Tuesday, 11 June 2013

Fixing the Calendar Mechanism On A Comete Diver's Watch

Earlier this year, during my usual daily perusal of eBay's used watches, I found this cheap wind-up watch and couldn't resist.


Looks great, it's reliable, and it's got a diver's bezel! There were two good reasons for it's low price tag though. 1: Condition - "well used" would be a generous euphemism for it's state. The following photo of the case back (hopefully) shows some of the deep scratches and dents that the watch has suffer when trying to be opened by and inept former owner.


2: The Calendar Mechanism: Didn't work, hence the reason for this post.
 
To start, the back is cracked open to show a nice, clean, simple and serviceable movement.
 

Out of the case with the dial removed:


And now to start the tear-down.


After the usual tear down, I found no problems so I flipped the movement over and started to investigate the calendar wheel.


I wound the crown forward and back, and now the calendar mechanism appeared to run smoothly. In watches that don't have a quickset function (usually one of the three crown positions that causes the clutch wheel to engage the calendar wheel - either directly or indirectly) there is a skip function between 9pm and 12 midnight where you wind forward and back to skip the calendar wheel forward. Watches that have a calendar complication, but no quick wheel advancement method (other than winding!) are quite rare.

To investigate why the wheel wasn't functioning whilst cased, but was when the movement was uncased, I wound the crown back to see exactly how the calendar wheel works.


As the calendar wheel rotated anti-clockwise (reversing the watch's time) the calendar wheel tooth recessed into the calendar wheel and a spring protruded from the opposite side of the wheel.


The calendar wheel tooth then slid over the date wheel index tooth.


And popped out, free to engage the calendar index tooth from the other side, to advance the date wheel when the crown was then wound forward.


So where's the problem? Well, the plate that holds down the calendar wheel tooth (which is actually a tooth on the end of a spring) is quite loose. It sort-of-but-not-quite floats on top of the tooth-spring.


I found that when even a little pressure is applied to this plate, the calendar tooth became jammed and did not have sufficient clearance to move. This then is the likely cause of the date mechanism malfunctioning. The remedy should simply be a more careful positioning of the dial which sits directly above the calendar wheel. The dial is not in perfect condition, so a little "persuasion" may be needed to generate the required clearance between the dial and the calendar wheel plate to prevent pressure on the calendar wheel tooth.


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