Mystery solved... For a couple of years now, we've had a problem with three different digital clocks. All of them mysteriously ran too fast, but intermittently. Over the course of a month, they might gain a half hour or so. The thing that's really weird about this is that three completely different digital clocks – different brands, different circuitry, different chips at the core – all did the same thing. Also, the problem happened whether the clock was in our house or in my barn office – and those two places have different electrical supplies (even different transformers!). I still suspected something amiss with our AC power supply, but then when I ran one from my UPS (a model that is continuously supplying inverted power) it still had the same behavior. I gave up, and started to think of explanations that included Harry Potter and divine intervention.
But then a few weeks ago, the problem suddenly stopped. WTF? Three separate clocks having a problem, and all of a sudden they all work? How could this be??
A few days ago I had the first thought of a possible explanation. The clocks all started working about the same time that the power company fixed the top part of our power pole (blogged here) – could that be what fixed my clocks? I didn't know exactly what they did beyond replacing the top crossbar. So I called the power company, hoping I might get some answers. And I did! Turns out they keep good records of what was found and what was done – and one of the notes they'd made on that day said “heavily corroded HV connection”. The “HV” means “high voltage”, the input into the transformers for my house and my barn – and those two transformers shared that same HV connection. Ah ha! Now there was a possibility! A heavily corroded connection might exhibit intermittent connections when, for example, the wind shook the power pole a bit.
So I rigged up a bit of an experiment with one of the clocks. I wired up an outlet whose power came through a wire that I cut, stripped, and then bound together with a rubber band. Then I tried wiggling that joint. Lo and behold, I was able to replicate the peculiar behavior. The clock didn't lose power, but it did pick up the intermittent connection as though it was additional cycles to be counted – and the result was the clock gained time, just as we used to observe.
I'm left with the mystery of why the problem still happened with a clock connected to my UPS. I think the most likely explanation is that the intermittent connection issue was transmitted through the UPS as coupled noise – which is certainly disappointing. The UPS works fine if I switch off its primary power, so I know the inverter it contains is working properly. I see no glitches on its output (using a 'scope) if I flip its primary power on and off. Nonetheless, the clocks malfunctioned when running on it prior to January 11.
I have to conclude that for our entire time in this house, up until the repairs on January 11, we've had flaky power and didn't even know it! The clocks have been rock-solid now for over a month. I'm very glad to have the problem fixed, but I'm still amazed that we didn't even know we had a problem!