Chinese 7750

[Uncout]

Hi, first real post to this forum, I'm hoping for some insight to a strange issue with a watch I have.

Per my intro post I'm planning on building a chrono using a Chinese 7750 (And hope in the future to upgrade it to a swiss one) and I got the opportunity to get a headstart using a rep IWC I picked up for cheap as the movement had some issues (Quickset day/date not working properly, doesn't bother me too much). (I know people have strong feelings about reps and I just want to say that I have no plans on keeping this as one, it just gave me a movement and case for less than either seperately which was a huge head start.)

For me the important part was the chrono functions which aren't perfect as one might expect.  First the chrono seconds was stuttering between 30 and 40 seconds which was solved with a small tweak of the clutch engagement, but now I can see it is running about 3 seconds fast per minute (!) against both the small seconds and phone time.

I can't get my head around a cause for this, if it was going slow I would think that it was slipping on the pinion or similar or if it was having an effect on the small seconds as well maybe affecting the gear train, but I can only imagine if it's running fast to the kept time that the gearing isn't 1:1, do you guys have any insight?

If this isn't fixable I'll live with it, but I can't find any info out there for a similar issue online and if it's an easy / cheap fix or adjustment I'm willing to give it a go.

Thanks!

[CaptCalvin]

It's been tampered with. The gearing between clone and original are different. Even different examples of clones might be different, with some being more faithful to original while others modified. Gears involved are 4th wheel, tilting pinion, and chrono seconds wheel. One of these might've been switched out.

Also if you're thinking of putting on your own hands you're going to have a lot of trouble finding a chrono seconds hand that would fit a clone 7750 without modification. The tube would need to be tightened substantially. You might need to resort to taking your own measurements and ordering hands not made specifically for 7750. The post sizes are different between clones gettin different factories too.

Edited August 18, 2020 by CaptCalvin

[Uncout]

13 hours ago, CaptCalvin said:

It's been tampered with. The gearing between clone and original are different. Even different examples of clones might be different, with some being more faithful to original while others modified. Gears involved are 4th wheel, tilting pinion, and chrono seconds wheel. One of these might've been switched out.

Also if you're thinking of putting on your own hands you're going to have a lot of trouble finding a chrono seconds hand that would fit a clone 7750 without modification. The tube would need to be tightened substantially. You might need to resort to taking your own measurements and ordering hands not made specifically for 7750. The post sizes are different between clones gettin different factories too.

Thanks, that's what I feared, doesn't sound like it would be a quick easy fix, I'll probably just live with it for now. Thinking I'll run it a few times and work out what the current ratio is for the chrono vs real time is so I can do a quick conversion when I time anything. 

Thanks for the info on the hands, I knew they were different posts, I was hoping to reuse the chrono hands until I replace the movement with a swiss one. If the current one's don't work with the dial I want to go with I think I found some on CousinsUK that will, but I wanted the movement in hand to double check the post sizes as there's conflicting information out there on what they were on the A7750s.

This whole project is going to be a bit of journey over several months.

[CaptCalvin]

5 hours ago, Uncout said:

there's conflicting information out there on what they were on the A7750s.

Problem with A7750s is that there's not just one A7750. There are several different factories out there making them and they don't care about standardization. There are differences in clones produced by different factories and even within the factories themselves tweaks might be made to different generations. You can't be sure of anything unless you got one in your hands and done your own measurements. A lot of  parts dont interchange either and spares don't exist. If you've got a problem with parts the movement is pretty much unsalvageable unless you've got an identical donor.

Embarking on this journey isn't going to be quick and easy. I started my own chrono build project with a genuine ETA 7750 and it took me almost a year to search for hands and dials and various other knick knacks to produce something I was satisfied with. Good luck and have fun with your own journey!

My final product:

Work and progress pics:

Edited August 19, 2020 by CaptCalvin

[Uncout]

I knew there were a couple different versions of the A7750 (Namely the 21,600 and 28,800 variants), but not that there were multiple factories of them. I might try and find a dead movement from somewhere to try and cobble a working A7750 together, if not then it's just waiting until I can afford the real thing.

I've been looking at case, dials and hands for about a month now and I agree, trying to do this was never going to be a quick thing, all of ones available just seem to have slight variations in size that makes it tricky to work out if they're going to work together, right pain but I'll get there with a sense of accomplishment.

Wow, that's a great looking watch, I considered the same case for a while but I didn't quite like the tachy scale, love the red mod you did! Also, that final dial was one I was considering for a different case, what were your thoughts on it?

[CaptCalvin]

8 hours ago, Uncout said:

I knew there were a couple different versions of the A7750 (Namely the 21,600 and 28,800 variants), but not that there were multiple factories of them. I might try and find a dead movement from somewhere to try and cobble a working A7750 together, if not then it's just waiting until I can afford the real thing.

I've been looking at case, dials and hands for about a month now and I agree, trying to do this was never going to be a quick thing, all of ones available just seem to have slight variations in size that makes it tricky to work out if they're going to work together, right pain but I'll get there with a sense of accomplishment.

Wow, that's a great looking watch, I considered the same case for a while but I didn't quite like the tachy scale, love the red mod you did! Also, that final dial was one I was considering for a different case, what were your thoughts on it?

Appreciate the compliment!

You'll need to pay attention to the dial opening diameter and outer track diameter. You can see that it took me a few tries. Once you've acquired a good fitting dial, you can kind of work everything else out from there.

If you decide to go with the dial I used, based on my measurements, outer track diameter is 31mm so dial opening in case needs to be at least that to avoid covering any of it, the minute hand will need to be 12.5mm to reach the minute markers, chrono seconds needs to be 13.5 to reach the outer track, and the sub dials require 4mm hands.

It's a finely made dial but a few flaws prevent it from being perfect. The lume is pretty average, the window size is tad bit small for the font size ETA uses for day wheel, and the outer markers for the hour counter is printed in a nonsensical division for a 12 hour counter (they count in 24 minute increments wtf?). To fix these as you can see I redid the lume, adding in heaps, but the day window is still a bit choked in the pics. I ignored this flaw for a while until recently I discovered that the font size on clone 7750 day disks display through the window nicely, but the center hole is too tight to fit on a genuine 7750. I had to ream it out so much that I suspect the stake is just barely holding the star wheel and disk together, but it's working ok so far. If you count from when this project started in beginning of 2018 to the day wheel swap just 2 weeks ago, this project had been over 2 years in the making! The nonsense outer hour counter markings I will just have to live with until I learn how to print dials.

Here's how it looks like now with the clone day wheel fitted.

 

Edited August 21, 2020 by CaptCalvin