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Curiosity question of a 30-year-old alarm clock that kept perfect time till lately.

18 months ago, in 2021, it started randomly speeding up. Runaway minutes then back to perfect time. With this alarm clock I was under the assumption that it may be dependent on grid frequency or harmonics or something causing interference in my house; I switched all off in the house and it still did it.

Out of curiosity, I spoke to the power company and they sent someone over to place a monitoring device over 7 days, yes, I was surprised! It happened several times during the monitoring, however the power company said nothing is wrong.

Put the clock away (I don't throw much out) and came across it 7 months ago, Nov 22. Plugged it in and left it—coming back a week later and still perfect time. Perfect time until a week ago, June 23. First it was a few minutes than quick runaways into hours.

Nothing on the premises changes or has changed and it can happen at any hour of the day or night.

As can be seen in the picture, there is not much to it. 28-pin IC, resistors, capacitors, diodes, transistor and transformer. Operating voltage is 240 VAC 50 Hz and all components look OK.

An LED bedside clock with the cover removed

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    \$\begingroup\$ Test the electrolytic capacitor for ESR and value. \$\endgroup\$
    – Tim Williams
    Commented Jun 8, 2023 at 10:02
  • \$\begingroup\$ Components change their values over time, electrolytic capacitors are often named first. Chips in plastic can degrade, too. \$\endgroup\$
    – the busybee
    Commented Jun 8, 2023 at 10:05
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    \$\begingroup\$ My suspicion would be that whatever is used to keep time, be it a counter on the grid frequency or a Pierce oscillator with a quartz crystal, is aging physically, and that leads to spurious counts of "clock ticks". It might not be an environmental unwise move to discard that clock - I, a couple of years ago, got rid of my childhood alarm and FM radio clock, after having it run a week on my energy meter. Turns out it uses nearly 5 W continuously, which does explain the warmth. 1 W over the course of a year at 0.33 €/kWh is pretty much 3€, so 5 W is 15€. Compare that to energy cost that is \$\endgroup\$
    – Marcus Müller
    Commented Jun 8, 2023 at 11:37
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    \$\begingroup\$ included in the street price for new alarm clocks that maybe use 3 W less, and it becomes quite likely that upgrading your alarm clock is the environmentally wise thing. (It's by the way something to do to your parents, if they have enough money to buy a new fridge: when visiting them, leave your energy monitor running on their decades old fridge, and pick it up next visit. Compare to energy consumed by better isolated newer fridge according to EU-standardized testing methods. Discuss return on investmentment over the course of 2 years.) \$\endgroup\$
    – Marcus Müller
    Commented Jun 8, 2023 at 11:41
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    \$\begingroup\$ If you tap it while it's on, does that affect the time? If so, it could be a loose solder joint. \$\endgroup\$
    – Andrew Morton
    Commented Jun 8, 2023 at 14:26

1 Answer 1

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These clocks often use the line frequency for their time base as it avoids the cost of a crystal oscillator and has quite good long term accuracy.

If that's the case with this particular clock it may be noise on the power lines causing it to count erratically, perhaps there is something plugged in nearby that has a switching type supply creating high frequency noise on the power lines. This wouldn't have been a problem when the clock was originally made, there were few switchers back then, but now they are ubiquitous and their quality can vary quite a bit.

You could try plugging it into a power strip that contains a line filter and see if that clears up the problem.

If the problem gets worse the longer the clock is plugged in it's probably a problem with one of the components in the clock itself and not outside interference.

Some clocks do use a crystal or ceramic resonator oscillator and this could be going bad causing intermittent spurious oscillations. You should be able to see a crystal or resonator on the circuit board if it's one of these types of clock.

Or it could be the clock IC itself going bad.

The power supply filter cap is also a possible source of problems, in a 30 year old device it should probably be replaced anyway.

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