What’s wrong with my chronograph?

This pops up now and again - your new chronograph is not doing what you expect it to do, and you want to know why....

The purpose of this article is to explain what happens in the chronograph section that affects the overall watch operation.

First let’s look at a simple castle-wheel chronograph, the ST1901. The design of these is pretty much the same as all castle-wheel chronographs, the parts are different shapes and the finish differs, but for the most part the operation of each part is the same.

Normal operation:

Depressing the start button advances the castle wheel clockwise by one index position, this is accomplished by the long sweeping lever stretching from 4 to 11.30 in the above picture. This allows the coupling clutch mount nose to drop in between the castellations the coupling clutch engages the second wheel, other castellations cam the reset hammers off the reset hearts to the cocked position where they are latched and lift the brake lever. After the second wheel rotates one revolution a tooth under the gear engages the slider gear and advances it one tooth position. The minute recording jumper prevents the minute recorder from advancing more than one position.

When the start button is pressed again, the castle wheel advances another position and the castellation cams the coupling clutch mount nose out and disengages the gears. It also allows the brake lever to drop in between two castellations and stops and holds the second wheel so motion of the watch will not alter the indicated second position.

The ST19 is a drop hammer design depressing the reset button releases the hammer, which under influence from the hammer spring falls on the reset hearts. It also lifts the sliding gear out of the way of the single tooth on the second wheel and lifts the brake.

(Some older chronographs have pusher operated hammers that when the pusher is depressed, the pusher lever acts directly on the hammer, against the hammer spring and the reset hearts. The hammer still performs the above functions, releasing the button allows the hammer to return to the cocked position. In these designs, a half hearted push on the reset button with not reset the hands to zero, which is why they were dropped in favor of the drop hammer design.)

Now, how to adjust the chronograph, it is best to start at the coupling clutch cam and work clock-wise:

The coupling clutch cam adjusts the depth of engagement of the coupling clutch gear and the upper 4th wheel. Too much engagement will put excessive drag on the movement and possibly stop it. If the engagement is too loose there will be excessive backlash between the gears and the second hand will stutter. If this cam is rotated 180 degrees out, it will move the coupling clutch nose too far away from the castle wheel and the chronograph will not start even after the start button is pressed.

The chronograph depth camadjusts the depth of engagement between the coupling cam and the second wheel. Again, too much engagement will put excessive drag on the movement and possibly stop it. Too little will result in stutter or second hand jump at chronograph start. The engagement should be 2/3 the tooth height.

The sliding gear depth camadjusts the depth of engagement of the sliding gear and the single tooth of the second wheel. Too little engagement will result the single tooth missing the gear altogether or not tripping the minute counter. Too much will result in the minute counter jumping two minutes or bouncing after indexing.

Minute recording jumper adjustment, when the minute counter is reset, the jumper detent should be resting evenly on two teeth. Loosen the screw and use the cam to adjust the up and down position of the jumper detent. If it is not adjusted properly, the minute counter will reset to one position, then move to another position when the start button is depressed.

Adjusting a lever operated chronograph is much th same, in fact the only difference between a cam operated chronograph and a castel wheel chronograph is that a two position cam replaces a continuously rotating castle wheel. The start lever moves the operating cam to one position generally the hammer cocked position, and the reset lever moves the operating cam to the hammer down position.

These have one additional cam, the chronograph disengagement cam which, when the hammer falls, allows the coupling clutch to be pushed out of engagement with the second wheel. If this cam is incorrectly set, the coupling clutch will be either too close and not disengage, or too far. If too far the coupling clutch will drop to far and cause second hand jump on engagement.

So, troubleshooting:

1) watch doesn’t run - - Something wrong in the watch section. - coupling clutch engaged too deep in upper 4th wheel.

2) watch stops when chronograph is engaged - - chronograph depth cam set too deep. - something jamming second wheel. - reset lever spring broken and hammer is not latched in cocked position.

3) chronograph doesn’t start when start button pushed - - start lever broken. - start lever spring broken and castle wheel not rotating. - chronograph depth cam not set deeply enough.

4) minute counter doesn’t index- - slider gear depth cam set too far out. - second wheel single tooth damaged or missing.

5) minute counter jumps more than one position at a time - - slider gear depth cam set too far in.

6) minute counter reset to one position, but moves when chronograph started - - minute recording jumper incorrectly set.

7) chronograph second or minute recorder resets erratically or not at all. - reset button not being pushed with enough authority (fly back or non-drop hammer designs only.) - reset lever damaged. - hammers not in full contact with reset hearts. - second hand or minute counter hand loose. - weak hammer spring.

8) watch stops when reset button pushed - - brake not disengaging. - slider gear not disengaging. - coupling clutch mount nose deformed and coupling clutch not disengaging.

I don’t think this is all the possible failures, but it is the most common ones.