Advice Requested: Tempermental Landeron 48 Chronograph

[Feedback]

Greetings All,

Hoping I can get some advice from resident experts on the Landeron 48 chronograph movement, or at least share my pain. I have worked on three Landeron 48s at this point and have struggled with 2 out of the 3 getting the stop/reset mechanism to work reliably. With all due respect to the responsble folks who created this movement back in the day at Landeron, it frankly seems like an impossibly bad design, but I'm hoping someone might have some tips on how to get it functioning properly and reliably.

For those who are unfamiliar, the Landeron 48 has a somewhat unusual operating scheme wherein one pusher starts the chronograph and the other performs both stop and reset functions. It's designed to work as follows (apologies if I use incorrect terminology for the parts):

• Stop Function: When the hammer is in the running position, depressing the stop/reset button pushes the "shoulder" of the stop/reset operating lever (circled in yellow on the image, although it is hidden underneath the hammer) against the top of the start operating lever, which returns the hammer to the stop position. The stop/reset operating lever is then designed to slide out from underneath the hammer under spring tension and return to its resting position.
• Reset Function: When the hammer is in the stop position, a subsequet press of the stop/reset button pushes the "finger" of the stop/reset operating lever (circled in red in the image) directly against the top of the hammer, moving it into the reset position against the cams of the chronograph wheels.

In practice, I find that the tolerances required for this to work properly seem to be overly strict. More often than not, the finger part of the stop/reset operating lever (circled in red):

1. Jams underneath the hammer after moving it to the stop position, failing to return to its resting position - i.e. the "finger" on the lever is seemingly too long to reliably work its way back out from under the hammer.
2. Fails to properly engage the top of the hammer when attempting to move to the reset position, wedging itself underneath the hammer rather than moving it - i.e. the "finger" on the lever is seemingly too short to reliably engage the top of the hammer.
3. Both - i.e. the operating lever finger seems both too long for the stop function and too short for the reset function.

In any event, this is frustrating because there don't seem to be any adjustments or eccentrics on the movement that affect how this mechanism works. I was able to get this working reasonably reliably on one of the three Landeron 48's I've worked on, but had to resort to carefully re-shaping the point of the hammer index spring to slighly alter the resting positions of the hammer, and this feels like it shouldn't be necessary in order to get things working properly.

In any event, any advice or insight (other than avoiding Landeron 48 movements ) would be greatly appreciated. The timekeeping functions of the movement pictured below are working great, so it would be a shame if I can't get the chronograph to stop & reset reliably....

[nevenbekriev]

OK, I will try to explain how it must work. I used the green color to put some marks on You picture.

From running mode, when stop/reset pressed, the tip of the stop/reset lever (the small green circle) must meet the hammers lever (where the other green circle is). Thus the movement of the levers must stop with the two green circles touched together while the index spring is still in the middle of the tooth. The pusher should stop sinking. The tip of the stop/reset lever should not go under the hammers lever.

When the pusher released, the tip should return up. Then the index spring must make the lever with the hammers advance to the stop position. Pressing the pusher again then will make tip press the hammers lever from above and will make it go to the reset position.

From what You say, the conclusion is that the tip of stop/reset lever is shorter than needed. But it can be bent a little to compensate for it and make it work correctly. Some local annealing will prevent from breaking

 

[Feedback]

Sorry, I should have been clear that the picture I posted above shows the chronograph with the hammer in the stopped position. Here is a picture of the chronograph in the running position. As you can see, the stop/reset operating level actually sits underneath the hammer in this position, so the movement to the stopped position cannot be effected by the tip, or finger of the lever (red), but rather by the shoulder of the lever (yellow) pushing on the top of the start operating lever as I described above: