Pictorial - Understanding A7750 Power Reserve & Rotor Problems.

[gbc]

Previously posted this on RepGeek ... decided to share it here to.

Someone handed me a dodgy A7750 to fix. Taking the opportunity to share some pics on understanding common problems with the A7750 - Bad power reserve, Wildy spinning rotors & Stripped gears.

Please note that the A7750 being repaired is A7750 "Lite" in Pam Luminor. The 7750's chrono guts were excluded during manufacturer as the model doesn't actually use any of the functions. If your watch is a chrono it will look different as there will be more parts. The simplied layout will make understanding the issues easier tho.

1st pic shows the rachet gear (gold with 3 holes) at the upper most part of pic. This is the gear that commonly strips when you hand wind. The watch original rachet gear was stripped tho the owner said he didn't hand wind ... in fact he didn't even know his watch could be hand wound. The gear you see in the pic is replacement gen ETA. Yes! You can swap gen parts in your A7750 ... well sort of. I'll explain later.

The 2nd pic shows the reduction wheel (gold & spoked) & reversing wheel (silver) being put back into place just above the center of the movement. These together with the ratchet wheel are the heart of the power train. The silver reversing wheel interfaces with the spining rotor & inertia is transfered & stored by the mainspring.

The gold reduction wheel is also a gen ETA part. There was nothing wrong with the Asian part but I find the A7750 winds smoother & quieter with these gen parts. I also has had a gen reversing wheel but it could not fit as the pivots were marginally bigger so the original Asian was maintained. So gen parts are swappable but its very touch & go with the A7750.

Sticky reversing wheels are often blamed for stripped rachet gears but personally i think the main culprit in many a A7750 is the click for the reversing gear. The 3rd pic proved my suspicions right as the watch appears to have a click that was damaged & too short. Damaged/missing clicks are very common on A7750 & is the source of many complaints we see here & the scourge of all rep chrono owners!

Damaged click is on the right & replacement gen ETA is on the left (longer). The click is really fragile & one of the hardest parts to handle on the 7750 as it needs to be friction fitted onto the winding bridge.

So why is this small metal shard so important? From the 4th pic you can see the mainspring click. Its function to hold mainspring from unwinding & basically forms the basis of your watch's power reserve. The click travels 4-5 notches clockwise from A to B afterwhich it falls back to A & locks the mainspring. This AB cycle repeats itself many times during the day when you wear your watch or put it in winder. This is how your automatic charges itself.

It takes about 5 to 6 complete rotation of the A7750 rotor to complete one AB cycle on the mainspring. This requires some effort as 6 rotations requires some rather spirited shaking. This is where the click on the reversing wheels come in.

From the 5th pic, you can see that reversing wheel click (circled in red) holds back the reversing wheel. This allows your watch to store the rotations of your rotor & allows it complete the AB cycle slowly & incrementally.

So if the reversing wheel click is damaged or too short the only way for your A7750 to charge itself would be to do 5 complete rotations "at once in its entirety". Any incomplete AB cycles is lost energy & the tension in the mainspring click will be sent back to your rotor as the spring snaps back. As the damaged reversing wheel click is not able to hold back the rotor ... it will spin wildly in the opposite direction.

You can imagine how hard it will be to do 5 complete rotations "at once" in the course of normal watch wearing. This one of the most common causes of bad power reserve ... your watch can't charge itself.

So if you your rotor spins wildly & your watch has crappy power reserves ... chances are its has a damaged reversing wheel click. The reversing wheel itself is rather hardy on the A7750.

There were some forumers that cured the problem by greasing the rotor but it actuality they were merely masking the symptoms. The oil specified for the rotor is Moebius 9010. This is a very light oil. A heavy lubricant like grease on the rotor actually adds drag so thats the reason the rotors spun less wildly.

You can easily check whether the reversing wheel click is the culprit because there is spy hole on 7750 movements. You don't even need to take off the rotor. But you need to use a loupe tho.

The pic below shows the almost complete reassembly of the A7750. The last thing to go on would be the rotor. The spy hole is circled in red. You can also see the exposed teeth of the reversing wheel marked RG. The teeth on the rotor will interface with the RV once screwed on.

Hope you guys find this informative.

[offshore]

Thanks for bringing this here.

I'm sure our people will find it most informative.

All info such as this is exactly what our home is about.

Offshore

[Logan]

Super informative post. My Big Bang has exactly these symptoms..

[freddy333]

Very informative, thanks.

[Toadtorrent]

Very informative post. Thanks! Do you find the substitution of gen ETA parts is consistent across all A7750 movements, or specific to each movement?

e.g. gen ETA ratchet gears always work, and gen ETA reversing gears NEVER work

OR

Gen ETA ratchet gears USUALLY work, and gen ETA reversing gears SOMETIMES (but not always) don't fit

Thanks for the effort and explanation.

