|Caliber Number||7A28, 7A28A|
|Low Battery Indicator?
|Driving System||Step motor with 2 poles|
||Rotary step switch|
|Functions||Central hours; central minutes; central chronograph seconds hand; small seconds subdial at 6:00; 30 minutes chronograph subdial at 9:00; 5/100 seconds chronograph subdial at 3:00 (labeled 1/10 on the dial)|
|Country of Manufacture||Japan|
|Known Models||Seiko Sports 100 (Please add your watch to the comments below…)|
The Seiko caliber 7A28 (7A28A) is a quartz chronograph watch movement with 15 jewels. This caliber was introduced in the Sports 100 line of chronograph watches in the early 1980s (1982?). Although discontinued now, the 7A28 was developed with dependability and longevity in mind. It is all metal with no plastic parts, repairable and serviceable, and can even be regulated thanks to the its rotary step switch for adjusting accuracy.
Some of the topics covered in this caliber listing:
How to set the time?
The time setting crown on 7A28A powered watches is located at 8:00. To set the time, pull the crown out two clicks and turn it clockwise or counterclockwise until the desired time is reached. It is ok to move the hands in either direction. Once the time is set, press the crown back in.
What are the other 3 buttons?
Most chronograph watches have 2 buttons, but the 7A28 has 3. Like other chronos, the stopwatch start/stop button is at 2:00, the reset button is at 4:00. The extra button at 10:00 is a split button.
How to reset the hands to zero?
If your chronograph hands become misaligned due to a battery change, you can reset them to zero with a few easy steps:
- First, make sure the hands are stopped.
- Pull the crown out one click.
- Press each button until the hands reach the zero position.
- Press the crown back in.
Seiko claims the caliber 7A28A has an accuracy rating of less than +/- 15 per month when the watch is operating in the normal temperature.
Can you adjust the accuracy on the 7A28A?
It is possible to adjust the accuracy on this caliber thanks to the rotary step switch, but it is not advisable unless you know what you are doing. Make sure the chronograph function is not running. Near the stem, look for the plus and minus symbols with hash lines between them. There is a knob-like device which can be turned in one direction or the other with watchmaker’s tweezers. Each “step” regulates the timing at 0.26 seconds per day.
“The 7A series is equipped with a multi-stage switch that allows adjustment in steps of 0.26s/day. The 7A28 technical guide calls it a ‘rotary step switch’.
Adjusting is done by changing the number of pulses to be counted for a time span of 10 second. Normally, the watch counts 32768 pulses per second, or 327680 pulses in 10 seconds.
If you count one pulse more or less (i.e. making every 10th second a pulse longer or shorter, for a total of 327681 or 327679 pulses), you adjust the timing by a fraction of 1/327680. For a day with 24*60*60=86400 seconds, this fraction is about 0.26 seconds. For a month (or 30 days) the adjustment ‘step size’ is 7.9s. So you’re very lucky to have a 7A that can be adjusted to 1s/month.
PS: There really is no trimmer capacitor. I’ve taken apart a dead 7A28 a few years ago (no pics taken, sorry). The switch is just a piece of metal with two contact ‘tongues’ that connects different pairs of a circular pattern of gold plated pads on the circuit board, depending on it’s position.” (Source: http://www.larrybiggs.net/scwf/index.php?mod=103&action=1&id=1189933858)
A new battery in the Seiko caliber 7A28A should last approximately 2 years. If you need to change the battery, it is possible to DIY. The battery number is 394 (SR936SW). This movement features a low battery indicator (EOL) which will make the seconds hand jump every 2 seconds when the battery is low.
This movement has long since been discontinued and it is nearly impossible to find a replacement in NOS condition. For service or repair, you may want to send your watch to Seiko, or you can always consider waiting for a parts/repair watch to pop up for sale and get what you need from it.
Watches with this caliber:
Seiko put out dozens of models with the 7A28 movement. Some are very desirable in the collecting community, such as this rare Seiko Sports 100 7A28-7009 watch worn by Sigourney Weaver in the 1986 James Cameron film Aliens. In 1985, Roger Moore played James Bond in A View to Kill, sporting a Seiko 7A28-7020 (SPR007).