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Benrus sky chief 72c


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Have had this piece for many years and am having issues. Just tore down and cleaned. Before I correct a few simple issues, there's a problem I cant seem to get a handle on.

I noticed the balance rotates more than 360 degrees, and the movement is loosing time quickly. I had replaced the mainspring to help with the slow time, and that's when I noticed the over rotation. I had removed all chrono functions to eliminate possible drag, but that didn't help the timing. I assumed the table jewel was missing, but it is still there.

Next I removed the hairspring and mounted the balance wheel to manual rotate. It appears the balance rotates 2 full turns before the pallet fork banks to the opposite pin and stops. Assuming that accounts for the overbanking when its running. Can't figure out why the balance is rotating 2 times. Could it be the roller table is wrong size? I do not know the spec for the 72c. Greatly appreciate any direction. Thanks Dennis

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Are your two rotations being measured from the extreme clockwise extent of the rotation to the extreme anti-clockwise extent,  or from the  rest point (I.e. with the impulse pin in between the pallet fork horns and central to the banking pins and the balance at rest) and either of the two rotational extremes? 

If the former then although a full 720 degrees is problematic (between the two extremes of rotation), 620 to 630 is fine as this equates to an amplitude of  310 degrees(ish), and could be mistaken for two full rotations at a glance. 

If the latter then it's an intriguing problem as it means that the impulse pin is somehow bypassing the pallet fork on the first rotation, but catching it on the second.  This would also have some significant implications for the hair spring which will be winding up and unwinding by twice the normal amount in each direction. 

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Hi there , just a thought have you checked end shake and side shake of the balance wheel .There maybe a damaged upper or lower jewel setting .Maybe banking pins need adjustment. Do you have a microscope ? I will have a look through my literature and see what I come up with .That is a great watch by the way.......... Cousins have this on there website https://www.cousinsuk.com/document/category/valjoux-movement-parts .Once I came across a similar problem with a movement and it turned out to be a slightly bent plate around the balance wheel area . What a nightmare that was ,anyway just a thought .

Edited by Graziano
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Thanks for the reply. Sorry it took so long .Learning the site.

Didn' think of the plate. Concentrating on the roller table. Just don't understand the 2 revolutions of the balance to lock.

BTW the cousins info is very helpful, Thanks again

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2 hours ago, dennis said:

Thanks for the reply. Sorry it took so long .Learning the site.

Didn' think of the plate. Concentrating on the roller table. Just don't understand the 2 revolutions of the balance to lock.

BTW the cousins info is very helpful, Thanks again

I had a look through my literature ,unfortunately nothing there .Looks like it's time to think out of the square . With out of the square problems ,it involves a lot of staring into the movement and concentration. sounds like something is misaligned in the escapement  

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4 hours ago, Marc said:

Are your two rotations being measured from the extreme clockwise extent of the rotation to the extreme anti-clockwise extent,  or from the  rest point (I.e. with the impulse pin in between the pallet fork horns and central to the banking pins and the balance at rest) and either of the two rotational extremes? 

If the former then although a full 720 degrees is problematic (between the two extremes of rotation), 620 to 630 is fine as this equates to an amplitude of  310 degrees(ish), and could be mistaken for two full rotations at a glance. 

If the latter then it's an intriguing problem as it means that the impulse pin is somehow bypassing the pallet fork on the first rotation, but catching it on the second.  This would also have some significant implications for the hair spring which will be winding up and unwinding by twice the normal amount in each direction. 

Having a real problem locating how to respond to posts. I apologize to all that responded, and thank you for your input.

I recorded the rotation in super slow motion. I would post if someone could tell me how.

The  2 full rotations are done manual without the hairspring. Stop to stop.

In the slow motion video the balance rotates about 1/4 past a full rotation of the balance.

The breguet hairspring is fine, and the balance rests perfectly between the pins.

 

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3 hours ago, dennis said:

Having a real problem locating how to respond to posts. I apologize to all that responded, and thank you for your input.

