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Thread: Omega 720Hz Megasonic running too fast

  1. #11
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    Nobody with an idea what might be the reason for the same fork performing well in one movement and not in the other?

    Of course it would be easiest to just use the NOS fork for this repair. But since it is the only one I have, I prefer to keep it for future trouble shooting. Also I would hate not to find out what the answer to this puzzle is...

    Come on let's solve this one together!
    watchabit likes this.

  2. #12
    WTF Full Member Chris Radek's Avatar


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    I'm not entirely sure I followed your summary of your tests.

    If a fork+coil pair works properly together in one movement, but not in your subject movement, then we must conclude there is something different about the rest of the watch. It seems to me that could be the mounting of the fork+coil (their relationship to one another) or it could be a "traditional watchmaking" type problem, such as lack of freedom in the train due to old oil, dirt, lack of endshake, pivot or jewel damage, particles stuck to a magnet, hand rubbing, etc. Maybe even magnetism? If you remove all the parts that should be magnetized, you could demagnetize the rest. Are the two nonmagnetic screws in the right places?

  3. #13
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    Hi Chris,

    thank you for the input. As the watch is running too fast(see my post from 8-26th for details), I think that rules out most of the "traditional watchmaking" type problems. Wouldn't they all rather lead to a loss in rate?

    The nonmagnetic screws are in the right places - close to the motor box on both bridges.

    I could indeed try to demagnetize all the parts besides fork, magnetic wheels and circuit board. I will go ahead and do thas tomorrow then.

  4. #14
    WTF Full Member Chris Radek's Avatar


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    Well I have to admit I don't know the answer to your question. An Accutron definitely can gain because of something like lack of endshake, because the indexing mechanism depends on the wheel being able to turn backwards because of the tiny force of the index fingers. This is very much like the draw in a lever escapement. The Accutron can't work correctly without the draw.

    But in the Megasonic I don't know if dragging on the far side of the magnetic bridge can cause misindexing. But I think you should check all those things anyway, not just discount them. If you don't have the answer to your problem yet, you know the answer is something you have discounted so far, right?

    Just in case, here are the pictures of the locations of the nomagnetic screws:

    nonmagnetic.jpg

  5. #15
    WTF Veteran Rob B's Avatar


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    I have worked on quite a lot of these watches over the years. If we have to start to match fork with coil and train to make one work, then clearly one of those parts is way out of spec, Even if we can get the watch working, it will very likely end up being a problematic watch some time down the track. My experience is that if the coil hums, but the watch runs fast, it is due to the index wheel in the micromotor being worn out (given that the fork rate is OK). A sticking micromotor or a problem in the magnetic train will make them lose time. The thing is, if the fork and motor is good, and the coil ok, then they will run very well and with a good latitude of battery variation without any problem.

    I recommend to anyone who has one of these watches only to run it if you are going to use it, any other time take out the battery as the micromotors do wear out. You can see the wear on the index wheel under a good scope. Incedentally, I don't recall ever seeing a motor that has lost its oil, though I have seen the odd one with a little bubble in the oil but that doesn't appear to make any difference to them.

    I realize that as spare good forks are extremely hard to find, we have to do our best when servicing one of these movements to make sure that we take into account everything that may cause a problem, but in the end, a worn motor is a worn motor.

    These are by far the most unique tuning fork movements, and having one in running order is a pretty cool thing to own.

    Rob

  6. #16
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    I have interesting news regarding this Megasonic running fast. As I wanted to make sure the results were consistent, I waited for several days to see if the rate was stable. I can now tell you that I have indeed managed to get the problem movement to run with correct rate. I had followed the idea of Chris to demagnetize all movement parts but fork/magnetic mobiles/coil unit. Indeed the reassembled watch then ran just fine!

    I was really surprised by this result as I could not imagine magnetic interference to play an important part here but obviously it did. I was much more convinced that the problem was within the motor box even though I had closely inspected the index wheel without seeing any damage or wear on the teeth. I used 80x magnification with a very good Olympus stereo microscope to really get a good look at the teeth - they looked just like on the NOS part I have.

    It still remains a mystery how the not then demagnetized movement did run ok with a NOS fork though...

    Thank you Chris and Rob for all your input here and also to Daniel who provided me with the replacement fork from a donor movement!
    watchabit and uscjake87 like this.

  7. #17
    WTF Full Member Chris Radek's Avatar


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    Thank you for coming back and telling us the result! Now we collectively know another quirky trick on these.

  8. #18
    WTF Veteran Rob B's Avatar


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    I strongly suggest you have jumped to the wrong conclusion here. You pulled the watch down, demagnetized it, and reassembled it. Then the watch works OK. That doesn't make any sense to me, tuning fork watches never need demagnetizing and small residual fields have no negligible effect on anything. This especially in your case as the rate of the fork was good. I think it is most likely that something else occurred during the dismantling and re-assembly, especially given that under high magnification, the index wheel looks unworn. Worn index wheels are the usual cause of running fast, but this one looked good. Worn wheels can be seen under a scope. I can suggest that there are two problem areas in megasonics which you may have solved accidentally, one is the clearance between the bottom of the motor and the 1st mobile bridge, and the second and more likely is the clearance between the magnets and coil. Even tiny slivers of magnetic particles sticking to the magnets can be a problem, as well as any fine particles attracted to the 1st mobile and stuck to the bridge. The fork magnets are hard to clean properly, you can get particles that stick to the magnets "behind" them and normal cleaning doesn't get them all.

    Rob

  9. #19
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    Hello Rob,

    thanks for sharing your thoughts. I agree that magnetized movement parts seem to be an unlikely cause for this watch running fast. Nevertheless the watch did run just fine after demagnetizing. It is possible that something else was changed during the process and that this unnoticed change was actually what cured the issue. But I would like to add that I had previously disassembled the magnetic gear train about 10-15 times already when checking different combinations of forks, electronics, mobiles etc. to rule out the cause. I think all changes that might occur from taking apart and putting back together would have occurred long before. Also all inspection and cleaning had been done long before the final demag operation.

    Let's just put it this way: if you have checked every other plausible cause for a cal. 1220 running fast you might want to try demagnetizing all parts but fork/mobiles/e-unit before you decide to run over the movement with a tank. Should it not cure the problem you can still revert to the original plan...

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