The pinion of the 7750 chronograph center wheel (right, 1) rides in a "plastic" bearing (2), reducing cost by saving a jewel. Lubrication
issues with this delicate part are thus eliminated.
By removing the chronograph plate, the conventional wheel train of the 7750 is exposed (left). These parts include (1) the
mainspring barrel ; (2) center wheel; (3) third wheel; (4) fourth wheel; (5) escape wheel. Note also the simple, stamped steel hacking lever (6), which arrests
the balance wheel when the crown is pulled into the hand-setting position.
As illustrated in the previous photograph, the center
wheel is not located in the center of this movement. This design eliminates the cost of boring the delicate center wheel pinion to carry the chronograph sweep hand pinion. Instead, the
motion works (and hands) of the movement are driven indirectly through an intermediate wheel (arrow, left) attached to the extended center wheel pinion. Another cost-saving design approach
is seen in the mainspring click (left). Rather than use a conventional click to prevent mainspring unwind, Valjoux has managed to use a simple bent spring anchored in a slot in the plate.
The spring itself (1) will ratchet as the ratchet wheel (2) winds clockwise. Counterclockwise rotation of the ratchet wheel is prevented by the spring butting against a corner of
the plate (3).
With the calendar plate removed (right), we can see the remainder of the movement. Parts include (1) the 12 hour accumulator wheel and
heart cam; (2) bottom plate mechanism for the hour accumulator; (3) and (4) calendar switching wheels; (5) keyless works for hand setting and winding; (6) intermediate
wheel for indirect minutes drive.
The 12 hour accumulator wheel uses a plastic brake (arrow) and simple
stamped steel levers to stop, brake, and reset the wheel. These levers are operated directly off of the lower case pusher rather than being mediated by the heart piece.
The hour wheel runs in a hole in the plate (right) rather than in a jewel or replaceable bushing. This construction reduces cost, but suggests that care
should be taken in overusing the chronograph, particularly without regular service.
Like other levers in the movement,
the keyless works components are made of stamped steel parts.
The Valjoux 7750 is now used in the vast majority of mechanical chronographs produced in Switzerland, and has allowed the mechanical chronograph
function in watches of modest cost. For a caliber obviously engineered from the ground up for economy of manufacture, the 7750 has proved itself a reliable and durable workhorse. Without the
7750, mechanical chronographs might be known only to the buyers of luxury watches.I wish to thank Bob Frei of the TZ Tool Shop and Frei & Borel for supplying the movement used in this review.
I wish to thank Bob Frei of the TZ Tool Shop and Frei & Borel for supplying the movement used in this review.