Longines L687 Chronograph Hour Hand Misalignment Issue

[Voyager]

Hi,

Trying to identify the cause for the chronograph hour hand misalignment for my Longines L687 movement.

Essentially, the hour hand will reset perfectly to 12 when reset button is pushed; however, after a couple days, the hour hand will be slightly misaligned (as shown in the picture). It doesn't appear to be gradually shifting over time but rather shifts very suddenly and doesn't get worsen beyond that point. So I am not sure if this is the hour hand creeping issue or something else.

Tried to demagnetize it but doesn't seem to help.

 

Any input is appreciated, thank you!

[rjenkinsgb]

My first guess would be a friction spring, something that helps hold it in place when not being driven?

If I'm understanding the diagram for that movement correctly, the lower centre hand is on item 52 in the drawing, "Hour counting wheel " - and that appears to be an assembly with a spring friction clutch included, between the driving wheel and hand / reset cam?

It's shown in the calendar works assembly drawing, page 22.

 

Or is the "reset" actuator supposed to remain solidly against the cams, and there is some slight loss of pressure on that somewhere, an eccentric screw or pin that needs adjusting? I've never worked on one of that style so I'm not 100% on the chrono disengagement or reset.

 

I'm looking at the ETA 7751 technical manual, linked via this page:

https://shopb2b.eta.ch/en/mecaline.html?p=3

 

[nickelsilver]

The hour counter wheel does have a clutch, which slips whenever the chrono is stopped. In the image the blue arrow is the hour counter wheel, the green is the hammer for resetting to zero, and the orange is the brake. The hammer is pivoted on an eccentric that goes through the watch and is moved by the cam; the cam movement causes the brake to be pulled away or come in contact with the hour counter wheel.

 

When the chrono is stopped, the brake is in contact. When you press the lever to zero, the hammer comes down and pivots the brake away while zeroing, then when the hammer comes back up the brake comes in contact with the counter again.

 

The spring that holds the hammer back is what is actually applying pressure to the counter wheel through the brake. If that is disengaged then the counter will run- but it will run all the time.

 

In this case it could be that the clutch wasn't lubricated and is too grippy to slip when the chono is off. The brake surface that touches the wheel is toothed, but it could still slip. On older 775X movements, the brake was smooth, and slipping was a common enough problem that they went to the toothed version.

 

 

Edited June 7, 2024 by nickelsilver

[Bonefixer]

Recently been working on a 7750 and the hour mechanism is the same. Essentially it is driven directly from the going barrel, but the driving wheel is friction mounted on the hour recording pinion, akin to a canon pinion. When the chrono is stopped, the little plastic part shaped like a boomerang stops the wheel moving, while the barrel continues driving the friction mounted driver wheel.

If it’s periodically jumping I’d guess that it’s a lubrication problem between the driving wheel and the pinion. If it sticks it will jump the little plastic teeth in the brake showing the slight shift you’ve got in the picture. 
 

Disclaimer - I’m pretty much a novice, and have quite possibly named these parts incorrectly and got things quite wrong.