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Discussion Starter #1
Hello everyone, I've recently purchased a Seiko 6139 which I want to service but since it's my first Seiko chrono I want some advice before moving on.

The only problem that the watch had when I bought it was the fact that the center seconds/chrono hand would sometimes not reset at 0 but stop 1-2 seconds before or after the 0 mark.

So far I've stripped down the movement but I'm not sure if I'm looking at some faulty parts. The movement was mostly clean and it had been serviced a few years ago but the job was done poorly as there were areas with excess lubricant.

I've read a little bit about this movement, particularly the chrono wheel (which looks pretty complex) and I wanted to know if I have a good wheel or not.

To my untrained eye I can't see anything wrong but I did notice something peculiar. Between the top wheel and the bottom one there is another one in the center which seems to be a bit wobbly. As you can see in the 2nd pic it wasn't perfectly horizontal. You can straighten it with the tip of the tweezers but you can also make it sit slightly sloped.

456998

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You've got some courage doing a chrono - good on you. They're a well designed movement - just be slow, careful and logical ;-)

That wheel looks fine ;-) A little lean doesn't effect it interacting with the lever faces.
 

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The characteristic of stopping at 0:58 when running is normal when the movement is in need of cleaning and re-lubing and sometimes when the mainspring is gunked up but mostly always a sign it needs a strip/clean/reassembly.

Good luck you are indeed a brave man to tackle the 6139 !
 

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Isn’t there a quick way to verify if the chronograph wheel is good? I’ve seen it done on a couple of Spencer’s videos.


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Discussion Starter #6
So, if I understand correctly, the wheel in the center of the whole chrono wheel ensemble should be tight and not rotate freely ? (Haven't checked that detail yet)
 

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Here's a better cross section diagram (borrowed from 'adventuresinamateurwatchfettling')
 

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the large wheel (and the clutch plate) rotates on the shaft/axle

the clutch assembly presses down on clutch plate (this is what the clutch spring is for) and the pressure keeps them together so that they (can) turn as one.

the clutch plate is staked to the same cog/pinion that the large wheel is staked to.

one test of the chrono wheel (removed from the movement) is to hold the large wheel between thumb and finger (as you would hold a coin to look at the two faces), then very carefully turn the golden finger. This should turn with a little resistance. Be VERY careful not to bend the golden finger, or to use force.

If you can't do the above, it means that the gear (pinion) that the large wheel is staked to is binding on the axle. This is usually caused by rust, but it could be very congealled oil.

If there is no resistance (the golden finger turns freely) then either the clutch spring is broken, or the clutch assembly is deformed, or the clutch plate has become loose (unstaked), or the large wheel has become loose (unstaked). All of these issues indicate a damaged / broken / faulty centre chrono wheel that will either behave randomly or potentially stop the movement.
 

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I trust Sir Alan implicitly on these chrono wheels as he has managed to rebuild a few. Given your pictures, me, I would not even think of using that wheel in one of my projects. Looks like the 4th wheel gear has become unstaked and thus the rake or lean of the gear. Everything should be in parallel and there should be no gaps in the clutch plates where they touch which would indictate a broken clutch spring.

The way I test these is to hold the long end of the chrono wheel pinion between my gloved index finger and thumb. Then take a very fine lens brush and gently brush the large 4th wheel gear. The gear should not move. If it moves slightly or spins this is indicative of a bad clutch.

Now, I have done this test on a chrono wheel that was soaked in oils and the clutch slipped. After soaking and cleaning in Naptha to remove the oils the clutch plate no longer slipped and the chrono functioned normally.


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This wheel needs to be corrected (I'm happy to be wrong - above). It can be moved on the pinion closer to the minute with the use of a staking set or reset heart.
 

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Discussion Starter #12
By the way, I forgot to mention that this is a 6139B. I've read that there are some slight differences...
 

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By the way, I forgot to mention that this is a 6139B. I've read that there are some slight differences...
Yes, there is a difference in an A cal wheel and a B cal wheel and they are not interchangeable. If my memory serves me, on the As, the lower clutch plate rides right up against the 4th wheel gear. On the Bs, there is a small spacer between the lower clutch plate and the 4th wheel gear.


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Discussion Starter #14
Coming back with some clarifications. The 4th wheel is not loose/unstaked. The only one that seems to be potentially wrong is the clutch wheel which can be slightly leaned onto one side if you use a pair of tweezers for example. When I turn the minute golden finger the clutch also turns and they only move if you apply a bit of force.

So far I've cleaned all the parts an will begin to reassemble the movement, soon. I've found some excess oil in some areas, and quite a lot inside the mainspring barrel. Haven't noticed anything else problematic.

From looking at various manuals I noticed that this movement has lots of adjustments that one can make for the chrono function. Is it possible that wrong adjustments could lead to the center seconds hand resetting at +/- 1-2 seconds instead of 0 ?
 

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If the sweep hand randomly resets + or - 0, dependent on being reset from a position before or after 0, would indicate a loose hand on the pinion, cracked or loose tube on the hand, or an issue with the heart on the clutch.
 
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