I recorded the rotation in super slow motion. I would post if someone could tell me how.

The  2 full rotations are done manual without the hairspring. Stop to stop.

In the slow motion video the balance rotates about 1/4 past a full rotation of the balance.

The breguet hairspring is fine, and the balance rests perfectly between the pins.

 

Hi there , to post a picture or video you have made if you look at the bottom of the post you are writing you will see drag files here to attach ,or choose files  .If you are on your pc and have the footage on your pc.Click on choose files , you can select from a window of your files which will appear .If you are on your phone the same applies just select the files from your phone and they will upload . The insert media on the bottom right of the page is for selecting web pages to attach.Hope this helps 

Edited by Graziano
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57 minutes ago, Graziano said:

The  2 full rotations are done manual without the hairspring. Stop to stop.

If I'm reading this right you are saying that the 2 full rotations are between the two extreme limits of rotation, and this is not observed with the watch running under its own power but by rotating the balance by hand until you reach each stop.

This is quite normal. Under normal running conditions, with the hair spring causing the balance to oscillate, it shouldn't reach the stops at each end of the rotation. Ideally you you want to see a balance amplitude of between about 270 and 310 degrees, which means a total rotation angle of between 540 and 610 degrees between the two extreme points.

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4 hours ago, Graziano said:

Hi there , to post a picture or video you have made if you look at the bottom of the post you are writing you will see drag files here to attach ,or choose files  .If you are on your pc and have the footage on your pc.Click on choose files , you can select from a window of your files which will appear .If you are on your phone the same applies just select the files from your phone and they will upload . The insert media on the bottom right of the page is for selecting web pages to attach.Hope this helps 

Thanks.will give it a shot. Some files may be too large.

 

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3 hours ago, Marc said:

If I'm reading this right you are saying that the 2 full rotations are between the two extreme limits of rotation, and this is not observed with the watch running under its own power but by rotating the balance by hand until you reach each stop.

This is quite normal. Under normal running conditions, with the hair spring causing the balance to oscillate, it shouldn't reach the stops at each end of the rotation. Ideally you you want to see a balance amplitude of between about 270 and 310 degrees, which means a total rotation angle of between 540 and 610 degrees between the two extreme points.

You are reading correctly. I posted a short video of the balance rotation. Not the best, but I will make a better one later, along with a video of another 72c I have that keeps excellent time to compare..

May be  dumb question but.....If the physical rotational degrees of the balance are twice the  amplitude degrees, what is the measuring source for the amplitude degrees?

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@dennisYou mentioned that you replaced the mainspring because it was running too slow. Did you replace it with the correct spring or is it somehow different? It seems as if the whole system has too much energy. The pallet seems to be delivering a ton of energy to the balance and appears to be causing it to swing to its full rotation. Does it look as if the roller jewel is hitting the pallet at the full extent of its travel? I would take a close look and see if it is.

Edited by FLwatchguy73
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18 minutes ago, FLwatchguy73 said:

@dennisYou mentioned that you replaced the mainspring because it was running too slow. Did you replace it with the correct spring or is it somehow different? It seems as if the whole system has too much energy. The pallet seems to be delivering a ton of energy to the balance and appears to be causing it to swing to its full rotation. Does it look as if the roller jewel is hitting the pallet at the full extent of its travel? I would take a close look and see if it is.

Yes I replaced the mainspring with a correct piece. I also removed all chromo functions to ease the drag on the train movement. That's when I observed the over rotation. Although the movement is still very slow.

I can see the pallet fork banking from pin to pin. Need to take a closer look, but it appears the fork rests on one of the  banking pins and seems to hit and bounce off the other.

It's torn down/cleaned and ready to check components. Plan to assemble train only, and go from there.

Thank you.

