Atlas SD7/9 N Scale
MC2 decoder installation

Photos and text by Jeffery Maurer. Installing a Train Control Systems MC2 in the Atlas SD-7/9 (non-DCC ready).


Remove body shell and handrails by gently pulling up on the shell.  Remove fuel tank.  Remove front and rear light assemblies.


Remove the two Phillips screws that hold the two halves of the frame together. Mark the top of the motor and then remove the motor from the frame.


Remove the plastic holder from the motor and cut off the two metal tabs used to provide power to the motor. Note the white dot added to identify the top of the motor.


Gently pry off the motor brush covers. NOTE: BE VERY CAREFUL or you will loose the small brush springs. The brushes (small copper cylinders in the photo) do not need to be removed but be careful that they don’t fall out.


Cut the orange and grey wires to length and solder to the brush covers. DO NOT try to do this without removing the brush covers first. Doing this can melt the plastic around the brush holders. Reinstall the brushes, brush springs and brush covers to the motor, orange on the bottom, grey on the top. Carefully route the orange wire as in the picture. Make sure the orange wire doesn’t touch the flywheel.


Using either a Dremel, a file, or a pair of diagonal cutters, remove the two frame tabs (shown in red in the photos).  Reassemble motor, frame, and trucks routing the orange and grey wires up between the two frame halves.

Track power can either come from the rear light board or (as I have done) off of the brass wipers. Red to right, black to left. If you removed the wipers in disassembly, make sure that they go under the metal tab in the picture.


Unsolder the resistor and the left leg of the LED. Solder the resistor to the left leg of the LED. On the other (rear) light board, cut the trace between the frame and the resistor. Solder the white wire to the other side of the cut trace. From the other side of the cut trace, solder a wire to the resistor side of the modified front LED and cover with heat shrink.


If power is going to be taken from the light board instead of the pick ups, solder the red and black wires to the indicated spots. Solder the yellow wire to the other side of the front LED and cover with heat shrink. Plug in decoder and attach to frame using double sided tape.  Reinstall shell and test. Since this is a Southern Pacific SD-7 without a steam generator, the extra fuel (water) tank was also removed while the chassis was disassembled.  

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