Horological Meandering

ei8htohms
8555

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could be a lot of different things

Hi Daniel,

Watch diagnosis from a distance is a losing proposition the vast majority of the time, but the symptoms you describe should not be too terribly difficult to suss out with a careful examination.  Stopping that quickly means it's almost certainly a problem with the balance, escapement or possibly the seconds wheel.  Any problems closer to the barrel than that would likely not cause such a sudden stoppage.

The first things I'd check would be the balance endshake and fit of the pivots in the jewels (check to make sure both cap jewels are properly installed for one thing).  If it's got good endshake and does not exhibit signs of having a broken pivot (which is I assume what you mean when you say "it was firm"), then I'd take a close look at the hairspring to make  sure it's perfectly flat and not too close to the underside of the bridge in one spot or another. 

I'd also want to check that the fork was engaging properly with the roller before taking the balance out of the watch.  You can stick a thin probe into the movement from the side (out of the case obviously) and check the proper end shake of the fork while also observing that the divisions (what WOSTEP watchmakers call the vertical alignment of the escapement components) are correct.  A little too much endshake of the pallet fork or the escape wheel could allow something to slip out of engagement when the movement is turned upside down.

If the hairspring is clear and free, and the divisions of the escapement look good, then take the balance bridge out and take a close look at the balance pivots and jewels.  A broken or damaged hole jewel could be the problem as could a bent pivot (although it would have to be pretty bent to stop the watch that quickly). Even some gunk in the upper hole jewel could be causing the problem.

The same process of elimination would apply for the pallet fork, escape wheel and seconds wheel, but in the 7750, you can't really get a good look at the escape wheel or any other wheels in the train without taking all the chono bits off.

I don't know if tech guides are available for the 7750 online, but you might be able to find a charitable watchmaker to scan some for you (I just looked and I don't have them).

_john