S. Epson Caliber YM90A

The S. Epson caliber YM90 (YM90A) is a quartz chronograph movement with simple stopwatch layout featuring a central seconds hand and a 60 minutes counter. Note that this minimalist design also means there is no running seconds hand, only a chronograph seconds hand.

Expected Battery Life:

S. Epson quotes a 5 year battery life on the YM90, but keep in mind that this is only the case when using the chronograph no more than 2 hours per day. If the chronograph seconds hand is left constantly running, the battery will drain more quickly. To do this, follow the instructions below:

• Resetting the Chronograph to Zero:
• After a new battery install, the chronograph seconds and minutes hands may need to be reset.
• Pull the crown out two clicks to time-setting mode.
• Press the top pusher to engage reset mode for the 60 minutes chronograph hand (the hand will make a full circle).
• Press the bottom pusher to advance the 60 minutes subdial hand to line it up at zero or 12:00.
• Press the top pusher again for 2 seconds to engage the central chronograph hand reset mode (the hand will make a fill circle).
• Press the bottom pusher to advance the hand until it lines up where you want it.
• Push the crown back in.

Crown/Stem Removal:

Before extracting the stem from a YM90A movement, the crown should be pulled out one click to date setting mode. In that position, the detent button will appear through the hole next to the arrow with the word “PUSH” beside it.

Service Notes:

The hour wheel is a plastic part. Avoid setting and removing the hour hand too many times as it can wear down the part and reduce the torque/friction needed to hold the hand in place. S. Epson recommends not changing the hour hand more than 5 times (although even this may be too much).

The center wheel on the YM90 has a safety feature to prevent the minute hand from being pressed too far down.

Examples of watches with this caliber:

At the time of this caliber listing, there are not many watch brands using the YM90A. If you know of any, please add them to the comments below…

One watch powered by this caliber is the Yema Spacegraf Zero-G. This model was introduced in 2020 as a collab with France’s Space Agency (aka CNES), to commemorate the 30th anniversary of French micro-gravity simulated flights.

At this time, it appears Yema has removed all references to this model on their English language sites, however, the Japan language page can still be found here.

Below are three colors: YMHF2019-3AA (black dial/black case), YMHF2019-AA (black dial/steel case), YMHF2019-GG (blue dial/steel case).

Additional Resources:

• More S. Epson mentions