Operation and Repairing the A7750 Hour Counter

[onzenuub 325]

Again one from Franciscos:

bought some days ago a second-hand TAG Carrera with the common problem of having the hour counter always running. When the chrono is off the hour counter is rotating, never stops. My idea was repairing it a showing in this tuto how to repair and works the hour totalizer.

 

A picture of the watch follows, showing the problem.

 

 

In addition, the watch has another flaw. Have a look to the date wheel. The numbers are not centred on the window. This is more obvious when only one number is shown, for example, the number 8. This flaw will be eliminated using a gen ETA wheel.

 

 

First step is to remove the mechanism from the case.

 

 

Following picture shows the double protection I am using lately to protect dial and hands. The hand extractor hugs the plastic and the plastic hugs the hands, reducing the risk of scratches.

 

 

For small chrono hands I use only the plastic.

 

 

The dial is removed and…surprise!… the week day wheel is there…it was not removed for this watch.

 

 

The day indicator is removed. In the following pictures the red arrow normally shows the piece that is going to be removed.

 

 

When removing the small plate is normal that a spring jumps. Red arrow shows the pieces that have been removed, showing the spring that jumped.

 

 

Next step is to remove another small plate and date indicator wheel, in this order, light red arrows. Next pieces to be removed, dark red arrows.

 

 

Next piece to be removed is the piece that moves the date indicator wheel when the date is changed using the stem (crown).

 

 

Next step is to remove the date platform. After removing 3 screws.

 

 

If the platform is removed with extreme care, you will see what next picture shows. If not, the spring F will jump.

 

 

In previous picture:

 

A is the hour counter of the chrono. It is a complex piece that is explained how works later.

 

B is a plastic piece that when pushes piece A stops the motion of the hour counter hand. Therefore, if the hour hand is always running probably there is a problem with this piece that is not pushing piece A when the chrono is off. If this happens, have a look to the places shown by the green arrows. Normally there you will see that these parts are not well assembled. However, the picture shows that everything is correct. Another surprise.

 

When the chrono is off and the reset button is pressed, the part of piece C shown by the arrow pushes the silver part of piece A, that seems a heart, and the hand returns to the zero position. Later a video shows how it works.

 

Piece A has two gears. The top one, shown as D, has the function of engaging with the teeth of piece B, to stop the hour counter. The bottom one, shown as E, it is always running because is directly connected to movement´s barrel. Therefore, gear E IS ALWAYS ROTATING. Summary, bottom part of piece A is always turning, top part only turns if piece B is not pushing.

 

Next picture shows where connects bottom gear E of piece A

 

Next video (please use full screen for the videos) shows how piece B fastens top part of piece A.

 

Next picture shows piece A. On the bottom pin is where the hand is attached.

 

 

The following picture shows the clutch of piece A. The triangle is the clutch that connects bottom and top parts. The tips of this triangle are brushing. Therefore, if piece B is not fastening gear D the hour hand will rotate, because the tips of the triangle are joining both parts of piece A. If piece B is fastening gear D, the tips of the triangle will slip, and both parts will not be connected. Hour hand will not turn.

 

 

Next video shows how the clutch works. The top part is strong fastened with the pliers. And the bottom part is rotated using a screwdriver.

 

 

And in the following video the pliers only fastens a bit the output pin, therefore, both parts rotates together.

 

Although all the parts seemed well assembled, I reassembled them again. Assembling spring F is one of the difficult steps of 7750 movement. I have found a way to do it. I guess each watchmaker finds the best for him/her. I think that following next video you will not have problems, after practising some time. A wood stick for watchmaking is employed. Be patient… finally you will do it.

 

 

After spring F is well assembled, the date platform has to be assembled. Again, this is a difficult step. You have to do it very carefully or spring F will jump. However, I find this step easier than assembling spring F. A hand was attached to the hour output, to check if is still always running. Yes, it was running again. The problem was not solved.

 

 

Next video shows the way to check that spring F and all the parts are well assembled. Just press reset button and see what happens.

 

 

Therefore, the breakdown was not a bad assembled system. I contacted the person who sold me the watch. He confirmed me that the breakdown occurred after 5 months of wearing the watch. That’s important. If the breakdown occurs in a new watch…95% chance that the pieces are not well assembled. Here the problem was different.

 

Following picture shows the problem. Piece B fails to fasten piece A. “Prev†is the original piece in this movement and “new†is the same piece of a movement I use for spares. Both movements run at 28800 bph. However, the pieces are different. “New†has longer teeth, fastens better and the hole is in a different position. When piece “new†was installed the breakdown was solved.

 

 

Ok, time to assemble back the movement. First step is to put the gen ETA date wheel to correct the not centred date font, and the piece show by the arrow. In addition, the gen ETA DW makes more perfect this rep.

 

 

Next, the pieces shown in the figure. The function of these pieces is to fasten the DWs after rotation. However, one of them is not necessary (no week day in this model). But it is convenient to assemble it in order to help the spring bar to work correctly. It is very convenient to oil these pieces following assembling instructions of ETA 7750.

 

 

Next step is to assemble the spring bar. Again the wood stick is employed. The spring end shown by the arrow is the first to be assembled.

 

 

And now the wood stick fastens the piece shown by the arrow. It is not easy, but finally the spring is assembled.

 

 

Now the small plate has to be assembled very carefully, or the spring will jump.

 

 

And now the next small plate.

 

 

Last work is dial and hands. The DW problem is solved.

 

 

A beautiful watch, repaired.

 

 

Thanks for reading this long tuto. I wish you can use it to repair your movement or to better understand how it works.

[greg_r 95]

Francisco really knows his stuff - another great tutorial

 

Thanks for re-posting, Onze

[SSTEEL 0]

Thanks for reposting this Onze, came in very handy today

[Haltec 0]

THX!!!

[Conex 0]

Wow. Simple.

[simonbolivar 1]

Exactly the issue with my 3717... I wish I had the balls (and patience) to attempt this .

[SSTEEL 0]

Done several now