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Tissot 781: hour hand not in sync with the minute hand

MartinOfPrague
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MartinOfPrague Member

MartinOfPrague

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Hello all, I have a Tissot cal. 781 with a strange problem. Let's say I will pull the crown and start rotating the hands at 12 o'clock, with both hands pointing exactly to the 12 index. After rotating the minute hand around the dial twelve times, the hour hand will now point to somewhere near "quarter-to-twelve" rather than to the 12 index. And if I go on, the "delay" accumulates.

The hour hand however seems to be moving very smoothly, there are no apparent stops, so I tend to believe it is not the matter of friction between the hour hand and the dial.

I disassembled the motion work and inspected the teeth and leaves, none of them is missing and they all look right. I took a picture of the gear, counted the teeth and leaves and the ratio is also correct - a 12 leaf cannon pinion to a 36 tooth minute wheel and a 10 leaf minute pinion to a 40 tooth hour wheel.

What should I look at next?

20240321_203407.jpg

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HectorLooi Distinguished Member

HectorLooi

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Doesn't sound like a loose canon pinion. A loose canon pinion results in loss of time but hand are still in sync.

Loss in syncing of hands is usually caused by missing dial washer, loose pinion of minute wheel or hands touching each other or something. 

 

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Nucejoe Distinguished Member

Nucejoe

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 Oh shoot, I hadn't understood OP's descriptions🙃   

OK minutes hand looses 15 minutes every twelve hours and the loss accumulates.

 The two adjacent gears shown above must either not belong to same caliber ( need a vulenteer to do some calculation because I am lazy ) or  a few gear teeth miss pinion leaves which must have gone unnoticed, a vid in slo mo shows this kinda stuff. 

 Loose revit of pinion on gear wont by itself explain the accumulation of those 15 minutes , but I check the revit for looseness anyway, it can facilitate gear teeth  to miss pinion leaves.

 

 

 

Edited by Nucejoe
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rjenkinsgb WRT Addict

rjenkinsgb

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6 hours ago, Nucejoe said:

The two adjacent gears shown above must either not belong to same caliber ( need a vulenteer to do some calculation because I am lazy )

12:36 then 10:40 ratios, looking at the photo.

/3 the /4, so correct 12:1 reduction..

 

It must be something slipping or skipping due to allignment?

 

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Nucejoe Distinguished Member

Nucejoe

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2 hours ago, rjenkinsgb said:

12:36 then 10:40 ratios, looking at the photo.

/3 the /4, so correct 12:1 reduction..

 

It must be something slipping or skipping due to allignment?

 

How about other gears in the MINUTE TRAIN? haha.

 

 

@MartinOfPrague There are walkthroughs on this caliber on WRT, where you might find a pic of the minute train, good to make sure you have the right gears as far as the Number of teeth. 781 Is originally an ETA with a host of calibers whose parts interfit. Worth a checking In case you found no gear teeth slipping.

Finally a vid in slo mo of the train  might be needed .

Rgds

Edited by Nucejoe
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MartinOfPrague Member

MartinOfPrague

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All right, so here is the most likely explanation: one of the dial feet is broken off, therefore the the dial is not well centered and the short tube which is a part of the hour hand probably rubs against the edge of the dial hole. Thanks again for all the suggestions, though!

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