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Insights on Timing Recalibration

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jadney

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Insights on Timing Recalibration

Post02 Feb 2021, 20:01

This is specific to my 95QGR-33, but it should apply equally to any quartz watch that has a 32,768 Hz crystal and an adjustable capacitor for calibrating the frequency.

It has previously been mentioned that just removing the back from the watch will change the oscillator frequency and so will connecting a probe to the test point on the watch, so simply setting the measured frequency to 32,768 does not result in good timing. Instead, we need to determine a change in frequency that will get us to the right place.

I had previously posted about setting the timing by trying to figure out the frequency error in ppm (parts per million.) That can work, but it's tedious. It occurred to me that an easier way would be to consider the fact that a frequency error of 1 Hz would result in a time error of 1 second after each 32,768 seconds. 32,768 equals 9.1002 hours. I wanted to find a simple way to use this to determine the frequency error over some integral number of days. It turns out that 3.05 days (~72 hours) comes to 8 "time periods" of 32,768 seconds. So here's what I did with my 95 QR that had been gaining a lot of time.

I set the watch to my computer time and then checked it again in exactly 3 days. (Same time of day, 3 x 24 hours later) If the frequency had been off by 1 Hz, the time error would have been 8 seconds. My watch was fast by 15 seconds, so the frequency was fast by about 1-7/8 or 1.875 Hz. I choose to reduce it by a bit more, 1-15/16 or 1.937 Hz to put it on the slow side, to allow for future ageing.

I opened the watch and connected my HP frequency counter via a low capacitance probe. It was difficult to get a stable reading but I managed to read 32,769.566 Hz.

Subtracting 1.937 gives 32,767.629 Hz as my goal, but I was not able to adjust it that finely. Instead I tried to shoot for something slightly lower, because normal ageing of a quartz crystal will always cause an upward drift in frequency.

After testing for an additional 3 days, this watch still appears to run fast by perhaps 1/2 second. I'm now letting it run for 30 days, which should give me a much better picture on what additional adjustment might be necessary, if any. If I'm right about being ~1/2 second fast over 3 days, that comes to 5 seconds over 30 days, which is pretty good compared to the original spec of +/- 30 seconds per month. Still, I'd like to get it a bit on the slow side, so that it will improve over the years to come.

Note that this method does not require an accurately calibrated frequency counter, but it does require one that can measure to an accuracy of 3 or more decimal places. It may be useful to me to record the frequency I end up at; this could be used in the future on other 95Qs if I use the same probe and if the counter has not aged upwards significantly.

Hope others find this useful, or at least interesting.
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Jim Adney
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Re: Insights on Timing Recalibration

Post04 Feb 2021, 18:44

Thx interesting read

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