Upgrading the Märklin 7286 turntable

My Märklin 7286 turntable was not moving so it was long overdue for a service. While I did that, I also decided to do some upgrades to make it work better than before.

I describe here how I carried out the following upgrades:

• Improved rotation
• Improved electrical contacts
• Added lighting to the control shed
• Added control desk direction indicators
• Added an operator in the control shed

Improved rotation

The turntable was designed to work with analog layouts. Track power is provided to the running rails and the center rail from below, and there are contacts at both ends of the bridge to transfer this power to the current spoke track. This allows one to only power the spoke track one is using, allowing locomotives to be left on other tracks unpowered. On a digital layout, all the tracks can be powered all the time, making all these contacts superfluous. Not only do these contacts add resistance to the rotational movement, they also make clicking noises and they even cause short circuits between the center rail contact  and the running rails of the spokes. I had previous defeated the connectors around the edge of the turntable by jamming them in. I decided it was time to take it a step further..

The center rail on the turntable has two metal plates that lie between the running rails. I cut the ends off both with hacksaw, filed them smooth and painted the ends black.

On the underside of the bridge, at the end of each rail was a small contact. I cut all four off with some small side cutters:

This results in a much smoother and quieter rotation.

Electrical contacts

The turntable bridge pivots about on a central support ("Königsstuhl") which has some concentric contacts. These contacts needed cleaning so I rubbed them with a very fine sandpaper and also cleaned them with some contact spray applied with a cloth.

I also cleaned the finger contacts on the underside of the bridge that ride along on the contacts:

Despite good contact on the supporting side, I could not get one of the running rail contacts to work, despite bending the finger downwards to apply more pressure, so I gave up and soldered a wire between the two rails.

Control shed

It always annoyed me that there was no light in the control room, so it was time to fix that. The shed unclips from the bridge providing good access to the interior. The glass windows are made from a single translucent piece that has a small dimple in the ceiling, a perfect size for adding a surface mount LED.

First I looked closely at the LED with a magnifier to determine which wire was connected to the end with a small dot. This is the side that has to be connected to negative. I tied a small knot in the other end of that wire.

I taped the LED into the top and ran the fine wires to the the corner of the building .

I then inserted the glass into the shed, keeping the wires between the glass and the outer frame at the corner so they cannot be seen.

There is enough room in the hole that the building clips into, to pass the two wires through to the underside of the bridge without having to drill a hole.

I decided to power the light using the power from the track as I did not want it only going on when the turntable was moving. Since the track contains the digital signal, it is AC and thus needs a diode to only allow the current to flow in one direction, plus the usual protection resistor.

The rails provide one side of the power source for the light, so I needed to get power from the center rail. I soldered a wire onto the small contact that conducts the center rail current to the parts between the tracks:

I drilled a small hole next to where the contact sits:

and threaded the wire though and soldered it to a 1N4001 diode.

The other wire was soldered to a 2K Ohm resistor soldered to one of the running rail contacts.

A quick test showed the light worked well.

Control desk direction indicators

I thought it would be nice to put red and green LEDs into the control desk so that when the bridge moved one way, a green light would go on, and the red LED would indicate the other direction.

I drilled a very small hole in the middle of the control desk.

I then connected the two LEDs in antiparallel. (I did not have a green SMD LED so I used a red SMD and a much bigger green one)

This illuminates the red LED when the current flows in one direction, and the green LED when the polarity is reversed. These were then inserted into the hole on the underside and one side was soldered onto a 2K Ohm resistor.

The pair of LEDs then had to be connected to the motor terminals, so that red will indicate one direction and green the other. One motor terminal is easy to connect to, but the other required a contact to be temporarily removed to solder a wire on.

It works:

however, the control room light is too bright for them to show up well. I need to increase the size of the resistor on the room light.

Operator in the control shed

I found a person that looked suitable and glued him in front of the control desk.