnight, and then a moment later switched over to internal power and started running again. If the power went out at 11:43 PM, that would be supporting evidence for my theory.
Quote:
Originally Posted by Fluke Starbucker View Post
I'd have to experiment

1) Set both of the clocks to the proper time
2) unplug one of them
3) see what happens

Of course, this might not explain why whatever happens happens, but it may give a clue. Boyo Jim has a strong hypothesis.
I'd say the test will decide, but I've never seen a device that worked that way. You're postulating a reset and a continuation under battery power from there, an event which I can't conceive has much practical value. Reset, yes -- shows the power was off. Continuing from last known time under battery power -- yes, hope springs eternal. Both -- serves only to confuse, as the time is always wrong, but you aren't informed about it.

Last edited by Musicat; 11-22-2013 at 03:27 PM.
  #15  
Old 11-22-2013, 03:45 PM
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I had similar issues when working at a remote location in Alaska that had their own power system. The clocks being incorrect was a common problem even without power problems. I would notice clocks being hp to +\- 15 minutes off after a couple weeks. We had a lot of brown outs and black outs occured fairly often for a while.
When I asked why my digital alarm clock was off I was told that surges from the power house cause clockes to either run quick or slow.
There was no supporting info and the person I asked had nothing to do with power generation on site.
  #16  
Old 11-22-2013, 03:48 PM
Boyo Jim Boyo Jim is offline
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I'd say the test will decide, but I've never seen a device that worked that way. ....
Well, IMO it shouldn't work that way, because if the backup power is supposed to keep the clock accurate, this would be a terrible way to do it. But it's all I could think of that might fit the known facts.
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Old 11-22-2013, 04:15 PM
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the clocks may not start keeping time with the backup battery upon power failure. the battery might start running its clock once the time is set.
If you had to set the time again to use the battery backup, that would make the battery backup utterly useless. The whole point of the battery is so that you don't lose the time.
  #18  
Old 11-22-2013, 04:34 PM
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We need to define things more narrowly here. Not all clocks work the same way. I have a digital-display clock that's 30 years old, and I'm pretty sure it syncs to the line frequency. Do the new ones work that way? I'm not sure.

I do know they used to, since the power companies were very careful to be accurate so they could exchange power.

I once lived in a remote area that was supplied by an old diesel generator. The operator claimed that if he ran the output at 60hz, it would fly apart, so he kept it somewhere in the 40-50hz range. As a result, all our clocks ran slow, our movie projectors ran slow, and our refrigerators weren't too happy, either.
  #19  
Old 11-22-2013, 04:41 PM
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Originally Posted by Boyo Jim View Post
My theory, unsupported by specific knowledge, is that when the power ran out both clocks reset to midnight, and then a moment later switched over to internal power and started running again. If the power went out at 11:43 PM, that would be supporting evidence for my theory.
Or, the backup batteries had failed in both clocks. The electricity was off for four hours, but came back ON at 11:43, at which point the clocks started running again, from 12:00.

Does that sound about right?
  #20  
Old 11-22-2013, 04:48 PM
Canadjun Canadjun is offline
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Really? I was always under the impression that most (all?) digital clocks that run on AC use the line frequency as a time base.
Wikipedia indicates both methods are used.
  #21  
Old 11-22-2013, 05:12 PM
ZenBeam ZenBeam is offline
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Wikipedia indicates both methods are used.
But are there many clocks that run on AC power that use internal oscillators even when they are plugged in?

I don't have a cite that most AC powered clocks use the line frequency to keep track of time. It was common knowledge, and I assumed it was true, since it made sense. If you already have a very accurate oscillator (the AC line), why bother adding the expense of another one?

That link does show that kanicbird's statement "no digital clocks use the electrical frequency to gauge time" is incorrect. ETA: Well, except for the citation needed part.

Last edited by ZenBeam; 11-22-2013 at 05:13 PM.
  #22  
Old 11-22-2013, 07:06 PM
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But are there many clocks that run on AC power that use internal oscillators even when they are plugged in?

I don't have a cite that most AC powered clocks use the line frequency to keep track of time. It was common knowledge, and I assumed it was true, since it made sense. If you already have a very accurate oscillator (the AC line), why bother adding the expense of another one?

That link does show that kanicbird's statement "no digital clocks use the electrical frequency to gauge time" is incorrect. ETA: Well, except for the citation needed part.
I could not find much on it either except that it does appear that some clocks do use the line frequency - to the point that line frequency is corrected to maintain accuracy.
see Long-term stability and clock synchronization in this link:
http://en.wikipedia.org/wiki/Utility_frequency

Last edited by kanicbird; 11-22-2013 at 07:07 PM.
  #23  
Old 11-22-2013, 08:35 PM
kayT kayT is offline
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I was just reading this thread and found a suggestion that variations in power could cause a clock to run fast. Maybe when our power came back on line it was "dirty" or some other thing, causing the clock to gain time?
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Old 11-22-2013, 09:01 PM
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I suppose it's possible, but doing Fluke Starbucker's test should give you a clear answer.
  #25  
Old 11-23-2013, 07:18 AM
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Originally Posted by kayT View Post
I was just reading this thread and found a suggestion that variations in power could cause a clock to run fast. Maybe when our power came back on line it was "dirty" or some other thing, causing the clock to gain time?
This is what I'm thinking too.

