Märklin Digital Crane 7651+7054

Some 14 years ago my wife bought me the Märklin 7651 digital crane. It is the classic rotary crane with a special decoder in the base to allow digital operation.

Preliminary tests indicated a nasty design flaw, the decoder allowed the crane to slew or raise/lower the hook but one cannot do both operations at once! I suspect that since there are two motors, one for each function, they designed it so that only one motor could be active at any one time - in order not to overload the decoder. Gebr. Märklin,  here is a solution:, use a decoder than can handle the load!

My first strategy to get around the silly limitation was to alternate the two motors using software but it tented to be a bit jerky so this last weekend I decided to go ahead and perform some surgery - I installed a second decoder to drive one motor and left their decoder in place to drive the other motor and switch the cab light on and off. Note that using the technique here, one can use two locomotive decoders to digitize an analog crane.

I used an old Delta decoder since I did not need any special functions, just some simple motor control.

I had also purchased the below baseboard mounting kit (7054) that allows the ugly motor assembly to be mounted below the layout surface so I decided to add the decoder, perform a major service and mount the motors below the layout all in one operation...

First I dismantled and lubricated all the gears and special armatures...

Then I added the Delta decoder. I de-soldered the leads that went to the hook motor and connected them to the new decoder. I soldered the ground and 'red' wire to the corresponding solder pads on the built in decoder.

This is how the 'Unterflur-Zurüstsatz' adds spacers for the below board mounting...

Note however, that when mounting it under the board you have to build downwards, adding the spaces, motor housing and eventually the digital base in that order...

Here it is mounted below the board. Note how the rubber joints in the spacers compensate for holes that are drilled not quite vertically.

I suspect that trying to service the motors in place is going to be next to impossible! The gray wire is the lead that takes power to the cab light. Having a plug in line allows the base to be removed for servicing.

Here you can see how clean it looks with the below baseboard mounting. When the landscaping is done there will be black coal dust everywhere, weeds etc. as befitting a coaling yard

Now all I had to do was write the software to control the crane using the two addresses.. this proved to be pretty simple and I came up with an interface that allows easy keyboard and mouse interaction to make the crane move left or right and up or down and switch the light on and off etc...

The crosshairs indicate no movement of the crane, clicking anywhere above the middle horizontal line makes the hook go up and below the line makes it go down,  left of the center line make it slew left etc.  The yellow block indicates the current setting so the screen shot above indicates that the crane is slewing to the right and the hook is also moving upwards. In addition to clicking with the mouse, the cursor keys can be used to move the yellow spot about and hence control the crane. Pressing 'S' or ESC or clicking on the Stop button moves the yellow spot back to the middle and all movement stops. 'L' or clicking on the 'Light and Magnet' area will switch the crane cab light on and off.

Since the crane decoder is of the old Motorola type, without absolute direction control, I also have a simple mechanism to toggle the decoder so that it is in synch with the user interface.

All in all a very satisfying project!