Casio CFX-20/200 Scientific Calculator Watch Page

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CFX-20 CFX-200 CFX-40 CFX-400

Quite possibly one of the finest engineer's watches ever made, combining a chronograph, stopwatch, alarm and ten digit scientific calculator with metric conversions all in a face that is around 1x1 inch. The CFX series watch was initially contracted from Casio by a company called "Best Products" and was sold mainly in stores like "Service Merchandise". Best Products is no longer around and Casio no longer manufactures a calculator watch with scientific functions. I discovered this in 2000 when my crystal fractured and frantically started to search for a replacement. You probably found this web page through a search engine while on the same mission.


You can get a watch from Casio with a built-in remote control, 3,000 entry database, GPS receiver, or digital camera, but they have left the engineering community out in the cold by failing to follow-up with a replacement scientific calculator watch. Many of the CFX 20/200 and 40/400 watches are now succumbing to old age. There is a high interest in a watch with these functions, as evidenced by the number of e-mail hits that I receive regarding this web page. My best advice to you is to mail Casio and beg them to re-issue a new generation of scientific calculator watch. I would also send copies to Timex, Armitron, and The Sharper Image, asking them to consider developing one. If Casio is not interested in filling the market demand, perhaps one of their competitors will.

Your second option is to buy a used one - if you can find it. E-Bay is a good place to look. Do a search for "casio calculator watch -CD" to look for potential sales. Prices have been pretty steep, with some watches selling for over $450. When bidding on such a watch, look out for missing buttons. The CFX watches were not particularly water proof and can fail from the intrusion of skin oils and sweat, especially if the watch has a gaping hole in it. Any watch with missing buttons will not last long and should only be worn infrequently. Also beware of the plastic case CFX-40 watches. They tend to crack along the line in the case where the two screws secure the back lid and make the watch susceptible to moisture.

Liquids and skin oils can get into the watch case fairly easily. Do not wear your watch when washing your hands, dishes, car, etc. If you are in a laboratory environment, do not wear it under latex or nitrile gloves. Do not wear it to bed. If it is functional and in good shape, treat it like a $500 Rolex, because that is essentially what you are now wearing on your wrist.

As of this writing, the watch exchange is defunct. Many people were interested in having their watch refurbished, but did not want to part with it and wait for others to contribute to the parts pool. This is understandable. Due to expanded work responsibilities and family time, I can no longer offer any repairs or cleaning services. Thanks to those of you who took avantage of the service in the past two years.

Wanted - any parts or non-functioning watches for parts. If you know of any sources (repair centers, depots) of spare parts or watches no longer working, please contact me. Maybe we can return a few more into circulation.

Need a new battery?

The original battery used in the CFX-20/200 series watches was the Eveready 2020. You will probably have a hard time locating one in the stores today. You can use the 2016 battery as a replacement. It is slightly thinner than the 2020, but has worked well for all who have recently installed them. For the 40/400 series, use the CR1616 battery.

Forgot how to set the date and time?

If you are handy with small tools and have access to a microscope, here are some tips to help clean and restore function to your watch keys, alarm, and battery clip. This requires disassembly of the watch module and may result in irreversible damage if not handled properly with the aid of a stereo microscope:


The calculator keys (on the cfx-20 and cfx-200) are operated by having one interdigitated set of gold contacts which are connected by a conductive rubber pad on the bottom of the key.  My experience has been that the buttons stop working when oils, dust, debris, and  human contaminants accumulate on either the gold contacts or rubber pad. 

The best way to clean them is to disassemble and clean the watch under a stereo microscope.  Make a small cleaning probe by taking a wooden Q-tip stick (Puritan brand), break off the cotton tip and sand the stick tip down to a fine, narrow point. Never use a metal tool, knife blade, or any metallic object to clean the contacts.  Sparkle is the cleaning solvent I use.  It is a purple-colored household glass cleaner that we use to clean laboratory optics (it is the only one we've found which does not damage fragile optical coatings).  Use a can of compressed HCFC air to remove any loose particulate debris from the contacts.  Use short controlled streams of air.  Avoid "blasting" the watch and try not to tip the can, as it will become a "chill spray" and may introduce condensation into the electronic module.  Next use the wooden probe and gently scrape off any large particles or residue on the gold contacts.  Clean the tip off with a lint-free cloth.  You may need to break off the tip occasionally and sand down a new point.  For the last step, dip the tip of your wooden probe in the sparkle and clean off the rubber pad surfaces.  This will restore operation to your buttons.  The side buttons are operated by metal spring contacts.  The cause of failure here is also oils, debris and human contaminants.  Use the wooden probe and clean off the two metal contact surfaces.

The alarm piezo grows faint over time because the contact surface where the spring makes contact oxidizes.  A little abrasion with either the wooden probe, tech wipe (fiberglass laboratory tissue) or clean eraser tip in the area where the spring touches the piezo will restore electrical contact. Use the sparkle to remove any abrasion dust/debris.  After 15 years, my cfx-200 had gone completely silent.  It's now as loud as the first day I bought it.

The battery clip appears to have been over-hardened on some watches.  My first cfx-20 clip snapped a tab the second time the battery was replaced. You can work around this problem by placing a strip of "capton" tape (electronic manufacturing grade) over the spring clip and battery.  The important thing is to make sure that it secures the clip and battery to the module and does not cover the piezo spring surfaces.  You can trim up the capton tape with an X-acto knife prior to assembly, so you can get the dimensions pared down to where the tape will fit well.


You've just replaced the battery in your cfx-40/400 watch, but the display is blank. Is your watch broken? No! The 40/400 watch module has a reset node that mst be shorted to the ground shield in order to bring it back to life. With the module back facing toward you, look in the lower right hand corner. Locate the gold circular pad with the "AC" stamped in the metal ground shield. This is your objective. Bend a paper clip into a "U" shape. Touch one end to the gold pad and the other to metal shield which covers most of the module back. Remove the paper clip and turn the watch over. The display should now be active.

CFX-40 face image by Gary Ross

Last updated 10/9/2006