Early AS 976

[Watchfixr]

Had a very early AS 976 come in for service. I mean very early, blued steel hairspring and no cap jewel on the bridge escape wheel pivot. Put a little pressure on the 2nd wheel and it started to tick, so it needed service. I disassembled it, ran it through the ultrasonic, reassembled and lubricated. I always do a preliminary time and amplitude test before putting on the cannon pinion just in case I need to tear it down for some reason. Dial down, 276 degrees amplitude and 4 seconds fast. Dial up, it stopped! Repeated tests. Again, 276 degrees and 4 seconds fast dial down; stopped, dial up. Now, I know what is wrong, but I want opinions from the gallery. I figured it out in the middle of the night after I had stopped at that point. It is simple and easily missed!

Anyone want to offer an answer?

[Khronos]

Ha!  I wouldn't have a clue as I have several watches that do that myself.   But then, I have watches that don't run in *ANY* position! 

 

[noirrac1j]

Is it a worn pivot drilling on the bottom plate since you mentioned it's not jeweled? It could also be a bent pivot on the balance.

 

J

[rogart63]

Did you replace the jewel? If so with the right one. As noirrac1j says it could be a bent pivot. Or hairspring touching the balance cook. Is it incabloc jewels. 

[StuartBaker104]

Did you forget to tighten the balance screw stud?

[anilv]

Dirty balance pivots/hole, or pivot at the balance cock end has deformed/mushroomed?

Anilv

[noirrac1j]

6 hours ago, StuartBaker104 said:

Did you forget to tighten the balance screw stud?

Good one!

[matabog]

Axial shake on escape wheel?

[clockboy]

Check the end play of the balance & end shake of the pivots. Something is obviously binding.

[Watchfixr]

Ok, a lot of good thoughts. The end shake of the balance was ok. That's what baffled me.

The cause of the problem is the very end of the cock pivot is broken off. I missed it during examination of the wheels and pivots. Too complacent! With a high power loupe you can see the pivot is broken. the end shake was good because the remaining part of the pivot would fit into the hole jewel far enough to limit the shake. It also was long enough to allow good amplitude and timing when the balance was running on the other pivot (dial down). When I  inverted the movement, the pivot would bind because it is too short to reach the cap jewel.

This problem is somewhat rare, but I thought I would pass it along as a lesson for others. We never stop learning in this business.