Recently I bought a Citizen 7RD009 that was not running properly. It starts up running normally but after a few seconds it produces an intermittent ticking and the outgoing gear slowed down to a near standstill.
From this excellent site I learned that this type of motor is a “Transistor Switched Balance Wheel Impulse Motor” and I also learned to understand the basic working.
In a nutshell: the motor has a balance wheel with an attached magnet. This magnet passes by two concentric coils that are attached to a small circuit. The “sense” coil detects the passing magnet and through use of a transistor causes the “drive” coil to be powered. The field of this coil pushes away the magnet and thus gives a push to the balance wheel. This happens each time the magnet passes the coil and that keeps the balance wheel swinging. The balance wheel then drives the gears through an “inverted Sully escapement” (that I learned here).
To find out what was wrong with my motor I made a short slow motion video. This showed that the motor exhibit what is called “overswing”. The balance wheel turns about 360 degrees in each direction and causing the escapement to bump into the gear again and push it back. As a result the little gear jitters only back and forth instead of progressing in one direction.
This put me in the right direction for a fix. As it seemed unlikely that this behaviour was caused by anything mechanically, I thought it must be an electronic problem. I traced down the circuit as shown below.
It has only few components. The most likely to be faulty are the two capacitors so I replaced these and that fixed the issue!
The swing of balance wheel was reduced to about 180 degrees in each direction and the gear moves steadily in one direction, as shown in this video:
From this excellent site I learned that this type of motor is a “Transistor Switched Balance Wheel Impulse Motor” and I also learned to understand the basic working.
In a nutshell: the motor has a balance wheel with an attached magnet. This magnet passes by two concentric coils that are attached to a small circuit. The “sense” coil detects the passing magnet and through use of a transistor causes the “drive” coil to be powered. The field of this coil pushes away the magnet and thus gives a push to the balance wheel. This happens each time the magnet passes the coil and that keeps the balance wheel swinging. The balance wheel then drives the gears through an “inverted Sully escapement” (that I learned here).
To find out what was wrong with my motor I made a short slow motion video. This showed that the motor exhibit what is called “overswing”. The balance wheel turns about 360 degrees in each direction and causing the escapement to bump into the gear again and push it back. As a result the little gear jitters only back and forth instead of progressing in one direction.
This put me in the right direction for a fix. As it seemed unlikely that this behaviour was caused by anything mechanically, I thought it must be an electronic problem. I traced down the circuit as shown below.
It has only few components. The most likely to be faulty are the two capacitors so I replaced these and that fixed the issue!
The swing of balance wheel was reduced to about 180 degrees in each direction and the gear moves steadily in one direction, as shown in this video:
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