The Seiko A639A is a reliable and versatile digital quartz movement, introduced by Seiko in the 1980s to support its expanding line of multifunction watches. The movement is well-regarded for delivering a full suite of features in a robust and easy-to-use module. It was often found in sporty, rectangular, or wedge-shaped watches that were built for everyday wear and light athletic use.

Technical Specifications

• Caliber Number: A639A

• Movement Type: Quartz (Digital LCD)

• Display Type: Liquid Crystal Display (7-segment)

• Time Format: 12-hour (with AM/PM indication)

• Functions:

  • Timekeeping (HH:MM:SS)

  • Calendar (Day and Date)

  • Daily Alarm

  • Stopwatch (1/100-second resolution)

• Backlight: Not built into the module (some watch cases may include external light feature)

• Battery Type: SR1120W (1.55V silver oxide)

• Battery Life: Approx. 2 years

• Accuracy: ±15 seconds per month

• Dimensions:

  • Width: ~26 mm

  • Height: ~22 mm

  • Thickness: ~5 mm

LCD Layout and Indicators

The Seiko A639A features a high-contrast LCD designed for intuitive use.

Layout:

• Top Line:

  • Day of the week (e.g., MO, TU, WE)

  • Function indicators:

    • “AL” (Alarm)

    • “CH” or “ST” (Chronograph/Stopwatch)

    • “PM” (if 12-hour mode and time is post-noon)

• Main Display:

  • HH:MM:SS in normal time mode

  • Stopwatch mode: MM:SS.1/100

  • Calendar mode: MM-DD or DD-MM (depending on market)

This layout keeps essential information front and center without overwhelming the user.

Functional Modes

The A639A cycles through four main modes, accessible via the mode button:

1. Timekeeping Mode

• Displays hours, minutes, and seconds

• Includes AM/PM for 12-hour format

• Default mode when exiting any other function

2. Calendar Mode

• Displays the day of the week and current date

• Date format may vary (MM-DD or DD-MM)

• No year or leap-year tracking—manual correction required

3. Alarm Mode

• One daily alarm

• Set by hour and minute

• Alarm ON/OFF toggle available

• Piezo buzzer provides audible sound

• “AL” icon shown on screen when active

4. Stopwatch Mode

• 1/100-second resolution

• Measures up to 59 minutes, 59.99 seconds

• Functions include Start, Stop, and Reset

• Useful for casual timing (e.g., workouts, cooking)

Button Configuration (Typical)

Watches using the A639A generally feature four pushers:

• Button A (Top Left) – Mode select

• Button B (Bottom Left) – Enter setting mode / select digit

• Button C (Top Right) – Increase value / Start/Stop stopwatch

• Button D (Bottom Right) – Reset stopwatch / confirm selections

The exact layout may differ slightly depending on the watch model, but this structure is consistent across most A639A applications.

Setting Instructions

To set time or alarm:

1. Enter the desired mode using Button A

2. Press and hold Button B to enter setting mode (flashing digits)

3. Use Button C to increase values

4. Press Button B again to shift to the next digit (e.g., from hour to minute)

5. Press Button A to confirm and exit

To toggle alarm ON/OFF:

• Use Button D while in alarm mode

Battery Replacement & AC Reset

After replacing the battery, an AC (All Clear) reset is required for the module to function properly.

Reset Procedure:

1. Open the case back carefully

2. Remove the used SR1120W battery with plastic tweezers

3. Install a new battery, ensuring correct polarity

4. Locate the AC terminal (marked on PCB)

5. Short the AC terminal to the battery’s negative terminal (case or spring) using metal tweezers for 1–2 seconds

6. LCD should flash briefly and return to default time mode

Failure to reset may result in a blank or non-functional display.

Maintenance Tips

• No lubrication required – fully electronic

• Clean buzzer contact if the alarm sound is weak

• If buttons become sticky or slow, open case and clean contact pads

• LCD fading or segment loss may indicate age-related damage; replacement of module is usually necessary

• Avoid moisture – unless marked “Water Resistant,” A639A watches should not be submerged

Watch Models and Use

The A639A was used in Seiko’s mid-range Alarm Chronograph watches. These watches typically featured:

• Rectangular or wedge-shaped stainless steel cases

• Integrated bracelets or rubber straps

• “Alarm Chronograph” text on the dial

• Popular references include A639-5000, A639-501A, etc.

These models were often worn by students, professionals, and casual wearers in the 1980s.

Conclusion

The Seiko A639A is a straightforward, dependable digital movement that delivered Seiko’s promise of functionality, precision, and user-friendliness. With a solid combination of alarm, stopwatch, and calendar features, it remains a reliable choice for collectors of vintage Seiko digital watches. Its durable design, intuitive interface, and nostalgic styling continue to make it a standout in Seiko’s digital era.