Perhaps, before the power went out and/or after the power came back on, there was a lot of noise on the line. "Lots of noise" could mean "lots of zero crossings," and each clock uses the zero-crossings as a frequency reference.
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Old 11-23-2013, 08:20 AM
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I was thinking that by time the power came back on, things would have been taken care of and it would be clean, but I like the idea that it was noisy before the power went out.
  #27  
Old 11-23-2013, 08:52 AM
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I had similar issues when working at a remote location in Alaska that had their own power system. The clocks being incorrect was a common problem even without power problems. I would notice clocks being hp to +\- 15 minutes off after a couple weeks. We had a lot of brown outs and black outs occured fairly often for a while.
When I asked why my digital alarm clock was off I was told that surges from the power house cause clockes to either run quick or slow.
There was no supporting info and the person I asked had nothing to do with power generation on site.
Yeah. I spent a summer in a remote location in northern Canada. At one time, the forest fire threatened the transmission line, and the local dam was "off the grid", ie. not syncing to the 60Hz AC power cycle. As a result, that wandered (not power surges) and the typical bedside clock radios would wander by as much as 10 minutes a day compared to official CBC beep time. IIRC so did the big analog office clocks (like the ones we remember from school).

Generally, the 60Hz cycle continent-wide is precisely controlled, so it is a reliable signal to run a clock off. Also, any wander is quickly compensated so there is no long term "drift". If the device is not continuously powered, obviously, it probably needs a crystal. Remember the first cheap digital watches (push button to see red LED) used cheap crystals, These got progressively better over time so that from the mid 70's to the mid 90's watches went from a few minutes a week to a few seconds a month accuracy. Crystals are cheap and reliable. However, they can suffer from drift, so a mains-driven clock is less in need of adjustment. (Unless there's a power failure.) So odds are a crystal that needs to maintain time occasionally in a clock-radio does not need to be extremely accurate and the makers went even cheaper on the crystal.

My clocks without the battery backup will reset to 12:00 (and flash) in the event of a power failure when power returns. Same with my stove. My microwave will scroll the message to set clock. Since many power failures are of short duration, it best to let you know the time may be off. (It's also convenient to tell you when the power came on...)

Last edited by md2000; 11-23-2013 at 08:54 AM.
  #28  
Old 11-23-2013, 09:05 AM
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I was thinking that by time the power came back on, things would have been taken care of and it would be clean, but I like the idea that it was noisy before the power went out.
I would tend to agree. But when the power comes back on, lots of wacky things could happen on the power line due to heavy loads coming back on line.

I did some google searching on using the 120 VAC / 60 Hz waveform as a clock timebase. There are many circuits out there, and the few that I looked at simply converted the 60 Hz sine wave to a low-level 60 Hz square wave, divided it down, sent it to a counter, etc. etc.

This works O.K. if you trust the 60 Hz is... 60 Hz. But noise on the line that crosses the zero-axis will increment the counter, thereby making the clock run fast.

A more sophisticated circuit would try and filter out the high frequency garbage and extract the 60 Hz signal. This approach might work if the noise is additive and uncorrelated. But if the frequency of the 60 Hz waveform itself is screwed up, there may not be a clean-and-easy way to do it.
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Old 11-23-2013, 09:15 AM
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The best solution, of course, is to buy a WWVB radio controlled clock.

"Atomic" alarm clocks are pretty cheap nowadays.
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Old 11-23-2013, 04:16 PM
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I was just reading this thread and found a suggestion that variations in power could cause a clock to run fast. Maybe when our power came back on line it was "dirty" or some other thing, causing the clock to gain time?
I doubt it. The explanation is probably simpler. Digital clocks controlled by the mains frequency often have an internal oscillator and 9 V battery for backup but the internal oscillator is crap, it is just good for short power loss, so that a micro blackout won't reset the time, it is not really expected to be accurate. If the battery is not in good shape that could affect the precission as well. In this case I think this is more likely than dirty power line. You can test it easily: just unplug the clock for some time and see what happens.
  #31  
Old 11-23-2013, 04:22 PM
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The best solution, of course, is to buy a WWVB radio controlled clock.

"Atomic" alarm clocks are pretty cheap nowadays.
If it is mains operated and the mains fails then it will make no difference while power is gone but it will set itself to the correct time when power returns.

