The Seiko A638A is a vintage analog-digital hybrid quartz movement introduced in the early 1980s. Part of Seiko’s “Digi-Ana” (digital-analog) lineup, the A638A combined the classic appeal of analog timekeeping with the practicality of digital functions, including a daily alarm, calendar, and 1/100-second stopwatch. This module catered to wearers who appreciated the aesthetic of hands on a dial but still wanted the benefits of a digital chronograph.

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Technical Specifications

• Caliber Number: A638A

• Movement Type: Quartz Hybrid (Analog + Digital)

• Display:

  • Analog: Hour and Minute Hands

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

• Functions (Digital):

  • 12-hour time format

  • Calendar (Day and Date)

  • Daily alarm

  • Stopwatch (1/100 second)

• Backlight: Not integrated (some case models may include external lighting)

• Battery Type: SR927W or SR1120W (1.55V silver oxide)

• Battery Life: Approx. 2 years

• Accuracy:

  • Analog: ±15 seconds/month

  • Digital: ±15 seconds/month

• Dimensions:

  • Width: ~26 mm

  • Height: ~22 mm

  • Thickness: ~5 mm

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Display and Layout

The A638A features a dual-display layout, with both analog and digital readouts visible on the same dial.

• Analog Section:

  • Traditional hour and minute hands powered by a stepper motor

  • Typically located in the upper section of the dial

• Digital Section:

  • Positioned at the 6 o'clock or lower half

  • Displays HH:MM:SS in time mode

  • Shows day/date in calendar mode

  • Stopwatch mode includes MM:SS.1/100 readout

  • Indicators such as “AL” (alarm), “ST” (stopwatch), and “PM” appear when relevant

This hybrid layout allowed for simultaneous analog timekeeping and digital function monitoring.

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Modes and Features

1. Timekeeping (Digital)

• Digital time displayed in 12-hour format with AM/PM indicator

• Analog and digital times are not always synced—they may require separate setting

2. Calendar

• Displays day of the week and numerical date

• Manual date correction needed for months with fewer than 31 days

• No year or automatic leap-year adjustment

3. Alarm

• One daily alarm with hour and minute setting

• Alarm ON/OFF toggle

• Audible via piezo buzzer

• Alarm indicator visible when active

4. Stopwatch

• Measures up to 59 minutes, 59.99 seconds

• Resolution: 1/100 second

• Start, Stop, and Reset functions

• Useful for basic timing tasks such as workouts or kitchen timing

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Button Layout (Typical)

Watches using the A638A module generally feature three or four pushers:

• Button A (Top Left) – Mode selection (cycles through Time → Calendar → Alarm → Stopwatch)

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

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

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

Some variants may use long-press combinations to enter or exit set modes.

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Setting Instructions

To set digital time:

1. Press Button A to enter time mode

2. Hold Button B until digits begin to flash

3. Use Button C to change values

4. Press Button B to switch between hour and minute

5. Press Button A to save and exit

To set analog time:

• Pull out the crown (if present) and rotate to adjust hands manually

• In some versions, analog and digital sections are independent

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Battery Replacement and AC Reset

After battery replacement, an AC (All Clear) reset is necessary to restore function.

AC Reset Procedure:

1. Open the case back with the proper tool

2. Remove the old SR927W/SR1120W battery

3. Insert a new battery, ensuring correct polarity

4. Locate the AC terminal (marked on the PCB)

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 flash briefly and reset

Skipping this step can result in a blank screen or display glitches.

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Maintenance Tips

• No lubrication required – the movement is fully electronic

• Check and clean buzzer contacts if alarm sounds weak

• Unresponsive buttons may require internal cleaning with isopropyl alcohol

• LCD fading or ghosting is often irreversible and requires a donor module

• Most models are not water-resistant—avoid submersion

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Watch Models

The A638A was featured in Seiko's "Digi-Ana" (Analog-Digital) watches, especially in the late 1980s. Common characteristics:

• Round or rectangular steel cases

• Analog dial at the top, digital window below

• Reference numbers like A638-5000, A638-501A, etc.

These models were often marketed as stylish yet functional timepieces for modern professionals.

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Conclusion

The Seiko A638A hybrid movement successfully blends analog elegance with digital utility. With features like a 1/100-second stopwatch, daily alarm, and calendar—combined with classic analog time—it offers a unique and practical experience. Today, A638A-powered watches are prized by collectors for their bold 1980s design, dual-display appeal, and Seiko's trademark reliability.

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