THE VALJOUX 7750 CHRONOGRAPH
BY WALT ODETS
OTHER FEATURES OF THE TOP PLATE
The Valjoux 7750 uses a sturdy and
convenient semi-fine rate regulation device. The index (right, 1) is moved to adjust the daily rate of the watch. Attached to an eccentric screw (2), this action causes the
regulator ring (3) to rotate via an arm (4), adjusting the effective length of the balance spring.
Like most contemporary ETA movements, the Valjoux 7750 uses an Etachron
regulator and balance spring stud assembly. Illustrated right, this assembly is very inexpensively constructed, but unlike some more expensive units, it allows excellent adjustment of the
regulator for minimum interference with the balance spring (1). The regulator (2) and its upper end (3) are held in the regulator ring by a clamp (4). Thus the
regulator can be both rotated and moved towards and away from the center of the spring with ease. It is a shame that more manufactures do not provide such adjustability.
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).
THE BOTTOM PLATE
The bottom plate (right) carries a
conventional calendar mechanism with date, day and date, or other complications. These are modular units that can be switched according to the caliber desired.
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 expresses an aspect of Swiss engineering skill that we do not
normally associate with the Swiss watch industry: economy of manufacture. The 7750 is a good representation of the new, simplified chronographs that began to appear from Ebauches S.A. (now ETA) at the
beginning of the 1940s and that provided serious, usually fatal, competition to smaller manufacturers of high-grade chronographs.
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.
© 2015 Bourne in Time Inc.