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You mean overbanking? I haven't observed that in a running state. When I manually rotate the balance it takes 2 rotations in either direction for the balance to hit a stop. Now that may be it hitting the outside of the fork horn. I'll have to check that Thanks

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@FLwatchguy73's illustration shows how you can get 2 rotations stop to stop.

Starting with the impulse pin in the fork horns as shown you can see how the balance can make an (almost) full rotation clockwise before the impulse pin contacts the the outside of the lower horn (in the pic) and stops. 

If however you go anti-clockwise instead then the pallet will move across to the opposite bank allowing the impulse pin through and the balance can then complete a full anti-clockwise rotation until the impulse pin contacts the outside of the upper pallet fork horn.

In other words, from the position shown, the balance can make an almost complete rotation in either direction.

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3 hours ago, Marc said:

@FLwatchguy73's illustration shows how you can get 2 rotations stop to stop.

Starting with the impulse pin in the fork horns as shown you can see how the balance can make an (almost) full rotation clockwise before the impulse pin contacts the the outside of the lower horn (in the pic) and stops. 

If however you go anti-clockwise instead then the pallet will move across to the opposite bank allowing the impulse pin through and the balance can then complete a full anti-clockwise rotation until the impulse pin contacts the outside of the upper pallet fork horn.

In other words, from the position shown, the balance can make an almost complete rotation in either direction.

When you say impulse pin are you referring the to the roller jewel. Also, you are using the banking pin center in either case correct?

I then understand the balance rotating twice if using the end rotation contact with the outside of the pallet horns.

Took a new video with a partial assembly. Rotation appears approx 3/4 .Also pic of timing.  Not reading very well as you can see

 

 

20200324_123500.jpg

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5 minutes ago, dennis said:

When you say impulse pin are you referring the to the roller jewel.

Sorry yes my fault, impulse pin and roller jewel are the same thing.

 

7 minutes ago, dennis said:

Also, you are using the banking pin center in either case correct?

Not quite sure I understand what you mean by that.

 

Your TG trace really is quite something, sadly there is nothing readable in it at all. This could be down to many things but I would start by checking that the escape wheel teeth and the pallet jewels are spotlessly clean, and that the hair spring isn't touching anything that it shouldn't. Also that the pallet jewels are secure and correctly aligned in the pallet. Once the trace cleans up a bit then you can get some useful information from it.

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Just from scrolling through both videos, one frame at a time, it looks as if you're getting around 340 degrees of amplitude, which seems excessive. My best guess is that you're getting too much power from the mainspring.

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19 minutes ago, FLwatchguy73 said:

it looks as if you're getting around 340 degrees of amplitude

It looks to me more like a total rotational angle of 340 degrees from extreme clockwise point to extreme anti-clockwise point, which equates to an amplitude of only 170 degrees.

Until you get the amplitude up to 220 or higher the TG trace will be of little use, and ideally you want to see the amplitude up above 265 to 270 degrees (total rotation of 530 degrees or higher) after a full service and with a new m/s.

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2 hours ago, Marc said:

It looks to me more like a total rotational angle of 340 degrees from extreme clockwise point to extreme anti-clockwise point, which equates to an amplitude of only 170 degrees.

Until you get the amplitude up to 220 or higher the TG trace will be of little use, and ideally you want to see the amplitude up above 265 to 270 degrees (total rotation of 530 degrees or higher) after a full service and with a new m/s.

Changed balance and hairspring. Removed pallet ok. New video. Rotation almost 540(270 ampl). Getting there

 

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unless the hairspring stud is on an adjustable arm, you'll have to move the hairspring collet to tighten up that beat error. That will likely also improve your amplitude. Excellent progress!:thumbsu:

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18 minutes ago, FLwatchguy73 said:

unless the hairspring stud is on an adjustable arm, you'll have to move the hairspring collet to tighten up that beat error. That will likely also improve your amplitude. Excellent progress!:thumbsu:

Thanks. Talking through and getting great advice goes a long way.

Will adjust the hs tomorrow. thanks again!

 

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