The Seiko G139A is a vintage quartz movement introduced during Seiko’s golden era of analog quartz innovation. Part of the G-series, this caliber was used primarily in men's dress watches from the late 1980s to early 1990s. With a clean, straightforward layout and practical features, the G139A stands out as a reliable workhorse designed for daily use with minimal maintenance.

Core Specifications

• Caliber Number: G139A

• Movement Type: Quartz (analog, step motor)

• Display: 3 hands – Hour, Minute, Central Seconds

• Battery Type: SR920SW (1.55V silver oxide)

• Battery Life: Around 3 years

• Accuracy: ±15 seconds per month

• Size: 10½ lignes (~23.3 mm diameter)

• Thickness: Approx. 3.1 mm

• Jewel Count: 1 jewel (for improved gear train performance)

• Hand Fitting Dimensions:

  • Hour: 1.20 mm

  • Minute: 0.70 mm

  • Second: 0.20 mm

Functional Features

What makes the G139A interesting is its simplicity with just enough refinement. While there are no complications like a calendar or chronograph, it includes:

• Center seconds hand for precise time indication

• Basic hacking feature – the second hand stops when the crown is pulled out

• Single crown operation with only two positions

This movement is ideal for analog watches with a clean, minimalist dial and a dependable time display.

Time Setting Procedure

The crown operates in two standard positions:

1. Pushed In (Normal Mode) – Movement runs and the time is displayed

2. Pulled Out – Time can be set by rotating the crown. The second hand stops, allowing for more accurate synchronization

There’s no quickset date function because the movement lacks a date complication altogether.

Battery Replacement and Circuit Reset

When replacing the battery:

• Use only plastic-tipped tweezers or anti-static gloves

• Clean contact points before installing the new cell

• After insertion, use metal tweezers to momentarily bridge the AC terminal and battery’s negative side. This resets the integrated circuit (IC)

Failure to perform the reset may result in erratic second hand behavior or failure to start.

Maintenance and Troubleshooting

• Keep movement dust-free – Always perform work in a clean environment

• Avoid excessive lubrication – The movement is primarily dry-lubed

• Inspect the coil and gear train – A damaged coil is a common reason for skipping or non-function

• Check for hand clearance – Misaligned or rubbing hands can cause stoppage or erratic ticking

Because of its simplicity, most issues are resolved either through cleaning or replacing worn parts. If damage is extensive, full movement replacement is often more cost-effective.

Identifying the G139A

The caliber number "G139A" is stamped on the main plate, typically near the battery holder. Watch cases housing this movement often feature classic 1990s Seiko styling—sleek bezels, thin profiles, and minimal dial markings.

Conclusion

The Seiko G139A embodies what Seiko does best—durable, practical, and timeless quartz engineering. Though basic in design, it delivers consistent performance, making it a dependable option for daily-wear watches and a good candidate for repair or restoration projects.