You probably have one. Or you've seen one. Or your dad has one in a junk drawer next to some old batteries and a rusted flathead screwdriver. The Casio F-91W is basically the cockroach of the horology world—it survives everything, it's everywhere, and it refuses to die. But here's the thing: despite being one of the best-selling pieces of technology in human history, almost nobody actually knows how to use the thing without trial and error. You start clicking buttons, the alarm goes off at 4:00 AM, and suddenly you're hunting for a Casio F91W instruction manual PDF at three in the morning.
It’s a tiny masterpiece of 1980s engineering. It’s also incredibly annoying if you don't know the "long press" secrets.
Let’s be real. The paper manual that comes in that tiny blue and white box is microscopic. It’s printed in a font size that requires a literal magnifying glass and is folded into a map the size of a postage stamp. If you've lost yours, don't sweat it. Most people do. This is the breakdown of how that Module 593 actually functions, minus the corporate jargon and the tiny paper.
Getting Around the Three-Button Layout
The F-91W doesn't have a crown. No twisting bits here. You've got three buttons, and Casio, in their infinite wisdom, labeled them with tiny text on the resin case that most of us can't read after the first year of wear.
Button A is at the top left. This is your "Light" button, but it's also the "Reset" or "Select" button when you're in setting mode. Button B is at the bottom left. This is the "Mode" button—your gateway to the different dimensions of the watch. Button C is on the right side. It’s the "Start/Stop" or the "12/24 hour" toggle.
It sounds simple. It isn't always.
If you want to change the time, you have to cycle through the modes using Button B. You’ll hit the Alarm screen first. Then the Stopwatch. Then, finally, the digits start blinking. That’s your cue. If you miss it, you have to cycle all the way around again. It feels like a 1980s video game cheat code. Honestly, the tactile "click" of these buttons is half the reason people keep buying these things for twenty bucks.
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How to Stop the Hourly Chime from Driving You Insane
This is the number one reason people go looking for a Casio F91W instruction manual. You’re sitting in a quiet meeting or a movie, and beep-beep. Every. Single. Hour.
To kill the chime, you need to navigate to the Alarm mode. Use Button B to get there. Once you see the "AL" icon, look at the bottom of the screen. You’ll see a tiny symbol that looks like a radar or a series of waves. That’s the alarm indicator. There’s also a tiny bell icon. That’s the hourly signal.
Press Button C.
Every time you press it, the watch cycles through four states:
- Alarm on.
- Hourly chime on.
- Both on.
- Both off.
If you want total silence, keep tapping until both icons disappear. It’s a relief. Truly.
The Secret "CASIO" Screen and Why it Matters
There is a weird bit of lore involving the F-91W. Because these watches are so cheap and popular, the market is absolutely flooded with fakes. Yeah, people actually counterfeit a $20 watch. It's wild.
But the Casio F91W instruction manual won't tell you the secret handshake to check for authenticity.
Hold down Button C (the right one) for about three to five seconds while you're on the main time-telling screen. If the watch is genuine, the word "CASIO" will appear across the display. If nothing happens, or if the screen just flickers, you've likely got a knockoff. It’s a built-in firmware check that Casio implemented to help users verify their gear.
There's another "hidden" feature. If you press all three buttons at once, every single segment of the LCD will light up. It’s basically a pixel test. If you see gaps in the numbers, your battery might be dying, or the zebra connector inside (the rubbery strip that connects the screen to the circuit board) is getting loose.
Setting the Time Without Losing Your Mind
Ready to sync with the atomic clock? Or at least your phone?
Press Button B three times. The seconds should start flashing.
Now, Button A is your best friend. Pressing it will move the "flashing" highlight from seconds to hours, then minutes, then month, then date, then day of the week.
When the thing you want to change is blinking, hit Button C to advance the number.
Here is the kicker: you can only move forward. If you’re trying to set the hour to 10:00 and you accidentally skip to 11:00, you have to go all the way to 12, then through the PM hours, then back around. It’s a test of patience.
One thing people always forget is the "Double Zero" trick. If you press Button C while the seconds are flashing and the count is between 0 and 29, it resets to zero without changing the minute. If the seconds are between 30 and 59, it resets to zero but adds a minute to the time. It’s a quick way to sync up if you’re slightly off.
The Infamous Light (Or Lack Thereof)
We have to talk about the light. It’s legendary for being terrible.
The green LED on the left side of the screen is a relic. It barely illuminates the left half of the display, leaving the minutes in a sort of shadowy mystery. People complain about it constantly. But honestly? It’s part of the charm. If you wanted a backlight that could double as a lighthouse, you’d buy a G-Shock with an Illuminator display.
The Casio F91W instruction manual doesn't give you tips on how to make it better, but the community does. Some people actually take the watch apart and put a tiny piece of aluminum foil behind the LED to reflect more light across the LCD.
Don't do that unless you’re prepared to lose a tiny screw and ruin your water resistance.
Speaking of water, the F-91W says "Water Resist" on the front. It does not say "Diver." It’s rated for 30 meters, which in watch-speak basically means "you can wash your hands or get caught in the rain." Don't go pressing the buttons underwater. That’s the quickest way to turn your digital watch into a miniature aquarium.
Actionable Tips for New Owners
If you just picked one up, here is the short-list of things to do immediately so you don't have to keep googling how the thing works:
- Toggle 12/24 Hour Time: Just press Button C once while on the main screen. That's it. No menus required. It’s the easiest feature to trigger accidentally, which confuses a lot of people when they suddenly see "14:00" instead of 2 PM.
- The Strap Tail: The resin strap is notoriously long for some wrists. Don't cut it. The material tends to split if you leave a raw edge. If the "keeper" (the little loop) keeps sliding off, a small black O-ring from a hardware store can act as a secondary keeper to hold the tail down.
- Battery Life: The manual says seven years. In reality, these things often last ten or twelve. If the screen gets dim when the alarm beeps, it's time for a CR2016 battery. Replacing it requires a #0 or #00 Phillips screwdriver and a pair of tweezers to short the "AC" contact to the back of the battery to reset the module.
- Cleaning: Resin absorbs sweat. Every few months, hit it with an old toothbrush and some mild dish soap. It prevents that weird "vintage watch" smell and keeps the buttons from sticking.
The Casio F-91W is a tool. It's not a status symbol, yet somehow, it’s become one among designers and engineers. It’s simple, it’s honest, and once you memorize the three-button dance, you’ll never need to look at a manual again. Just remember: Button B is your map, and Button C is your action. Master those, and you've mastered the most popular watch on Earth.