ESA 9210 Tuning Fork Chronograph Movement

ESA 9210 New Old Stock

ESA 9210 New Old Stock

Anyone who has worked on the Omega f300 Speedsonic movement will have nightmares thinking about the chronograph module 🙁 .  It was ESA who secretly started work in 1970 on the development of a tuning fork chronograph. They took their existing ESA 9162 movement, as used in tens of thousands of Omega f300s, Tissot Tissonics, Longines Ultronics etc, and added a chronograph layer to the movement.

ESA sub-contracted the design and manufacture of this chronograph module to Dubois-Depraz — this company specializes in making bolt-on chronograph modules for the major watch manufacturers and are still around today. A good read can be had here and on their website here —>

It was finally released in 1972 and is known as the ESA 9210 although many watch makers allocated their own calibre number to it — the Omega version is known as cal. 1255.

Longines L749.2 Service Manual

Longines L749.2 Service Manual

Back in 2011, I had cause to “open up” the chronograph module on one of my Speedsonics. It came apart really easily 🙂 . But it took me 3-4 days to get it back together again and I vowed never to work on this module again. So when people asked me to service their Speedsonics, I would always say “Yes, fine, as long as the chronograph module is working correctly, as this part is not included.” and, to be fair, the chronograph module usually doesn’t play up unless battery corrosion has got to it.  So when I bought a Baume & Mercier Tronosonic Chrono in June 2013, I put its rather sad, broken ESA 9210 movement to one side and replaced it with a NOS movement.

But with some spare time, I decided to have another go and restore the old B&M movement. I had one advantage this time; a service manual for the Longines L749.2 movements (Longines branded ESA 9210). And I have to say, with the manual, things went a whole lot easier and that 3-4 days reassembly shrunk to less than half an hour!  I’ve can now stripped down and re-assembled the ESA 9210 movement with confidence!

Side View of the Modules

ESA 9210 Side View

ESA 9210 Side View

If you click on the image to the left, you’ll see how modular this movement is — it is built up in layers.  In the photograph, the dial side is at the bottom (although the dial is not attached to the movement).  Starting from the top, the first layer is the Oscillator (Upper Plate 101) containing everything to dive the movement: tuning fork, coils, electronics, index wheel. Then comes the Main Plate 100: this has the hand setting works. After that, the Chronograph Mechanism Platform 8281 that house ones half of the chronograph mechanism.  Lastly, the Calendar Platform 2551 that has the second half of the chronograph mechanism as well as all the day and date wheel works.  In the following sections, the plates and platforms that make up these layers are separated out and described more fully.

View of Plates from the Oscillator Side

 View of Plates from the Dial Side

ESA 9210 Chronograph Workings

It is quite difficult to describe in words how the chronograph part of this movement works. And many things are not obvious even after examining this module several times.  In the photo below, I show the chronograph mechanism is the running position i.e. the start button has been pushed.  Some notes:

  • The chronograph mechanism shown below is in the running position.
  • Green Arrows : the direction of moving parts when the Start button is pressed.
  • Red Arrows : the direction of moving parts when the Stop button is pressed.
  • Blue Circle : the part that gives rise to the “click” when the Start / Stop button is pressed.
  • The Fly-Back-Yolk Bolt prevents the Reset button being pushed when the chrono is running.
  • The two brass foils are on the top of the Hour and Minute chronograph counters (see below).
  • All chronograph wheels are permanently engaged with the rest of the movement. The Start / Stop button operates, via the Clutch-Operating Lever, a clutch on the centre second wheel to control whether the chronograph is engaged or not.
  • The constant second wheel is not in the photo below.
ESA 9210 Calendar Platform 2551 Oscillator Side Built Up

ESA 9210 Calendar Platform 2551 Oscillator Side Built Up

These next three photographs show the 3 rather complicated chronograph counters with their :

  • zeroing hearts — first disc with single hole after the hand post
  • floating clutch base — large disc with the three large holes
  • foil spring — between the gear wheel and the clutch base

These are new items which is why they are nice and shiny. In the stop and reset positions, the large 3-holed floating discs get pushed up towards their gear wheel.

The main centre second chronograph counter is the one with the longer hand shaft; the bottom counter in the second photo. The brass wheel on this counter drives the rest of the chron mechanism.  When assembled into the movement, this counter has a copper coloured Chronograph Driving wheel (8058) pushed onto the top stub of the counter — you can see this in the fourth photo; the driving wheel is right in the centre of the movement.  It is this copper coloured Chronograph Driving wheel that is affected by the clutch mechanism — in one position, it is locked to the rest of the counter assembly and therefore drives the chono mechanism, but in the other position, it rotates freely at the top of the counter without driving it and therefore the brass wheel below it does not drive the rest of the chrono mechanism.

Difficult to explain in text!  My paragraph above is probably clear as mud. 🙂