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Improving the Precision of the A7750 Minute Counter

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Again one from Francisco.

As you know, the Asian 7750 movement is a copy of the ETA 7750. The Chinese factories have copied all the pieces of the Swiss 7750. However, some pieces are different, some are still compatible, some not, or need modification.


These differences are some times visible in the functioning of the Asian 7750. One of the differences in some movements (there are several factories) is that the minute counter increments one minute before than the Swiss movement. Next video shows the problem. Please check that minute counter moves before the seconds hand reaches the 12 position.

This tutorial shows how to solve this problem. Following the steps of this tuto the chrono will work like the next video shows, like the gen ETA 7750.

In some movements no mod is needed. However, in other movements the read of the chrono may be wrong due to this issue. For example, if the correct read must be 2 minutes and 58 seconds, the chrono will show 3 minutes and 58 seconds… error of 1 minute!


The first step is to remove the central chrono seconds hand. The piece we are going to swap is the piece where this hand is attached.




The rotor is removed. Follows removing the spring that the red arrow shows:




The steps to disassembly this part of the A7750 mechanism are the same that the steps shown in the tuto to repair a 7750 when the rotor spins when manually winded. Therefore, the reader may be interested in consulting that tuto as well, to have both explanations and different pictures. The tuto is here:


First, the spring is pushed out and then lift it up, resting on the bridge:




The spring is removed and the 3 screws (red circles):




Then the bridge is removed, very carefully, trying not to produce any horizontal movement. Following picture shows what you will see. Green arrows show the two gears that are to be easily removed.




Next piece to be removed in the hammer, shown by the green arrow:




The picture shows that the hammer has been removed and the next part to be disassembled, a spring. Indeed this spring could be removed before, and may be better to do it before, after removing the first spring, to avoid this spring jumps if you are not careful.




The red arrow in next picture shows the next piece to be removed, the clutch. However, the green arrow shows a spring that is above the clutch. Therefore, when removing the clutch you have to be careful to not damage this spring. You do not have to remove this spring.




Finally we are able to remove the part shown by the red arrow. This wheel rotates the seconds chrono hand, and after one complete turn, pushes the minute counter, incrementing one minute. This is the piece to be replaced by the gen ETA 7750 part.




Part removed. The disassembling has finished, no difficult.




Next picture shows a comparison between the Asian and Swiss wheel.




As shown, the green arrow point at a wire. This is the wire that after one complete turn pushes the minute counter (really pushes an intermediate wheel). It is possible to see that the wires are not completely equal. This small difference in the shape produces that the minute counter does not rotate in the same moment, as the videos shown. Therefore, swapping these pieces will solve the problem.


Part reference for cousinsuk.com store:


VAL77508000 Valjoux 7750 8000 Chronograph Wheel £11.90


Let us begin the assembling. The ETA wheel is assembled. However, you have to press a bit to fit it. The pin where the second hand is assembled has larger diameter in the ETA part.




And now is time for assembling the clutch. Remember that the spring has to be above the clutch




It is sure that the oscillating pinion that is under the clutch will not be well assembled (red circle). You have to assemble it now. This is not easy. Be patient.


Next video shows how a small screwdriver is employed to lift the clutch up, and with the tweezers the oscillation pinion is placed in its correct position


Next two pictures show the final result.





The pin of the oscillating pinion must be visible trough the hole of the clutch. Check it carefully, many times seems well assembled, and it is not.


The following assembling steps are the same like in the tuto of repairing a 7750 that rotor spins when manually winded. I am not going to repeat the instructions. The tuto is here:


Therefore, following pictures only show the piece that is assembled each time, employing green arrows.










After these steps the movement is assembled. However, a final task must be done. The hand output has larger diameter, as consequence, the second hand does not fit, unless you have a 21600 version. We have to modify the hand tube.


I have seen a tutorial showing how to do this, placing the hand over the output pin and hitting it. However, I do no like this procedure. I prefer to modify the tube outside the movement.


Next photos show how first I use a needle, and later the tips of the tweezers to make bigger the tube. Then, a bit of epoxy in the tube…and just fit the hand on the output.




The final result is this:


Now we have a chrono without reading errors.


Thanks for reading.

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:lol: onze

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Thanks to Francisco for a great post.


And thanks to Onze for sharing - well done, mate :lol:

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Your dedicatition is second to none Onze... :lol:

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