Vintage Seiko Grand Quartz 9940-8000 Wristwatch Twin Quartz 1978 Brushed dial

SPECIFICATION

Reference: Grand Quartz QGN010
Movement: Twin quartz 9940
Age: August 1978
Case Size: 35mm
Case Thickness: 8.5mm
Lug to Lug: 41mm
Lugs: 19mm
Condition: Used Seiko watch
Box & Papers: No
Case Material: Stainless Steel
Warranty: 12-Months Warranty

 POINTS TO MENTION

• In fantastic pre-owned condition, with light wear from use
  
• Case and bezel have some very light scratches and mark
  
• White vertically brushed dial
• Serviced Movement
• New battery fitted
• Quartz signed crown
  
• Paired with an unworn grey leather strap
  

THE WATCH

The Seiko Grand Quartz 9940-8000 is a vintage high-end quartz wristwatch from Seiko’s late-1970s Grand Quartz series, part of the brand’s premium quartz lineup developed during the height of the quartz revolution. It is fitted with Seiko’s Calibre 9940 Twin Quartz movement, which uses two quartz crystals and a temperature compensation circuit to significantly improve timekeeping precision compared with earlier single-crystal quartz movements. This Twin Quartz system allowed the 9940 movement to achieve exceptional accuracy, far surpassing standard quartz accuracy measured per month.

Today the Grand Quartz 9940-8000 is appreciated by collectors of vintage quartz watches and Seiko enthusiasts not just for its historical significance as part of Seiko’s high-accuracy quartz development, but also for its combination of accuracy, build quality, and period-typical styling that reflects Seiko’s technical leadership in the era when quartz fundamentally reshaped the watch industry.

TWIN QUARTZ

Twin Quartz is Seiko’s temperature-compensated quartz system, introduced in the late 1970s. Unlike standard quartz watches that use a single quartz crystal, Twin Quartz uses two crystals along with a specialized circuit that compares their oscillations and corrects for temperature-related timing errors. This was important because temperature changes are the main reason quartz watches drift over time, and Twin Quartz was designed to eliminate that issue.

The way Twin Quartz works is fairly clever: the two crystals are cut so that their frequency shifts in opposite directions when temperature changes. As the temperature rises, one crystal’s frequency increases while the other decreases. The circuit compares the two frequencies and adjusts the output, effectively canceling out the temperature drift. This made Twin Quartz far more accurate than typical quartz watches of the era.

Twin Quartz first appeared around 1978, and it was used mainly in Seiko’s high-accuracy Grand Quartz models. The most famous Twin Quartz movement is the Caliber 9940, which powered watches like this Grand Quartz 9940 and similar references. These watches are now prized by collectors for their technical significance and their role in Seiko’s quartz innovation.