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Improve the Accuracy of your G-Shock

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Materials required:

  • 1. G-Shock to be adjusted (the “target” watch).
  • 2. Accurate time source (e.g. or a clock that synchronizes with atomic time).
  • 3. Case opening tools (page 9).
  • 4. Stopwatch (not the G-Shock that is being adjusted).
  • 5. Notepad.
  • 6. Pen.
  • 7. Patience!

Step 1 - Set the target watch time

Set the target watch from an accurate time source (the “base time”). If you are using an atomic clock as your source of base time, ensure that the clock has successfully synchronized within the last twelve hours.

Step 2 - Check the target watch time

You need to check exactly how close the target watch is to the base time. If you have performed step 1 perfectly, then you should find that the target watch time is exactly the same as the base time. You could just try looking at the target watch and the base time simultaneously, but unless the time difference is large you probably won’t be able to spot it this way. So here’s a better way to do it.

  • 1. Look at the base time, and start the stopwatch when the base time seconds reach 00.
  • 2. Look at the target watch, and stop the stopwatch when the target watch seconds reach 05.

If you had set the target watch very carefully from the base time, the stopwatch should now read exactly 05 seconds. So if there’s any difference from 05 seconds, then the target watch is slightly offset from the base time.

To get the target watch offset, simply subtract 5 from the stopwatch reading. So:

Offset = Stopwatch reading - 5

For example, if the stopwatch reading is 00:05:20 (i.e. 5 seconds and 20/100ths) then you calculate 5.20 - 5 = 0.2. So the target watch is running 0.2 seconds slow.

On the other hand, if the stopwatch reading is 00:04:70 (i.e. 4 seconds and 70/100ths) then you calculate 4.70-5 = -0.3. So the target watch is running 0.3 seconds fast.

In your notebook, write down today’s date and the target watch offset. (Don’t forget the + or - sign!)

Step 3 - Determine the target watch accuracy

Wait one day, or as near as you can get to 24 hours later.

Repeat step 2.

As the target watch has now been running a day since you synchronized it, the new target watch offset enables you to calculate how fast or slow the watch is running.

In your notebook, you’ll now have something like Table 1.1.

Table 1.1: Accuracy chart

Day Target Watch Offset (seconds)
Monday -0.1
Tuesday -0.5

Now subtract the first figure from the second one. In the example, -0.5 - -0.1 = -0.5 + 0.1 = -0.4. This figure is the number of seconds the watch has lost in a day. In this case, the figure is negative, so the watch has gained 0.4 seconds.

Step 4 - Note the target watch accuracy

Write down the date, time, and how much the target watch gained or lost. Your notebook will look something like Table 1.2.

Table 1.2: Accuracy chart

Day Target Watch Offset Change
Monday -0.1
Tuesday -0.5 -0.4

Step 5 - Adjust the trimmer capacitor

Remove the back cover of the target watch. Find the trimmer cap (indicated in Fig. 1.1).


Figure 1.1: Finding the trimmer cap

The trimmer cap may be in a slightly different location in your watch. You don’t have to remove the module from the case in order to get access to the trimmer.

Turn the trimmer cap about 1/32 of a turn in one direction. The precise amount doesn’t matter, but it should be a very very slight adjustment. No advice can be given on which direction to turn it; depending on the precise position of the trimmer, clockwise may speed up the watch, or may slow it down. Just try clockwise first, and if you find the change is going the wrong way, next time try counter-clockwise.

Now repeat the whole process, and keep repeating until you are happy with the accuracy. As the watch gains accuracy, you should make smaller and smaller adjustments to the trimmer each day. It is very hard to judge these tiny adjustments until you get more experience, so you may find that sometimes you have gone a little too far in one direction. That’s fine; the next day, you can just make a small tweak to reverse it.

You may find that after a while you have to leave two or three days in order to detect a difference between the target watch and the base time. You might feel that this is a good time to stop!

With a lot of patience, and experience, you can end up with a G-Shock that remains accurate to within a second or two over the course of several months.

Who needs atomic time?

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