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The Encyclopedia of Horology

Movement record · Japan

Seiko calibre 7A28

Verified

The first analogue quartz chronograph movement, using three separate stepping motors so that the chronograph hands run independently of the timekeeping train and can be reset instantly.

Technical specifications

Diameter
Not recorded in a source consulted here
Height
Not recorded in a source consulted here
Jewels
15 jewels
Frequency
Quartz, with independent stepping motors for the chronograph hands
Power reserve
Battery powered
Winding
Not applicable; battery powered
Setting
Crown setting
Escapement
None; quartz oscillator with stepping motors
Balance
None; quartz oscillator
Complications
Analogue chronograph reading to one-tenth of a second, Independent chronograph hands driven by separate motors

Identification features

  • Calibre marking on the movement and case back
  • Fully jewelled quartz movement, unusual for the type

Known variants

  • 7A38 with calendar
  • 7A48

Servicing notes

Service parts for the 7A family are long discontinued; specialists generally work from donor movements rather than new components.

General guidance only. Antique movements should be serviced by a qualified watchmaker.

Historical significance

A landmark in quartz engineering, showing that the analogue chronograph could be rebuilt around electronics rather than adapted from a mechanical design.

Related photographs

Black Seiko watch close-up
Black Seiko watch close-up — gcardinal from Norway, CC BY 2.0. Wikimedia Commons file page
Dial detail of a Seiko Speed-Timer Chronograph Automatic 6138-0040 ("Bullhead"), 1976
Dial detail of a Seiko Speed-Timer Chronograph Automatic 6138-0040 ("Bullhead"), 1976 — Dnalor 01, CC BY-SA 3.0. Wikimedia Commons file page
Seiko SKX781 Orange Monster diver watch (2026-02-13)
Seiko SKX781 Orange Monster diver watch (2026-02-13) — Olgierd Rudak, CC BY-SA 4.0. Wikimedia Commons file page

No confidently matched photograph of this subject has been found yet. These photographs show a closely related record, and are not examples of the type described on this page.

Sources & references

  1. Seiko technical literature and museum descriptions of the 7A28
  2. Some widely cited online specification tables for this calibre are community-compiled rather than documented by the manufacturer.

Where sources disagree, the disagreement is stated rather than resolved silently.