Seiko Caliber A904 Watch Movement

Manufacturer	Seiko
Caliber Number	A904, A904A, A 904
Movement Type	Digital, quartz
Diameter	27.4mm
Height	4.8mm
Jewel Count	No jewels (0)
Accuracy Rating	+/- 20 seconds per month
Battery Cell #	CR2016 (lithium, 3.0 V)
Battery Life	~7 years
Functions	See below…
AC Reset?	Yes
Country of Manufacture	Made in Japan
Known Models	Seiko Alarm-Chronograph (Ref: A904-5019, A904-5199, A904-5100), Seiko Sports 100 Alarm-Chronograph (Ref: A904-6000, A904-506B), Seiko Sports 150 Alarm-Chronograph (Ref: A904-5200) (Add your watch to the comments below…)

The Seiko caliber A904 (A904A) is a digital quartz multi-functional watch movement produced in circa 1980s ~ 1990s.

Some of the topics covered in this caliber listing:

Functions:

The display of the calibre A904 offers a readout of the day of the week, date, time with seconds, AM/PM, and alarm on/off indicator.

Other functionality includes:

12 or 24 hour time display
Stopwatch
Hourly time signal (ability to beep every hour)
Illumination (light)

LCD Screen:

The display on the cal. A904 is a nematic liquid crystal display ( with Field Effect Mode. It has a multiplex liquid crystal driving system (here is an interesting article that discusses this).

Learn More: What are Nematic Liquid Crystals? A Brief History and Applications

Changing the battery:

The A904 takes a large size CR2016 battery cell. A new battery should last up to 7 years.

To swap the battery, you will need to undo a clamp that is part of the top battery cover/insulator. Use your tweezers or smallest screwdriver and gently pry outwards.

Install the battery positive (+) side up. When putting the insulator back over the battery, hook the inner clamp first, then hook the outer clamp in place (reverse of what you did to remove it).

Note that there is also an insulator that goes under the battery. Be sure to clean and old battery acid off of it before installing a new battery.