In Europe they sell them prety cheap too and I have a few but they use DCF77 in Germany.
  #32  
Old 11-24-2013, 12:22 PM
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Originally Posted by Boyo Jim View Post
My theory, unsupported by specific knowledge, is that when the power ran out both clocks reset to midnight, and then a moment later switched over to internal power and started running again. If the power went out at 11:43 PM, that would be supporting evidence for my theory.
I really don't see the pint of a battery backup that lets the clock reset to 12:00 and then keeps time from there. The only advantage I can see at all for this is if I needed to know exactly how long power was out. I'd think most people would rather know the time.
  #33  
Old 10-16-2014, 01:36 AM
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I do not care so much why this is happening, but would like to know how to correct it. We had a seven-hour power failure when a transformer died. When the power came back on, all clocks except two were keeping time perfectly (including clocks on t.v. cable boxes, telephones, computers and individual electric clocks). However, the clock on the Jennair oven and the GE microwave are now running fast. I have reset them multiple times, but to no avail. The transformer has not yet been replaced and our whole block is being run by a generator (now for about 5 days!). Perhaps once the new transformer is installed, the kitchen clocks will slow down. But for now, it is confusing. Isn't this odd?
  #34  
Old 10-16-2014, 02:26 AM
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It is probably using a different frequency or isn't a sine wave or something. I know when I used a UPS once for an alarm clock I ran into similar issues. I think some are even marketed as havering "pure sine waves" or something.

I am willing to bet that once the transformer is replaced - you will be fine. I have one of those watt measured devices that shows power usage and it will also show frequency - sometimes it is off a little bit - if memory serves like 59.98. If you think about it - it wouldn't take much to make a clock go off 60.6 for example is only a 1% variation, but that adds up to 14 minutes a day.

And stuff like your cable box is probably getting the time over the cable line. I know I never have to change mine during DST - other items have their own timing circuit and aren't using the line frequency as an oscillator. There is nothing you can do until the transformer is replaced - unless you see an engineer/tech by the generator - he might be able to adjust the frequency.

Last edited by DataX; 10-16-2014 at 02:31 AM.
  #35  
Old 10-16-2014, 02:45 AM
Duke of York Duke of York is offline
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I really don't see the pint of a battery backup that lets the clock reset to 12:00 and then keeps time from there. The only advantage I can see at all for this is if I needed to know exactly how long power was out. I'd think most people would rather know the time.
I was always under the impression that "battery backup" was only intended to save the alarm settings, and the station settings.

I have a DAB alarm clock which does reset back to the correct time.

Last edited by Duke of York; 10-16-2014 at 02:48 AM. Reason: Add extra info
  #36  
Old 07-23-2015, 06:03 AM
Ericcellis Ericcellis is offline
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I know this is an old thread but....

Hi, I'm an electronics engineer and thought I'd throw in some ideas of my own to help out. Ok, not all digital clocks use the line frequency (60hz in US) but most do. The power company is pretty good at keeping the the generator spinning at a constant 60 revolutions per second. That's why our line frequency is 60 hz. The clock counts out 60 cycles and knows 1 second of time has gone by. Now when the power fails, the clock must produce it's own signal. Some might use a crystal for this, but the loss expensive clocks will use a ic chip like a lm555. This chip allows you to take a DC voltage (DC = Direct Current = 0 Hz) and have a pulse frequency of whatever you design. You change the frequency (and other things that don't matter here) by changing the resistors and capacitors at the input. I'm simplifying this as far as I can so let's pretend it only takes one resistor at the input and is 10000 ohm aka 10k ohm. So at 10k ohm you get a nice 60 hz pulse. Now you build the next one. You put in another 10k ohm resistor but instead of 60hz, you are getting 58 hz. Losing 2 seconds per minute. You read the resistor with a ohm meter and it reads 9923 ohm, not 10000 like it should be. You build another one. Now you get 63hz. You are gaining 3 seconds. Now your resistor reads 10856 ohm. Most resistors have a +/- tolerance of 10%. So just the resistors alone could cause you to gain or lose time. But now we have to realize there are a few resistors in this circuit. Each one adding to the error. Sometimes the high value resistor gets offset by a low value resistor in the same circuit. Other times they are both high or both low so the error is larger. Next you have to take into account that the capacitors have +/- tolerance too. The ic can add a little more. In the end you have a bunch of smaller errors adding together to a big one. If the error was +/- 2 seconds per minute, you'd be looking at +/- 8 minutes in 4 hours. This is looking at it from really high up. I didn't want to go into electronic theory and do calculations. I'm to tired for that. Lol
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Old 07-23-2015, 10:43 AM
Learjeff Learjeff is offline
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Aha: the temporary generator is running fast. That is, it's generating higher than 60 hz. Clocks using crystals are keeping good time; clocks using line frequency are running fast.
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