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HP Forum Archive 21

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[WP34s] Accurate Clock
Message #1 Posted by Katie Wasserman on 8 Apr 2012, 12:48 a.m.

I added a crystal and load capacitors to my WP34s and was bothered by the inaccuracy of results as others have reported here. While you might get a lucky and find an accurate crystal with the stated 20 or 30 ppm of most of these 32K crystals you typically end up losing or gaining about 2 seconds per day -- not terrible but not as good as a $10 Timex watch.

I decided to change one of the load capacitors to a tiny trimmer (variable) capacitor and used that to "pull" the crystal to better accuracy. I found that there was enough room inside the calculator to use thru-hole devices for the crystal, fixed capacitor and variable capacitor:

While a little messy, it works just fine and with some trial and error over the course of many days I was able to adjust the clock to within about 1 second per week. The oscillator is not temperature compensated so this accuracy is only obtainable at a constant temperature. Also this won't last forever as the crystal will age, but it's much better than when I started with two fixed 6pf capacitors.

For the record I used the following parts:

Digikey 300-8301-ND - Citizen 32,768Hz 6pf load xtal

Digikey 490-3752-ND - Murata 6pf ceramic capacitor

Mouser 242-2820 - Xicon variable capacitor 2.8 - 20 pf


Edited: 8 Apr 2012, 1:34 a.m. after one or more responses were posted

Re: [WP34s] Accurate Clock
Message #2 Posted by Walter B on 8 Apr 2012, 1:25 a.m.,
in response to message #1 by Katie Wasserman

Thanks, Katie, for sharing :-)

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