Difficulty in servicing 7750 compared to non-chrono movements

[rb0087]

A question for the watchmakers here, specifically those who have dealt with 7750 or similar movements. I have never serviced a chrono movement before, so I'm wondering: how much more difficult is servicing the 7750 than something like a 2836 or 2892? Is it just more time consuming and tedious, or are there skills that are unique to chronos or this movement that make it a difficult learning process? Also, how much risk of breaking parts would you say is involved in trying it out for the first time?

Some background: I'm somewhat competent at servicing movements; I'd say that I'm either on the high end of the beginner category or the low end of intermediate. I've bought a few reps on here and vintage Swiss movements from the bay. I started with a 6497 until I could service that easily, and moved on to some Swiss ETA 2892-2's, and then later some 2836's. I can service and regulate all of those fairly well, although I must admit that I've had some trouble breaking balance pivots on the last two Asian 2836's that I worked on (I really hate those Novodiac style shock springs, I think that in dealing with those I broke the pivots).

Anyway, I am now buying an 116520 Daytona with an A7750 29j seconds at 6 movement. Unless amplitude is poor out of the box, I'm not planning on giving a service to this movement as soon as I get it, but I just want to know what I'm getting into, and how big a risk I run of getting to the point of needing to buy another donor movement to fix my mistakes. Thanks for any advice!

 

[cwazy1]

there are more 'spring' parts in the 7750 and a higher risk of things shooting off. There are also a few tricky tension areas where a spring holds 2 pieces together but the spring isnt anchored (eg. hour chrono brake)

All in all, its just more parts and more oiling areas. Difficulty is about the same, the size of screws are the same, the parts are no smaller. Just use rodico to secure the tension springs to make sure that they dont go flying.

 

[rb0087]

Thanks, cwazy. That's kinda what it looked like to me from the technical docs, etc., but I've definitely underestimated these things before.

And on an unrelated note, your pam 6497 de-casing / disassembly video was one of the first things I checked out before starting to work on my pam homage, so thanks for that!

 

[Wiz]

If you already know the basics then you shouldn't face any real issue, it's just another movement. Some points are more tricky than others, but you'll easily figure out what to do.

However if you learnt everything on the job and lack some basics, it might be a little more difficult. For example, you don't oil a part that is always moving like one that is rarely moving, or not making full rotation.

Other than that you should be fine.

 

[Ko67]

Go to eta web site and get the tech sheets. They also have an animation that shows assembly and disassembly order. If you can follow a tech sheet you can work this out . Be very careful with the screws on the Asian mvts. I have snapped the heads off of several and that is why I do not work on them for others. Too much risk and not enough parts availability

 

[rb0087]

Thanks for the input, guys. I've taken a look at the tech sheets and everything. I haven't yet broken a screw on an Asian movement, but thanks for the heads up; I'll definitely watch out for that. I've broken screwdrivers on corroded vintage movements, but that's another story.
I have picked everything up as I've been going, so I may very well lack some fundamentals. I've looked up lubrication guidelines as they've come up, e.g. oiling recommendations for jeweled pivots, main spring barrels, etc., but I haven't come across many guidelines for quantities of lubrication on cams, levers, etc. What is the key difference when oiling components which do not move continuously or complete a full rotation? I would have guessed that the only difference would be that you could be more generous on the lubrication of contact surfaces, since the parts aren't moving quickly, and that you would use higher viscosity lubricants (although those are specified in the ETA tech docs). Am I anywhere close there, or missing something important?

 

[Wiz]

Be carefull with these tech sheets and animations, they give the impression that you have everything you need, but really they lack lots of informations, basics mostly.

About parts that don't move continuously:

Modern oils are supposed not to gum anymore if you listen to manufacturers, but that's just marketing, they gum less than other oils, but they still gum. Also oil is where all the dust and metal dust (from keyless work friction for example) ends up, making some kind of paste on the long run.

If a pivot is running continuously, this will be spread all around, which is not much of a big deal. If a pivot is rarely moving, or not making full rotation, it will accumulate on certain areas, and this will be a problem.

You need to remember, if a movement is well made and well assembled, in theory it can run great without oil. The primary function of oil is to prevent wear.

So if you have parts that are not moving all the time, and more important that are never or rarely making full rotation, it's better not to oil them at al.

Same goes for the pallet pivots, you never oil them.

At least that's what I've been told by a guy who is a teacher in a top watch school.

 

[rb0087]

Great tip, Wiz, thanks a lot. That makes a lot of sense, but I wouldn't have guessed it on my own. The "less is more" philosophy when lubricating movements seems to hold pretty much everywhere, but it's exactly the opposite of the beginner's instinct.

That's the sort of info that is tough to find on the internet. You're right on the tech sheets; I've noticed the same thing. The tech sheets have all of the movement-specific information, but assume a level of knowledge on the basics that's hard to find without formal training or advice from a more experienced watchmaker. I've looked up a bit on my own (for instance proper lubrication of pivots, etc.), but it's hard to find the information if you don't know what you're missing.

 

[Wiz]

About how much oil to use, the best investment you can make is getting an automatic oiler. 1A is ok for wristwatch movements.

If you never had anyone to show you, you're much likely to apply to much oil with a regular oiler, even if you're under the impression of already using very little.

 

[rb0087]

Thanks, I'll keep that in mind. Those automatic oilers do look nice, but they've always been a bit too pricey for me. So instead I've been going by the guidelines here so far:

http://www.bhi.co.uk/Documents/certificate/Tech/PractLub.pdf .

It has good information about lubricating pivots and pallet stones, but sparse info on chronograph components. Not as good as seeing a job done correctly in person, but hopefully enough to give me a rough idea. I'll admit, though, it's still sometime tough for me to gauge how much oil will be on the pivot based on what the drop on the oiler looks like, so I could probably still benefit from one of those automatic oilers.

 

[Wiz]

I know they're pricy, but it's worth every penny. When you'll see how liitle oil the auto oiler applies, you'll say "really???". Guaranteed