The Seiko A639A is a quartz digital LCD movement released in the mid-1980s, part of Seiko’s continuing push to offer robust, multifunction digital watches with sporty and practical appeal. The A639A builds on earlier A-series chronograph movements, offering a full suite of features including timekeeping, calendar, daily alarm, and a 1/100-second stopwatch. Known for its accuracy, user-friendly interface, and iconic layout, the A639A remains a respected and collectible movement in Seiko’s digital legacy.

Technical Specifications

• Caliber Number: A639A

• Movement Type: Quartz (Digital LCD)

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

• Primary Functions:

  • 12-hour time format

  • Full calendar (Day and Date)

  • Daily alarm

  • 1/100-second chronograph (stopwatch)

• Backlight: Not included in the module (some watches may feature case-mounted lighting)

• Battery Type: SR1120W (1.55V silver oxide)

• Battery Life: Approximately 2 years

• Accuracy: ±15 seconds per month

• Module Dimensions:

  • Width: ~26 mm

  • Height: ~22 mm

  • Thickness: ~5 mm

Display Layout

The A639A features a logical and efficient LCD layout, focused on legibility and intuitive navigation.

• Top Row:

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

  • Indicators: “AL” (Alarm), “ST” (Stopwatch), “PM” (for 12-hour mode)

• Main Display Area:

  • Time: HH:MM:SS in normal mode

  • Stopwatch: MM:SS.1/100

  • Date: MM-DD or DD-MM (regional variants)

The interface is clean and functional, consistent with Seiko’s digital design language from the period.

Operating Modes

The A639A includes four primary modes, accessible via button cycling:

1. Timekeeping Mode

• Displays current time in 12-hour format

• AM/PM indication provided

• Seconds reset capability

• This is the default standby mode

2. Calendar Mode

• Displays day of the week and date

• Date format may vary depending on market (MM-DD or DD-MM)

• No year tracking or leap-year correction

• Manual correction needed for short months

3. Alarm Mode

• One daily alarm

• Alarm time set in hours and minutes

• Alarm ON/OFF toggle

• Sound is emitted via a piezoelectric buzzer

4. Stopwatch Mode

• Measures elapsed time up to 59 minutes, 59.99 seconds

• Resolution: 1/100 second

• Features Start, Stop, and Reset functions

• Suitable for general timing and sports events

Button Layout (Typical)

Watches using the A639A generally feature four pushers, arranged around the case:

• Button A (Top Left) – Cycle through modes (Time → Alarm → Stopwatch → Calendar)

• Button B (Bottom Left) – Enter/set mode / select field

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

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

Some models may include labeled bezels or markings next to the buttons for added clarity.

Setting Instructions

To adjust the time or alarm:

1. Enter the desired mode with Button A

2. Press and hold Button B to enter setting mode (digits will begin to flash)

3. Use Button C to advance the value

4. Press Button B to switch between hours, minutes, and seconds (in time mode)

5. Press Button A to confirm and return to normal display

Alarm toggle is available by pressing Button D while in alarm mode.

Battery Replacement and AC Reset

After battery installation, the AC (All Clear) reset is required to initialize the module.

AC Reset Steps:

1. Open the case back using the appropriate tool

2. Remove the used SR1120W battery with plastic tweezers

3. Insert a fresh battery, observing polarity

4. Locate the AC terminal marked on the circuit board

5. Use metal tweezers or a conductive tool to short the AC terminal to the battery’s negative terminal (case or spring) for 1–2 seconds

6. LCD should flash briefly, then reset to the default time mode

Without this reset, the module may not function or the display may stay blank.

Maintenance Tips

• No lubrication required – this is a fully electronic movement

• Piezo buzzer contacts may need cleaning if the alarm is faint

• Sticky or unresponsive buttons often benefit from internal contact cleaning

• LCD fading or missing segments is usually permanent and may require a donor module

• Most watches using this module were not waterproof—avoid water exposure unless case markings state otherwise

Watch Models

The A639A powered Seiko watches in the Alarm Chronograph and Sport lines, particularly:

• Rectangular or wedge-shaped stainless steel cases

• Bold bezels with function labels

• Model references such as A639-5000, A639-5029, etc.

These watches were globally distributed and aimed at professionals, students, and sports enthusiasts.

Conclusion

The Seiko A639A is a reliable and straightforward quartz digital movement from Seiko’s golden age of LCD technology. With a full suite of core functions—including 1/100-second stopwatch, alarm, and calendar—it delivered the right balance of function and form. Today, A639A-powered watches are appreciated by collectors and vintage digital watch fans for their clarity, performance, and durable design.