Monday, June 29, 2009

7750 chrono

Taken from a forum...

User guide to the 7750 automatic Chronograph movement

Pity the poor 7750, of all the movements, it has to be the one that is discussed the most on these forums, yet it’s probably the most misunderstood and certainly the one movement that everyone has an opinion on. There are so many myths and legends and un-truth’s about this movement, that I though I would share with you those that I remember.

Myth or Truth??

1. You should run the chrono for 24 hours after you first get your 7750, to help “distribute” the oils…

This is totally untrue, not only for a 7750, but for any watch movement. If you understand how a movement is oiled, where it’s oiled, and how the oil is applied, you will know that this is impossible. A movement doesn’t have oil “sitting around” waiting to be splashed and distributed inside the movement when it’s running. When speaking about lubrication in a movement, the type, quantity and how it works is not like any other mechanical thing. It’s certainly not like a car engine, or even the engine in your lawnmower.

In a properly lubricated and serviced movement the oil is held in place between the pivot and the jewel via capillary action. If the oil is held in place, then it has no where to go or be distributed to…

For a movement to be correctly oiled, it first needs to be perfectly clean and dry. Oiling any movement without a full teardown and starting with a clean and dry jewel and pivot, is another myth. Once you have a perfectly clean and dry movement, a measured drop of the specific type of oil is inserted into each jewel, between the jewel and pivot. The measured drop of oil then gets sucked in by capillary action, and the oil fills in the space between the jewel and the pivot. And there is sits, forever. Once the oil fills the space, it can’t move, or be distributed or anything, capillary action keeps it secured in place. The oil will stay there until the watch is cleaned or disassembled. Time only causes the oil to deteriorate and dry up and in about 5 years (makes no difference if the watch is running or not) the oil needs to be replaced with fresh oil.

Since all the oils that are used in lubricating a movement are held in place within the jewels by capillary action, what oil is exactly being “distributed” when you run your chono for 24 hours? The answer of course is “none”.

Myth – Busted

2. You should only run your chrono with a “Fully Charged or fully wound movement”.

The friction in a movement is carefully planned. There is a certain designed friction built into each and every movement. Part of that friction is of course the oil type and quantity applied in each pivot and jewel during servicing. If you over-oil, or under-oil, or use the incorrect type of oil when you service the movement, the running of the movement can be affected dramatically. Over oiling a movement can stop it running completely, under oiling can cause over-banking of the balance wheel (spinning to far and hitting the impulse jewel).

If the power reaching the balance wheel varies, the balance wheel will swing more or less (more power = larger swing, less power = smaller swing, less swing = faster, more swing = slower). If the balance swings more or less, the rate of the watch will vary, if it varies, the time keeping is not accurate.

Therefore the design of the 7750 chronograph, like any well designed movement, is such that the balance wheel can’t tell the difference between a running chrono, and a chrono that is stopped. If the basic movement friction doesn’t vary from chrono ON to chrono OFF, it makes no difference as to the state of wind of the movement when the chrono is engaged.
You can run the chrono at any time, whenever you want, it makes no difference as far as the movement is concerned.

Only two things happen when you start a 7750 chrono

1. The tilting pinion (which is running ALL the time) tilts over and engages into the center chrono seconds gear. The center seconds chrono gear has slight friction, in fact it’s very free to rotate, any friction would be the actual gears meshing. The only real friction is when the center seconds gear ticks over a “minute” on the 30 minute counter. If your movement is serviced correctly, and the chrono adjustments are all within the specifications, there would be about 2 seconds of actual engagement between the center seconds and 30 minute counter, this quick engagement and disengagement isn’t enough to slow down the balance wheel and affect the rate.
2. The 12 hour wheel wants to run all the time, why? well because it’s connected directly to the bottom of the mainspring barrel. As long as the watch is running, the mainspring barrel is spinning round and round. The reason the 12 hour counter doesn’t run when the chrono is OFF, is because a break lever is preventing it from running. Turning the chrono ON, releases this break lever and the gear starts to spin.

That’s it for what happens when you engage the chrono, very simple and easy.

So starting the chrono causes the tilting pinion to swing over engaging the center seconds gear, at the same time the 12 hour wheel is released and starts to spin.

What does the movement see? Nothing really, it doesn’t know if the chrono is ON, or OFF, because there is no change in the friction or running. So if the movement can’t tell if the chrono is running or shut off, what does it matter as to the state of wind of the actual watch? It doesn’t matter, you can engage and run the chrono any time you like, and even leave it running until the mainspring runs out of power…it doesn’t make any difference.

Myth - Busted

3. Don’t set the date between 7 pm and 2 am

This is true for all 7750’s, Asian or ETA. There are two type of calendar changeover gears, Instant, and Slow.

Instant Date Change. A small gear with either a built in moveable spring or a spring connected to it, changes the date “instantly” when it snaps over at midnight.

Slow Date Change. A fixed gear rotates along with the hour gear and slowly engages into and switches the datewheel. Since this gear has a fixed tab that engages into the Datewheel, if you try and change the date when this tab is engaged into the Datewheel, you will damage the Keyless works. It’s not a design defect, it’s just the way it is on this model, if you want a date function, the size of the movement only allows a certain type of date change gear, in the case of a 7750, a fixed gear is installed.

The date gear starts to engage with the date wheel around 7pm and disengages totally by 2 am, if you attempt to do a quick date change during the engagement, you will damage the watch.

Myth – Confirmed

4. The Rotor on my 7750 is noisy

The 7750 rotor only winds in one direction. The winding direction has resistance since the rotor is trying to wind the mainspring. In the other direction there is no resistance at all, and the rotor is free to spin around, and around…and around… If the rotor bearings are dry, there is nothing preventing the rotor from spinning like crazy. Lubricating the bearings helps to quiet the rotor and prevent such a free spin in the non-winding direction.

The noisy rotor is normal, it can be made quieter during servicing when it’s lubricated, or you can lubricate it without a full service.

Myth – Confirmed

5. Don’t re-set the chrono with the seconds hand anywhere but between 10 and 2 on the dial

On an un-serviced 7750, the reset hammer slams down very hard onto the heart cam’s for the Seconds and Minute counters. This force can and will cause the seconds hand to slip, if the hand is sitting anywhere but between 10 and 2. Even genuine watches can suffer from this problem. A slipping center seconds hand is totally preventable by the owner, all you have to do is never reset your chrono between anything but 10-2.
Myth – Confirmed

I think that’s about what I can remember, if you have any further items that you think should be added to this list, please let me know and I will address them.

And as always, all of this is based on my personal experience with these movements as well as standard knowledge with movements and their servicing.