Mysterious watch gain while cased

[GeorgeC]

I have a gts Wittnauer wristwatch 76/3G I have overhauled.  When the movement is out of the case or only in the case back, the regulation is normal and adjustable.  Currently -4 to 5 sec/day.  Once I put the upper case on the movement and check regulation, the watch appears as though it is magnetized.   I have demagnetized the movement, case back and separately, the upper case.  No impact on the regulation behavior.  If I set the watch with a digital clock, the watch, when fully cased, will gain a couple minutes in an hour.

Hands seem to be clear of crystal, hairspring is not sticking or touching anything, the top and bottom of the case is assembled with normal pressure.  Any thoughts on what is going on??

 

[mikepilk]

Have you checked if the movement out of the case gain a couple of minutes an hour too ?

I wonder if it's incorrect timegrapher readings causing confusion - the top pic shows 2.4 ms beat error, the bottom 0.4 ms. Why? Maybe just a weak sound when cased.

It's best to put the crown against the metal bit of the timegrapher, especially when cased, as it gives a stronger signal (The crown connected to the mainplate transmits the sound better than through the case).

[GeorgeC]

Mystery solved.  Turns out that the caseback inside wall was tapered at the edge by design and was coming in contact with the hairspring stud screw head.  I forgot to mention that I did install a balance complete.  The new hspg stud must not have allowed the stud screw to thread in as far as the prior stud did.   Also, I noticed when I uncased the movement and placed it back on the timer, I would see a gradual increase in timing indicating that things were running very slow when cased (binding taking place).  I don't know how that would result in the watch gaining time though?

I ended up giving the case back a few small hammer taps to a round nose staking tool which provided enough tolerance for the hairspring stud screw head to not touch.

Thanks for the look.

Edited May 29, 2022 by GeorgeC