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M4 Turret movement resistor puzzle.

Simon Allen
 

I am seeking to restore an M4 turret movement.  Robert Miles in his book says that it did not have a resistor connected in parallel with the coil.  My one does.  Across the two terminals connected in parallel to the coil is a 250 ohm resistor.  What was its purpose?
John Hubert
 

Two points here.  Firstly Robert (Bob) Miles book was compiled from the material he had available.  There are quite limited records for Synchronome.  It is possible he worked mainly from the hardware he had access to rather than detailed drawings and specifications.

Secondly, the ’norm’ (Synchronome) is to have resistor approximately 10 times the resistance of the coil in parallel for spark suppression.  Possibly that was fitted in your case.  Do you know the coil resistance?

I believe (and someone will no doubt correct me if I am wrong) that it was common in later days to have an “mercury delay” for big dial movements - and this may have removed the worry over sparks?

On 26 Jul 2021, at 19:53, Simon Allen <simon.sallen@...> wrote:

I am seeking to restore an M4 turret movement.  Robert Miles in his book says that it did not have a resistor connected in parallel with the coil.  My one does.  Across the two terminals connected in parallel to the coil is a 250 ohm resistor.  What was its purpose?

Darren Conway
 

Hi

A resistor was used to suppress sparks across the contacts as they opened. Sparks erode the contacts.

If they had diodes available then, they would have used them.  That is what you should use instead of a resistor.


Regards

Darren Conway
New Zealand
On 27.07.21 6:53 am, Simon Allen wrote:
I am seeking to restore an M4 turret movement.  Robert Miles in his book says that it did not have a resistor connected in parallel with the coil.  My one does.  Across the two terminals connected in parallel to the coil is a 250 ohm resistor.  What was its purpose?

Virus-free. www.avast.com

John Hubert
 

As a further thought, is it possible that the large movements - that would be installed very often large existing dials might have the resistor fitted by the installer when completing the installation, rather than fitted to the movement prior to delivery?

On 26 Jul 2021, at 20:21, John Hubert via groups.io <jfphubert@...> wrote:

Two points here.  Firstly Robert (Bob) Miles book was compiled from the material he had available.  There are quite limited records for Synchronome.  It is possible he worked mainly from the hardware he had access to rather than detailed drawings and specifications.

Secondly, the ’norm’ (Synchronome) is to have resistor approximately 10 times the resistance of the coil in parallel for spark suppression.  Possibly that was fitted in your case.  Do you know the coil resistance?

I believe (and someone will no doubt correct me if I am wrong) that it was common in later days to have an “mercury delay” for big dial movements - and this may have removed the worry over sparks?

On 26 Jul 2021, at 19:53, Simon Allen <simon.sallen@...> wrote:

I am seeking to restore an M4 turret movement.  Robert Miles in his book says that it did not have a resistor connected in parallel with the coil.  My one does.  Across the two terminals connected in parallel to the coil is a 250 ohm resistor.  What was its purpose?


Peter Torry
 

When switching high current and or inductive loads it was usual to fit a mercury switch as then can cope with most "nasty" loads particularly when breaking high currents.  They were also fitted in order to stretch the impulse to  the turret movements to allow sufficient time for the slower movement to operate correctly.  If it is of help I can post an image of one - when I can find my camera !

Regards

Peter


On 26/07/2021 20:21, John Hubert wrote:
Two points here.  Firstly Robert (Bob) Miles book was compiled from the material he had available.  There are quite limited records for Synchronome.  It is possible he worked mainly from the hardware he had access to rather than detailed drawings and specifications.

Secondly, the ’norm’ (Synchronome) is to have resistor approximately 10 times the resistance of the coil in parallel for spark suppression.  Possibly that was fitted in your case.  Do you know the coil resistance?

I believe (and someone will no doubt correct me if I am wrong) that it was common in later days to have an “mercury delay” for big dial movements - and this may have removed the worry over sparks?

On 26 Jul 2021, at 19:53, Simon Allen <simon.sallen@...> wrote:

I am seeking to restore an M4 turret movement.  Robert Miles in his book says that it did not have a resistor connected in parallel with the coil.  My one does.  Across the two terminals connected in parallel to the coil is a 250 ohm resistor.  What was its purpose?

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