Atlas S2 Decoder Installation

Photos and text by Scott Russell (CR and S Trains) of Sandy Hook, CT

Atlas S2 (Roco Drive)


The older Roco drive in the Atlas Alco switchers had a headlight only. By removing the board, an M1 decoder can fit in the notch over the motor and not have to go in the cab. You can also add Golden White LED’s for the headlight and backup light in the cab.

If possible, carefully remove the handrails from the model. This may take a while, but you’re less likely to break them while installing the decoder. At least remove the ones from the steps to the back of the cab.

Remove the body by carefully prying off the cab from the bottom then tilt the body forward to disengage the front latch.

Remove the PC board (Fig 1) by removing the screw and unsoldering the wires.



  1. Remove the weight and put Kapton or electrical tape on the four places shown (Fig 2) that may contact the chassis.


  1. Turn the chassis over and put tape on the bottom of the motor to insulate it from the chassis. Replace the motor screw with a Kadee Delrin screw or other non-conductive 2-56 screw (Fig 3). The hole in the motor is metric, but the 2-56 screw can be forced in fairly easily.

  1. Replace the weight and solder the gray decoder wire to the motor brush. Do this quickly to not melt the plastic brush retainer. Solder the red wires to the right side pick-ups and the black to the left. The decoder wires are long enough that if you strip some insulation from near the middle, you can solder it to the front wire and fold it back and solder to the rear wire. Make sure you put the shrink tubing on the correct ends of the wires before you solder them. Cut the orange wire to reach the mounting screw, tin it and bend a loop to go under the screw head. Reattach the weight. (Fig 4)


  1. The decoder will fit in the notch with the flatter side of the decoder facing up. The large component fits next to the motor brush (facing down). Hold the decoder and wires in place with tape. (Fig 5)

  1. Cut a Golden White LED to fit using the original PC board to see how far the LED should stick out to almost touch the light pipe to the headlight. Solder the blue wire to the positive lead (the smaller part of the LED lead frame that you can see is the positive lead, so you can tell after you’ve cut the leads to fit) and run an extra blue wire to the rear of the engine with the yellow wire.. Solder a 1K ohm resistor to the other lead and then to the white wire. Shrink tube all the leads. I use DAP BlueStik (also called Fun Tak) or Duco StikTak, etc. You can get it in a craft store in several colors. It sticks to almost anything, but it’s removable and reusable. It’s great for sticking down decoders and wires. (Fig 6)

  1. Solder leads and a resistor to another LED and stick it in the cab with BlueStik, lining it up with the backup light lens (Fig 7). Run the leads down and hook them to the yellow and blue wires, observing the correct polarity. By using an LED here, you won’t melt the cab roof with a hot bulb. The cab looks backward in the photo because I flipped it up to forward after I connected the wires.


  1. Put the hood back on, tuck any extra wires under the hood and snap the cab back on.


  1. Program the decoder and you’re ready to go. For a switcher, I program “Rule 17” (CV 49=8 and CV 50=24); “Dim Stopped/Opposite Dim” (CV 61=48) and “Dimmed Brightness’ (CV 64=6 for an LED). For this particular model, it ran best with CV 2=25 (start volts) and CV 56 =1 (dither frequency) and CV 57=10 (dither voltage). You might have to play with these values to get it to run to your satisfaction.