The Seiko A358A is a multifunction digital quartz LCD movement introduced in the early 1980s, designed for Seiko’s popular multi-alarm digital watches. A direct successor to earlier A35x-series modules like the A354A, the A358A is capable of handling up to three independent alarms, along with core digital functions like timekeeping, calendar, and a 1/10-second stopwatch. Its improved usability, modular layout, and clear display made it a favorite for users needing versatility in a compact wristwatch.

Technical Specifications

• Caliber Number: A358A

• Movement Type: Quartz (Digital LCD)

• Display Type: 7-segment Liquid Crystal Display

• Primary Functions:

  • Timekeeping (12-hour or 24-hour format)

  • Calendar (Day of week and Date)

  • Three Independent Daily Alarms

  • Stopwatch (1/10-second resolution)

• Backlight: Not integrated (some watch cases may include a side-mounted bulb or light feature)

• Battery Type: SR1120W (1.55V silver oxide)

• Battery Life: Approx. 2 years

• Accuracy: ±15 seconds per month

• Module Dimensions:

  • Width: ~26 mm

  • Height: ~22 mm

  • Thickness: ~5 mm

Display Layout

The A358A has a functional and easy-to-read display layout:

• Top Section:

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

  • Icons/indicators such as “AL1,” “AL2,” “AL3” for each alarm

  • “ST” for Stopwatch

  • “PM” (for 12-hour format) or “24H” for military time display

• Main Display Area:

  • Time in HH:MM:SS

  • Calendar mode displays date in MM-DD or DD-MM format, depending on region

  • Stopwatch mode displays MM:SS.t (tenths of a second)

This layout provides clear separation between primary timekeeping and secondary modes.

Functional Modes

1. Time Mode

• Displays hours, minutes, and seconds

• Option to toggle between 12-hour and 24-hour display formats

• AM/PM indicator visible in 12-hour mode

• Time setting includes hours, minutes, and seconds

2. Calendar Mode

• Shows day of the week and numeric date

• Manual adjustment required for months with fewer than 31 days

• No year or automatic leap year handling

3. Alarm Mode

• Supports three separate daily alarms

• Each alarm programmable with hour and minute

• Individual ON/OFF toggles for AL1, AL2, and AL3

• Piezo buzzer provides audible alert

4. Stopwatch Mode

• Measures elapsed time up to 59 minutes, 59.9 seconds

• Resolution: 1/10 of a second

• Functions include Start, Stop, and Reset

Button Configuration (Typical)

Most watches using the A358A movement feature four side-mounted buttons:

• Button A (Top Left) – Mode selector (Time → Calendar → Alarm 1–3 → Stopwatch)

• Button B (Bottom Left) – Select digit or toggle alarm ON/OFF

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

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

This layout was designed for quick one-handed operation.

Setting Instructions

To adjust time, date, or alarms:

1. Use Button A to enter the desired mode

2. Press and hold Button B to activate the setting mode

3. Use Button C to advance the digit

4. Press Button B again to switch fields (e.g., hour → minute → date)

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

For alarms, each (AL1, AL2, AL3) is set independently in its respective mode.

Battery Replacement and Reset (AC)

The A358A must be reset after battery replacement using the AC (All Clear) contact.

Steps:

1. Open the case back using the correct tool

2. Remove the used SR1120W battery with plastic tweezers

3. Install a new battery, confirming correct polarity

4. Locate the AC terminal (typically labeled on the PCB)

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

6. The LCD should briefly flash all segments, then revert to the default time display

Failure to reset the AC may result in a blank or unresponsive screen.

Maintenance Notes

• No lubrication is required – the module is fully electronic

• Clean piezo buzzer contacts if the alarm volume fades

• Button unresponsiveness may be resolved by internal cleaning

• LCD fade or segment loss is irreversible—replacement requires a donor module

• Most A358A watches offer only basic splash resistance

Compatible Watch Models

The A358A was used in several Seiko Multi-Alarm digital watches during the 1980s, particularly those with:

• Rectangular stainless steel or chrome-plated cases

• "Multi-Alarm" text printed on the dial

• Slim profiles with clear bezel labeling

• Often marketed globally under reference numbers like A358-5000, A358-5019, etc.

These watches were positioned as professional tools for people needing dependable daily alarms and stopwatch capability.

Conclusion

The Seiko A358A is a capable and reliable digital quartz movement offering multi-alarm functionality, calendar, and a precise stopwatch in a compact form. It remains a favorite among collectors of vintage Seiko digital watches for its utility and build quality. With regular care and the occasional AC reset, A358A-equipped watches continue to deliver performance decades after their release.