Watch losing 20-30 minutes when changing date

[Reacher01]

Movement is a ST96-4 I've just serviced.

I didn't change the mainspring and I'm using an app for timegrapher, but timekeeping seems good (around 15 seconds difference between positions, beat error around 0.5ms).

Now, strange thing, even when this problem is happening (from around 10.30 pm) the timegrapher app keep saying that timekeeping is good.

Also, when setting the time, the crown is very very light and it gets heavier around 10.30 pm, when the date wheel gets engaged.

What kind of problem should I be looking for?

After further research it looks like a loose cannon pinion may be a cause.

Question, would using thick grease instead of oil on the cannon pinion make a difference?

[Nucejoe]

 Grease is for high torque low speed gears/ arbours, oil is not suitable for canon pinion as it often wont stay where put .

Did you lubricate all parts that move in date change mech?  Best to follow data sheet which shows oiling of each part.

Are springs to the date mech correct ones? 

Rgds  

[JohnR725]

17 hours ago, Reacher01 said:

using an app for timegrapher

Just a word of caution often times apps are not reliable it's better to spend the money and buy a Chinese timing machine and avoid the headache of tracking down nonexistent problems because of faulty apps.

17 hours ago, Reacher01 said:

Movement is a ST96-4 I've just serviced

For those of us who like to see what were talking about here is somebody servicing one

https://watchguy.co.uk/service-avia-st96-4/

17 hours ago, Reacher01 said:

Now, strange thing, even when this problem is happening (from around 10.30 pm) the timegrapher app keep saying that timekeeping is good.

Also, when setting the time, the crown is very very light and it gets heavier around 10.30 pm, when the date wheel gets engaged.

What kind of problem should I be looking for?

After further research it looks like a loose cannon pinion may be a cause.

Let's look at the clues the app if it's reliable says Watch is running fine. This is because the app and timing machines only can listen to what the balance wheel is doing they can't see the rest of the watch they don't know if the hands or even on the watch whether the moving right whether the calendar changes they only know what it's hearing and it's hearing that the watch sounds like it's running fine which is a clue.

Then when you put the cannon pinion On you are supposed to try it out. Ike pull the crown out in the sending and set the watch once that's on how did it feel? Yes I know this is subjective test but if it felt super loose well that would be a problem.

Then there is the other problem notice from your description around the time that you're probably starting to drive the calendar mechanism he gets heavy? So if you're having a calendar mechanism problem that could be loading down the gear train possibly. If the cannon pinion Is not tight that is definitely an issue. Then conceivably could be all of the above calendar mechanism loading things down the causing of the slippage that you're getting. Basically ago I have take the hands and dial off and examine the calendar mechanism and of course check to see if the cannon pinion Does really feel right does it feel tight or loose?

Then also follow the Suggestions from Nucejoe.

Then out of curiosity what lubrication's were you using?

 

[Reacher01]

Forgot to write that I solved this problem.

The cannon pinion wasn't tight enough. Checked everything else before tightening it (spring, oiling, etc.) but only crimping the cannon pinion made a difference.

Problem solved.