[RWG Technical]

Very informative, thank you.

I haven't had any luck with ETA gears into asian 7750's. The ratchet gears always have too large a pivot for the A7750, so I will have to turn them down on the lathe.

I have also tried replacing the reversing click gear with an ETA, it doesn't fit either...

I have never noted any real problems with the reversing click spring that I couldn't fix, normally they are loose so I glue them in place, and if the tip of the spring doesnt' engage into the gear, I bend it to make it engage, besides this, they have been fine.

Thanks for sharing your expertise.

RG

[southcoast68]

Awesome information!!!!!!!!!

Thank You!!!

[FxrAndy]

Thanks GBC

[ChipWrecked]

Thanks GBC,

A 2836-2 (probable clone) movement is exhibiting similar symptoms you describe in the A7750. Obviously different anatomy, but is the clickspring a likely defect in this situation as well?

[gbc]

Very informative post. Thanks! Do you find the substitution of gen ETA parts is consistent across all A7750 movements, or specific to each movement?

e.g. gen ETA ratchet gears always work, and gen ETA reversing gears NEVER work

OR

Gen ETA ratchet gears USUALLY work, and gen ETA reversing gears SOMETIMES (but not always) don't fit

Thanks for the effort and explanation.

I can't say for sure as per my post. There's a little trial & error involved & you settle on the arrangement that works for that particular watch.

I never had any problems with the swapping in the ratchet wheel personally.

So I'll go with "Gen ETA ratchet gears USUALLY work, and gen ETA reversing gears SOMETIMES (but not always) don't fit"

You can make mod Gen parts to fit if required as highlighted by The Zigmeister.

Most fit issues relate to the pivots that are marginally too large in the gen & they can be smoothed down by hand.

[gbc]

Very informative, thank you.

I haven't had any luck with ETA gears into asian 7750's. The ratchet gears always have too large a pivot for the A7750, so I will have to turn them down on the lathe.

I have also tried replacing the reversing click gear with an ETA, it doesn't fit either...

I have never noted any real problems with the reversing click spring that I couldn't fix, normally they are loose so I glue them in place, and if the tip of the spring doesnt' engage into the gear, I bend it to make it engage, besides this, they have been fine.

Thanks for sharing your expertise.

RG

Thank you. I have also learn much from you.

You are a great service to us all

For the ratchet wheel, you can also swap the pivots over as there are friction fitted to the rachet wheel.

[gbc]

Thanks GBC,

A 2836-2 (probable clone) movement is exhibiting similar symptoms you describe in the A7750. Obviously different anatomy, but is the clickspring a likely defect in this situation as well?

The 2836-2 has a different anatomy so no one can say for sure without inspecting it.

Generally,the winding train on the 2836-2 is pretty hardy.

Does your watch keep good time & have healthy power reserve?

The 2836-2 has bi-drectional winding meaning it can wind CW & CCW.

It has 2 click wheels & the click is build into the wheel itself unlike the A7750 with an external click.

I would guess if it is click problem it might be due to the crown wheel ... which is different issue.

[dluddy]

Great post gbc. Thanks for sharing

[chronoluvvv]

Post of the MONTH right here ... muchas muchas gracias, senor !

[AstroAvia]

Very informative and very encouraging also. After reading this thread, I have disassembled my A7750 (first time I have ever done that!) auto winding module so as to look for the culprit for the issues my watch is having (I have a thread on this at RG). I have found the reversing wheel is only erratically engaging the auto-wind gears, whereas the click is intact and working perfectly. One thing I noticed though that differs in my A7750 from what GBC has masterly described is that it only takes 2 to 3 complete turns of the rotor to complete one "AB cycle", as evidenced by the audible and visible click of the mainspring click.

THANK YOU VERY MUCH, GBC!

[aliates88]

Thanks very much, that's exactly the information I need. Very good pictures and explanations.

[gbc]

Very informative and very encouraging also. After reading this thread, I have disassembled my A7750 (first time I have ever done that!) auto winding module so as to look for the culprit for the issues my watch is having (I have a thread on this at RG). I have found the reversing wheel is only erratically engaging the auto-wind gears, whereas the click is intact and working perfectly. One thing I noticed though that differs in my A7750 from what GBC has masterly described is that it only takes 2 to 3 complete turns of the rotor to complete one "AB cycle", as evidenced by the audible and visible click of the mainspring click.

THANK YOU VERY MUCH, GBC!

Hi! AstroAvia,

I've been following your post on RG concerning your PDs.

I can understand your frustration.

I've posted a possible solution there that you wanna try.

http://www.repgeek.com/showthread.php?t=34201&page=2