How a Chronograph Works
There are two parts to this page. The first part includes a description of how a mechanical chronograph works: click on the first image to read the details. The second part shows, on this page, how this movement is disassembled in stages. The chronograph on this page is a Swiss Valjoux made for Wittnauer (Longines). It is similar to the Valjoux movement in the Omega Speedmaster, but the Speedmaster has a higher grade version. Working on chronographs should be left only to experienced professionals. This page only provides an introduction. For more information, I recommend Donald De Carle's book "Complicated Watches and their repair."
Remove the chronograph bridge.
Remove the hammer spring and the hammer (lever). Then remove the minute recording wheel and the chronograph runner. Also remove the drive wheel, which requires a special gear puller. Be very careful here. Do not change the settings of the three eccentric studs, which are used to adjust the chronograph parts.
Remove the screw holding the operating lever in place and move the lever aside, as shown. Under this lever is the reset lever, which should be removed to expose one of the three screws holding the plates together. Let the power down (the click is under the hour clutch lever). Remove the screws holding the plates together and remove the bridge plate with the chronograph parts.
Remove the bridge above the gear train. Notice the pinion gear next to the mainspring barrel. This pinion gear is the drive wheel for the hour recording wheel.
Here is a view of the gear train.
The next photo shows the pillar plate after all the gears were removed. Note that the balance wheel should be removed first, not last: I replaced the balance wheel only for the purpose of these photos.
Below is the bridge plate with the chronograph parts,
and a view underneath. The chronograph parts do not need to be disassembled unless repairs need to be made to the chronograph mechanism.
Finally, remove the hour recording wheel. Also remove the winding parts and the balance jewels before cleaning in an ultrasonic watch cleaning machine.