Omega 861 Speedmaster stopping

[AlexeiJ1]

Hi guys. 

Recently tackled a 861 Speedmaster mark 2 for a mate and it was running well. 

It came back shortly after a brief wear and I found that the chrono stops at 58 seconds after after 3-5 hours of running. 

I've adjusted the spring pressure on the minute runner lever. Have inspected and cleaned centre wheel, the minute counter finger is in good nick and springs back well. No rust there. 

 

What's next? Really sick of it as I've done all the other chronograph controls. I fitted a NOS chronograph driving wheel as old one was a bit worn and warped. 

 

Should I fit a new centre wheel? 

 

Cheers 

Alex 

[JohnR725]

just a little background history did you service the entire watch?

[Endeavor]

It would be helpful, at least to me, to rewrite your question and naming the components by their names as in the Omega service manual, which I included.

Omega 861 service manual.pdf

 

Edited May 23, 2023 by Endeavor

[nickelsilver]

Stopping at just before 60 indicates something with the minute counter. The minute counter wheel needs to be absolutely free, and unlubricated; you should be able to nudge it forward or backward until a tooth just comes over the jumper tip and it should snap into place cleanly under the jumper tension. The jumper itself- clean, totally free, and the jumper spring should be nice and straight with the jumper removed.

 

The sliding gear should be set so that a tooth on its wheel points directly at the center of the chonograph runner. If it's off a little bit counter clockwise the finger on the runner can jam (though hard to happen on this caliber).

 

One thing I've encountered on this caliber is someone in the past screwed down the chronograph bridge without checking that the intermediate minute counter wheel has its pivot in its hole. That wheel is unjeweled; if you screw the bridge down in that condition (and don't break the pivot) it can make a divot in the bushing, sometimes slightly closing the hole. This can cause enough friction to to cause the minute counter wheel to not snap cleanly tooth to tooth with the jumper, or, even stop the watch with the chronograph running.

 

Have a look through, and try to post some clean close up pics of the chrono mechanism.

[Waggy]

I'm sure you have considered this, but I had a similar situation with a non-chronometer watch (so maybe it's not applicable in this situation?), where it turned out (at least this was my conclusion) that the barrel arbor hole was too slack (and this could be seen upon closer inspection) and lead to the barrel binding up. At high wind the spring force could overcome this additional friction/binding but as the spring wound down it had less and less energy to overcome this problem and it eventually stopped. I fixed the slack in the plate and the error did not re-appear, so some circumstantial evidence at least that my working theory was correct. It's a relatively easy check and therefore easy to rule in or out - just remove the wind from the spring and see if the ratchet wheel can 'rock' back and forwards - I learned this from Kalle Slaap of Chronoglide who always does this as part of his initial disassembly checks. Anyhow, something to try and (hopefully) remove from the suspects list .

Just thinking it may also be worth re-checking all the jewels for cracks or chips, but this would require a full disassembly of course.

 

[Endeavor]

Another question; what's the amplitude of the watch with and without the chronograph engaged? It seems, according to your information, that it runs for 3-5 hrs and then it stops with the seconds recording hand just at 58seconds, just at the point whereby the running works experience an additional load.

In simple terms; are the running works powerful enough to drive the chronograph or is it running out of steam after 3-5 hrs and can't bear the additional load anymore when it has to drive the minutes-recording wheel at 58 seconds?

What's the amplitude when you disengage the chronograph after it has stopped running (3-5hrs) and what's the amplitude at that point, after you zeroed the chronograph, engage it again and measure the amplitude between the 0-58 seconds?

You can also visually observe the amplitude of the balance wheel at the moment when it has to drive the minutes-recording wheel.

Perhaps there is nothing "wrong" with the chronograph. Perhaps it's a combination of a "heavy" running chronograph, and a not strong enough running work, both in desperate need of a proper service?

Which leads back to JohnR725 question above plus; what was the complain / reason when/that you got it?

Edited May 23, 2023 by Endeavor