I needed to understand the internal working of my turntable so that I could optimize its digital operation. I have documented what I found below.
The Märklin turntable (7286) is a rebadged Fleischmann product and adapted for 3-rail running. It is a very nice prototypical looking turntable and has 48 possible track positions at 7.5° intervals.
It can be controlled manually on an analog layout and digital control is also possible, but not out of the box. (More on that later.)
Both ends of the turntable bridge have contacts that make connections with tracks around the edge. This allows transfer of power from the turntable to spoke tracks, ideal for analog layouts, but it can cause a problem with digital layouts due to contacts shorting to the always powered tracks as it turns.
To remove the bridge, unscrew the screw between the rails in the middle. Lift up the screw and a contact clip and the two metal plates that make up the center rail.
Two edge slots, three slots apart, have to be removed from around the perimeter in order to lift the small end of the bridge out.
The bridge sits atop a set of six concentric contact rings. Below the turntable they are connected via a ribbon cable to a small interface board. Some of the connections are bridged with capacitors on the underside.
The contacts are as follows:
On the underside of the bridge is a motor mechanism. The entire mechanism can slide a short distance along the length of the bridge. This allows a driving gear at the outer end of the motor mechanism to engage with, or be disengaged from, the teeth around the side of the turntable pit. A small hand lever can be pushed towards the center of the turntable to disengage the mechanism, allowing the turntable to be freely repositioned. A small spring reengages the motor mechanism when released.
The motor mechanism has a relay which is connected in such a way that when the relay is energized, it will close a switch in the motor circuit. In addition, the relay is mechanically connected to the main drive gear so that the switch is held closed except at 4 positions of the main drive gear, 90° apart. Thus, the motor circuit switch is closed and operable, if either the relay is energized or the drive gear is not at one of its 90° positions. 90° of rotation of the drive gear corresponds to 7.5° of rotation of the turntable bridge. There is also a third way the motor circuit switch can be closed, the hand lever that disengages the motor mechanism can also be pushed towards the outer edge of the turntable.
Rotation of the turntable bridge is achieved by energizing the relay and applying a voltage to the motor. To stop at the next turntable position, the coil is de-energized. The bridge continues to turn while the mechanical latch holds the switch closed. When the drive gear reaches one of its 90° positions, the latch is pushed into a notch on it by a spring, which opens the switch in the motor circuit, stopping the turntable. It is locked in place until the coil is energized and voltage applied to the motor.
To stop at a specific track beyond the track adjacent to the starting track, the coil must remain energized until it is within the last 90° of rotation of the drive gear. There is no feedback mechanism to know the speed of the motor so control systems using this setup have to rely on a time delay before de-energizing the relay.
Negative voltage on coil (4) will engage the relay if either motor line (2/3) is positive.
If the motor switch is closed, a positive voltage on 2 (and negative on 3) will drive the turntable clockwise and a positive voltage on 3 (and negative on 2) will drive the turntable counterclockwise.
The wiring schematic in my turntable is as follows:
Note that there are numerous versions of the turntable sold through various companies, and the wire colors and connection labels vary. The principle of operation is however, the same.
Inputs to control the turntable
Electrical input is direct current (12 - 16 Volts DC). If you want to use readily available 16VAC you should convert it to DC using a bridge rectifier (there is one you can use in the manual controller that comes with the turntable).
Controlling the turntable using a K84 type module.
One can use 3 addresses (A, B & C below) of a k84 module to control the turntable. Here is an example:
Set port 1/contact 4 to negative (K84 C = Green) to stop at the next position
Multiple position movements
In order to move a specific number of positions (for example 24 positions to move 180°) automatically you will need to time how long your turntable takes to move one position, and multiply that by the number of positions you want to remove, and subtract one half period, to determine when to set port 3/contact 4 to negative. For example, say it takes 2s per position, to move 5 positions you wait for 2s x 5 - 2s/2 = 9s.
Note that turntable speed can vary with temperature and even direction of movement.
Order of k84 commands
Other control methods
A number of third party companies have produced control systems. The most sophisticated I have read about is from the DSD2010 Digital-Bahn but you do have to modify the mechanism extensively. It also provides lighting for the control room and outputs for signals, and an option for a sound module.
Märklin used to sell a digital controller (item 7687) but early versions were very unreliable, and a number of people have opted for the k84 approach instead.
To get reliable turning to any specific track position, there has to be feedback from the turntable as relying on timings is fraught with sources of error. One approach I am considering is placing 48 reed contacts, connected to a single sensor, in a ring on the underside of the turntable and mounting a single magnet under the bridge. As the bridge moves, successive reed contacts will be closed and software can count the slots traversed and know how far the bridge has turned.
In my layout control software I can define turntables as follows:
And then it looks like this when running:
I can optionally display radio buttons to select any track directly, or even any slot that is 180° away from any track.
If the turntable is not motorized, the software makes a voice prompt to move the turntable to the required position.