Seiko 2B21A – Technical Guide

The Seiko 2B21A is a compact analog quartz movement with a central seconds hand and date display, developed primarily for ladies’ and midsize watches. As part of Seiko’s 2B-series, the 2B21A was built to be energy-efficient, service-friendly, and highly reliable. It provides practical features such as a quickset date and a three-hand layout while maintaining a slim profile suited for elegant case designs.

Technical Specifications

Key Features

• Slim construction suitable for small, dress-style watches

• Quickset date for user convenience

• Three-hand layout (hour, minute, second)

• Low power consumption, extending battery life

• Cost-effective design with zero jewels for affordable production and maintenance

Movement Layout

Front (Dial Side)

• Calendar system: Date wheel, date jumper, driving finger

• Motion works: Cannon pinion, minute and hour wheels

• Setting mechanism: Sliding pinion, clutch wheel, and setting stem

Back (Circuit Side)

• Quartz oscillator: 32,768 Hz crystal

• Integrated circuit (IC): Controls step motor impulses

• Coil block and step motor: Drives gear train at 1-second intervals

• Battery contact system: Includes terminal and insulator for power delivery

Battery Replacement Procedure

Battery Type: SR621SW (364)

Step-by-step:

1. Open the case back using a case opener appropriate for snap or screw-type backs.

2. Remove the battery using plastic or non-magnetic tweezers.

3. Inspect the contact points and clean with pegwood or a fiberglass brush if corroded.

4. Insert a new SR621SW battery, ensuring correct polarity.

5. Check for hand movement—typically begins within 10 seconds.

6. Set the time and date using the crown.

7. Close the case back and inspect the gasket if water resistance is needed.

⚠️ Caution: Avoid touching the quartz crystal, IC, or coil with metal tools. Use anti-static precautions when possible.

Servicing Notes

While the 2B21A is considered non-repairable at the electronic level, several mechanical components can be inspected or replaced:

What can be serviced:

• Date mechanism: Clean and lubricate jumper and driving wheel

• Hands and dial: Remove for cosmetic restoration or alignment

• Setting system: Clutch wheel and stem can be replaced if worn

• Battery contact: Can be re-tensioned or cleaned if not conducting properly

❌ No trimmer: Timekeeping is regulated by the IC and cannot be manually adjusted.

Common Issues & Fixes

Applications

The Seiko 2B21A was widely used in:

• Ladies’ quartz watches

• Small dress watches (round, rectangular, tonneau)

• Entry- and mid-level analog watches with a clean, minimalistic dial

Most models using this movement feature:

• Central seconds hand

• Date at 3 o’clock

• Applied dial markers

• Polished cases and slim profiles

Conclusion

The Seiko 2B21A is a compact and efficient quartz movement that balances functionality with minimal dimensions—perfect for dressy, smaller wristwatches. With its three-hand layout, quickset date, and reliable Seiko engineering, it delivers practical precision with minimal servicing demands.

Though simple, its design ensures long-term dependability, making it a favorite among Seiko technicians and collectors of vintage quartz models alike.