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Discussion Starter · #1 ·
Picked up a 60s-70s Tissot T12 Navigator Worldtimer a while back. had my watchmaker clean and overhaul the movement. I also bought what I thought was an identical donor watch so I could replace the dial and hands.
My watch guy does a nice job swapping and all is good until until I notice that the date is changing twice every 24 hours at the 6 o'clock position. This is when I noticed that this is supposed to be a 24 hour watch.
Now, my repaired watch has a Tissot 784-2 (12 hour) movement. The identical donor watch has a 788 (24 hour) movement. Both are of the same time period and both look very similar.

Original watch with 784-2


Identical (same dial, hands, etc.) donor watch


This is what my original looked like before I swapped dial and hands:



This is it after I used dial and hands from the donor watch.



Questions:
Is it possible that this watch came in two variations, a 12 hour and a 24 hour?
Are the date wheels different for the two?
My watchmaker made no mention.

Unfortunately, I cannot use the 788 movement because it has a lot of rust damage. I don't know how much is salvageable.

Can the 12 hour 784-2 be adjusted to be a 24 hour movement?

Any thoughts or ideas for this perplexing problem are welcome.
 

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Hi mrsnak,

the worldtimer with 784-2 was a Franken. The hour hand should turn once a day, and of course increment the date once per revolution. And precisely that's what the 788 distinguishs from the 784-2: It has a 24h display and an according date gear.

What your watchmaker realized should be called Franken-Franken. It has a 12h display on a 24h dial and the date gear from the 788.

If you still want 12h display on a 24h dial, you can remove one of the two pins of the date driver; then it will advance the date only every second hour-hand revolution. But the clean solution is to mount all gears (hands and date) from the 788 onto the dialside of your movement, which ever it is.

And an additional advice: Look for a real watchmaker.

Regards, Roland Ranfft
 

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Discussion Starter · #3 · (Edited)
Roland, I knew i could rely on your expert (and frank) assessment. I had assumed as such that it should have had a 788 movement but was puzzled by how seemlessly the 784-2 had been integrated.
This came from a well-known Ecuadoran eBay seller, and was filled with problems from the beginning. No clue how this guy manages to get the 100% feedback and I swore I'd never buy a vintage from a country with high humidity. This seals the verdict.
The watchmaker friend that I had work on this is not what I would consider a watch purist. I love him dearly as a friend, but he often does not have regard for making sure a watch is correct.
To do this right, I really need to resurrect the rusty 788 movement and see what good parts I can pull from the 784. I could have the date wheel converted to a 12 hour, but I wouldn't be happy.

The donor watch was correct, and I had the watchmaker take the hands, dial and date wheel from that one foolishly thinking that mine was also a 788. I only discovered for myself that the movements were different after I picked up the watch. He merely did what I told him to do and charged me so little, it's not a problem.

Fixing the 788 is going to be a task- look at the rust -and that's just on the surface.




I do have another watchmaker who is insane for his high standards, so I will be visiting him this week. I'm hoping that between cleaning the 788 and using parts from the 784-2, I can end up with a good 788.
 

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Hi mrsnak,

most parts to turn a 784 to a 788 are made of brass, thus should be still usabnle. And the date driver is the same for both anyway. So there should be no real problem.

Regards, Roland Ranfft
 

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Discussion Starter · #5 ·
Thank you much for that. I will report back to this thread when I have more to report. I'm enjoying the challenge.
 
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