7750 chronograph service

[Bonefixer]

I’ve been working on a 7750 with some success - so far not broken or lost anything and have the base movement up and running well. I’ve reassembled the chronograph mechanism and I have a problem with the minute recording. The finger on the centre wheel is at the wrong angle - it hits the minute driving wheel and the watch stops, unless I give a little nudge and then it carries on. 
 

The tech sheet says to check for 0.02mm clearance (figure B) which I haven’t got. Can I adjust the finger on the chrono wheel, or should I get a replacement? Is there anything else to consider?

[nevenbekriev]

You can adjust the finger, no need to replace anything. But firs You need to red and understand well this

[Jon]

It's a common problem and usually created by someone setting the finger and not really knowing what to aim for and it gets bent out of shape.

There are two bends on the finger. The first bend is adjusted to suit that 0.02 mm gap at the 'start' when the hammers come away from the hearts. The end bend can be manipulated to turn the sliding gear tooth properly. I'm guessing by what you have said that the finger is too deep with the tooth of the sliding gear and gets jammed when trying to turn it. Also check that the sliding gear is free to turn by blowing it with a blower and you'll see how free it is and make sure the minute runner turns freely inside the bush. Again fit the runner on its own and give it a blow to see it turn freely. You may find that the runner doesn't turn because of what I have said and it may not have to do with the finger. The drawing from the tech sheet is all you need to adjust the finger properly, but as @nevenbekrievhas said, it's important you understand the theory and marry that with the practical adjustments of the chrono mechanism.

[Bonefixer]

Thanks both. I read the lesson, and am getting a better understanding of the chrono mechanism. 
 

Practically speaking, how should I adjust the finger? Bend it a bit with two pairs of tweezers? Is this possible with it in situ, or does it need to be taken off and on repeatedly until it's right?

[Bonefixer]

As a follow up I've had some success. I adjusted the finger a bit so it misses the tooth and I've managed to get the minute wheel clicking over nicely. I've also adjusted the Minute wheel jumper spring and the eccentric screw to dial in the meshing of the Chrono driving pinion with the Chrono wheel. 
 

All good. The last issue is the chrono hand. It 'stutters'. This happens on a Bremont I have (based on a 7753 movement) and I sent it back for adjustment. It still stutters. 
Any suggestions?

[Mercurial]

A stuttering second hand on a 7750 will likely be due to either the depth of engagement of the oscillating pinion with the chronograph wheel being too shallow,  and/or the friction spring that prevents backlash and stabilises the movement of the chronograph seconds hand needs adjustment. 

The friction spring looks a bit like a dog’s bone, and is found in the centre of the movement. Here’s a picture : 

 

The spring could be worn, not providing enough tension against the wheel or could be dirty/oily. 

Hope that helps,

Mark

[Bonefixer]

5 hours ago, Mercurial said:

A stuttering second hand on a 7750 will likely be due to either the depth of engagement of the oscillating pinion with the chronograph wheel being too shallow,  and/or the friction spring that prevents backlash and stabilises the movement of the chronograph seconds hand needs adjustment. 

The friction spring looks a bit like a dog’s bone, and is found in the centre of the movement. Here’s a picture : 

 

The spring could be worn, not providing enough tension against the wheel or could be dirty/oily. 

Hope that helps,

Mark

I adjusted the depth of the oscillating pinion, and I think I have it right. The dog bone spring I hadn't thought of 

I think now I'm more familiar with the movement and how it works I might strip down the Chronograph module, clean it all again, and re-build. Maybe get it running better second time round. 

[Bonefixer]

Just to complete this thread - I got it all working. Runs well, good amplitude, accurate. Self winds. Date changes over at midnight. Chrono works as it should and all resets to zero. 
 

I learned a lot. 

[Mercurial]

Well done, great to hear you got it running so well.

Chronographs can be intimidating to begin with!

Best Regards,

Mark

[Knebo]

@Bonefixer, what did you do to stop the chrono seconds from stuttering? Was it the dog-bone-spring? Did you replace it or bend it somehow? 

[Bonefixer]

5 hours ago, Knebo said:

@Bonefixer, what did you do to stop the chrono seconds from stuttering? Was it the dog-bone-spring? Did you replace it or bend it somehow? 

To be honest I’m not sure. I think after the 3rd rebuild I just got it right. Never found an issue with the dog bone spring, just adjusted the depthing as best as I could and made sure the minute counter jumper spring was centred by adjusting the eccentrics. 
 

Probably just lucky