Omega Mark 40 Valjoux 7751 Rotor Behaviour and Chrono Reset Position

kaplan · Jun 9, 2022

Hi everyone,

I got my brother a Mark 40, It was sold as overhauled - main issue I feel is the rotor behaviour, it spins freely like a Miyota movement, coming from 500 series movements it was a surprise to me, however seeing this comment made me believe it was ok: https://www.reddit.com/r/Watches/co...?utm_source=reddit&utm_medium=web2x&context=3

Is it?

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Secondary small issue is the Chrono resetting a tick after 0 - in the sale photos it was resetting perfectly, so I'm not sure whether they just stopped it there instead of resetting it - is it possible to fix this without removing the chrono hand?

If not is it safe to just stop the chrono in the 0 position instead of resetting it and use the watch with chrono stopped instead of reset?

I guess it could be a sign of an internal issue if it moved afterwards, so maybe they didn't overhaul and just serviced it

Donn Chambers · Jun 9, 2022

The rotor movement is fine. Look up “Valjoux Wobble” for more info. It only winds in one direction and when it spins in the other direction there is no resistance and less friction so it can keep moving for quite some time and create a gyro effect on the wrist. First time you feel it you might think something is coming lose inside, but its perfectly normal.

Can’t comment much about the chrono reset issue, but I take service claims by seller with a grain of salt without proof (receipts with dates).

Archer · Jun 9, 2022

Yes, and no.

Cheers, Al

Frank J Malloy · Jun 9, 2022

Archer
Yes, and no.

Cheers, Al

I saw three questions from OP - 1) rotor spin is ok?, 2) fix chrono reset without removing hand?, and 3) safe to stop the chrono instead of resetting? I see two answers from you...want to be educated...

Archer · Jun 9, 2022

F Frank J Malloy
I saw three questions from OP - 1) rotor spin is ok?, 2) fix chrono reset without removing hand?, and 3) safe to stop the chrono instead of resetting? I see two answers from you...want to be educated...

Been covered many times, but yes, no, yes.

kaplan · Jun 10, 2022

I think another good solution is keeping the chrono running, I've timegraphed the watch, and it seems the amplitude doesn't significantly drop when I engage the chrono, it's around 250-590 - so I assume it's safe to keep chrono running as well, watch looks better that way too

To iterate on my initial query, the chrono reset position is moving even further, so without knowing much about the movement, I'm pretty sure it's a part that was needed to be replaced but wasn't replaced, or maybe a tension issue, so basically the "overhaul" wasn't perfect

I think another issue is handwinding being a bit coarse, I don't know the movement but maybe a reverser wheel issue as well

I was happy for my brother because the price was good especially considering the "overhaul" - but even with the issues, it wasn't that bad of a deal, he's happy

I was fancying the 3520.53 myself but will probably stay away, it's too complicated for my taste 😀

Archer · Jun 10, 2022

kaplan
To iterate on my initial query, the chrono reset position is moving even further, so without knowing much about the movement, I'm pretty sure it's a part that was needed to be replaced but wasn't replaced, or maybe a tension issue, so basically the "overhaul" wasn't perfect

I think another issue is handwinding being a bit coarse, I don't know the movement but maybe a reverser wheel issue as well

7750 hand winding is coarse, so without being able to experience it in person, it's likely fine.

If the hand is drifting after each reset, it's likely loose on the post. The hand can be pressed on further, or if need be tightened.

kaplan · Jun 10, 2022

Thanks for the insight

Non deterministically it's moving a tick further, so it's definitely not each reset, maybe once every 20 resets