The Seiko A638A is a digital quartz LCD movement released in the early-to-mid 1980s and is part of Seiko’s specialized A-series. The A638A is most recognized for powering dual-display watches, combining a traditional analog dial with a digital LCD display—a design that offered the best of both worlds. It was marketed to users who appreciated the timeless look of analog timekeeping while benefiting from the added functionality of digital features like alarm, calendar, and chronograph.

Technical Specifications

• Caliber Number: A638A

• Movement Type: Quartz (Analog-Digital Hybrid)

• Display:

  • Analog: Hour and minute hands (quartz step motor)

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

• Functions (Digital):

  • 12-hour time format

  • Calendar (Day and Date)

  • Daily alarm

  • 1/100-second stopwatch

• Backlight: Not integrated in the module (some models may have case-mounted illumination)

• Battery Type: SR927W or SR1120W (depending on model)

• Battery Life: Approx. 2 years

• Accuracy:

  • Analog: ±15 seconds/month

  • Digital: ±15 seconds/month

• Module Dimensions:

  • Width: ~26 mm

  • Height: ~22 mm

  • Thickness: ~5 mm

Display and Interface

The Seiko A638A uses a dual-display layout:

• Analog Display:

  • Traditional hour and minute hands

  • Quartz-controlled motor for smooth, consistent movement

  • No second hand in most models

• Digital Display (lower or side screen):

  • Time shown in HH:MM format

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

  • Date in MM-DD or DD-MM format

  • Mode indicators: AL (alarm), ST (stopwatch), PM (12-hour format)

This hybrid format provided analog legibility with digital convenience.

Functional Modes

1. Timekeeping Mode

• Digital time displayed in HH:MM format

• AM/PM indicator in 12-hour format

• Analog hands synced to quartz circuit

• Both analog and digital times can usually be adjusted independently

2. Calendar Mode

• Displays day of the week and date

• Month-end and leap-year corrections must be done manually

3. Alarm Mode

• One daily alarm

• Settable by hour and minute

• Alarm ON/OFF toggle

• Audible buzzer via piezo element

4. Stopwatch Mode

• Measures up to 59 minutes, 59.99 seconds

• Resolution: 1/100 second

• Start, Stop, Reset using assigned buttons

• Great for general timing, though not intended for professional use

Button Layout (Typical)

The A638A-powered watches generally feature three or four buttons, often located on the case sides:

• Button A (Top Left) – Mode select (Time → Calendar → Alarm → Stopwatch)

• Button B (Bottom Left) – Set or select digit

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

• Button D (Bottom Right) – Reset stopwatch / Confirm selection

In models with only three buttons, some button functions may be combined via press-and-hold.

Setting Instructions

To set digital time:

1. Enter time mode with Button A

2. Press and hold Button B to enter setting mode

3. Use Button C to increase digits

4. Use Button B again to switch between hour and minute

5. Press Button A to confirm and exit

To set analog time (if not auto-synced):

• Pull the crown (if present) and turn to set manually

• In some versions, digital time is synced to analog, while others allow independent control

Battery Replacement and AC Reset

After battery replacement, an AC reset is recommended for full functionality.

AC Reset Steps:

1. Open the case back using a proper case knife or tool

2. Remove the depleted SR927W/SR1120W battery using plastic tweezers

3. Insert a fresh battery with correct polarity

4. Locate the AC (All Clear) terminal on the circuit board

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

6. LCD will briefly flash, then return to normal time display

Without the reset, the digital display may remain blank or behave erratically.

Maintenance Tips

• No lubrication needed – fully electronic hybrid

• Synchronize analog and digital time if drifting occurs

• Clean buzzer contacts if alarm volume fades

• Sticky buttons may require cleaning of internal contacts

• LCD fade or ghosting over time is often irreversible

• Not all models are water-resistant; avoid exposure to moisture unless otherwise marked

Watch Models and Applications

The A638A was used in Seiko analog-digital hybrid watches, often part of the "Digi-Ana" line. Typical models include:

• Square or round stainless steel cases

• Analog dial with digital window at 6 o’clock

• Reference numbers like A638-5000, A638-5029, etc.

These watches were popular with users who appreciated both elegance and utility.

Conclusion

The Seiko A638A is a remarkable example of Seiko’s ability to blend classic analog design with digital innovation. With its hybrid display, solid quartz performance, and practical features like alarm and stopwatch, it offered versatility for everyday wearers. Today, A638A-equipped watches are admired for their vintage appeal, unique styling, and reliable dual-display functionality.