Replacing one component made a $2 Casio F91W from AliExpress 95% more accurate than 10x more expensive original Casio
Casio F91W is probably the most sold wrist watchin the world with 3-4 millions of pieces sold every year. It offers the most basic features, which did non changed since its presentation in 1989: a watch, stopwatch, green backlight and plastic case with a plastic strap.
Casio sells this time tracking unit for at least $22.95 and claims the accuracy of the Casio F91W model around +-30 seconds per month. Looks like Casio does not use top grade components for its F91W model, as this is two times lower accuracy than the industry standard of +-15 seconds for watches with 32,7 kHz quartz.
The standard quartz-driven watches (either with time hands, or with a digital screen) use oscillators that work with the frequency of 32768 Hz (it is basically a heartbeat of the watch). Such resonators demonstrate an accuracy of 0.0006% daily. In more daily terms this results in an accuracy of +-15 seconds per month.
In reality, even more affordable models perform better. But how affordable a watch can be to demonstrate timekeeping accuracy outside of the specified accuracy margin?
This particular fakest of fake Casio F91W was purchased for less than $2 with a free shipping provided. However, it looks like with such a low price you can not buy a watch. You can buy a wrist bracelet that resembles a watch with an LCD screen.
The accuracy of the purchased 2-dollar Casio F91W was unacceptably low. The time.is website was used as a baseline. A short measurement conducted for six days resulted in 1 minute 3 second inaccuracy. That would result in an inaccuracy of 5 minutes 18 seconds per month or 318 seconds per month.
Luckily, by replacing one element - the quartz - a digital watch can be made much more accurate. If you have a broken quartz-based watch, you will be able to increase the accuracy of the Casio F91W for free. Alternatively, 32.7 kHz quartz can be purchased for $0.2 per piece.
A replacement quartz was taken from broken fake Rolex Submariner. It took two touches of the soldering iron to fulfill the quartz transplantation procedure. After which the Casio F91W started to exhibit the inaccuracy of 3 seconds per six days, which converts to 15 seconds per month. That is well within the standard accuracy range, and is two times better than claims of the original Casio F91W, which is over 10 times more expensive.
The watch can be made even more accurate by adding a picofarad capacitor to the oscillator. For now the achieved watch accuracy is enough, and adding a capacitor may be implemented in the future.
Here is the replacement oscillator placed above the factory installed quartz.
Here is the video on the soldering of the replacement oscillator. The old low quality oscillator is visible below:
The quartz oscillator taken from the Rolex Submariner exhibited stable frequency of 32.7 kHz
The factory-installed oscillator of this 2-dollar Casio F91W exhibits frequency spikes up to 32,9 kHz. The frequency was not measured before quartz replacement, that is why this bad quartz was measured inside the Rolex Submariner mechanism.
Sucess for free
In the end, this $2 fake Casio F91W is now just as accurate as a high-quality quartz watch - all for free, using a salvaged component. Visually identical to the original Casio, this simple modification proves that even a budget watch can perform at a top-tier level with a little tweaking.
This knockoff watch has become twice as accurate as Casio’s official claim for the original F91W, making this $2 unit more precise tan watches that cost 10 times more.
Meanwhile, Casio continues to sell its F91W for over $22 with inferior timekeeping performance.