Special note: The current version of the Atlas U25B locomotive is DCC-ready, and will accept either the Atlas DA120, Digitrax DN146A or DN163A0 Plug 'n Play decoders. Do not follow the procedures below for current version locomotives. Follow the instructions provided with the above decoders.

The following detailed description covers installation of a Digitrax DN93 decoder in an Atlas original production (Kato) GE U25B diesel, as actually performed by the author of this publication, and reflects the experiences encountered during the installation.

The Digitrax DN93 decoder was chosen because it was the most advanced decoder available at the time of installation. Other decoders that could be used are Digitrax DN93FX, DN140, and DZ120 as well as N scale decoders from Lenz, NCE, TCI, etc.

The most important factor to remember in performing an analog to digital conversion is to ensure both motor brushes and the decoder orange and gray wires are insulated from the frame. Any contact of the brushes and/or these wires with the frame will result in virtually instant destruction of the decoder.

The first step in each of the descriptions which follow is to test the decoder for proper operation, following the instructions provided by the manufacturer. The purpose of this step is to ensure any non-operational or dead-on-arrival decoder can be repaired by the manufacturer under warranty. While all decoders carry a warranty for some period of time, usually one year, that warranty expires the minute you cut the wires, which, of course, you must do to install the decoder in a locomotive.

As you carry out the detailed instructions below, also refer to the assembly diagram and parts list provided by Atlas with the locomotive. It will help with laying out the parts and understanding the terminology used.

This installation makes use of the Aztec Manufacturing Company TM3004 TrackMaster™ factory frame precision milled to accept Digitrax decoders, in this case the DN93. A N Scale of Nevada Retroframe (RRR-U25) and Southern Digital AU25B Digi-Frame could also be used for this locomotive.

To install the decoder and modify the frame you will need the following tools:

If you do not wish to purchase one of the Retro-frames, above, you can modify your own existing frame to accept a DCC decoder. If you have the tools, time and inclination, this approach can save the extra cost of purchasing another frame in what is already an expensive process.

The following process applies to the Atlas frame. Refer to the detailed installation instructions for the locomotive in the next section, and follow those instructions to the point where you have the frame disassembled. Then proceed as follows:

• Remove the insulating bushings and place in a safe place.

• Assemble the bare frame halves together without the insulating bushings. Align the halves before the final tightening of the screws.

• Using the DCC decoder as a guide, mark the rear portion (assumes the cab and short hood are the front of the locomotive) of the frame to indicate the metal to be removed to accommodate the decoder.

• Hold the frame in a vise, and use a hacksaw and/or motor tool to cut out the marked portion. The cut does not need to be super-accurate because it will be dressed up with a file. Remove all sharp edges with a file.

• Use a Dremel #106 1/16" diameter rotary engraving cutter or equivalent in a motor tool to grind the necessary channels for the decoder wires (remember to wear eye protection). The channels need only be as deep as the diameter of the wire. Keep all other parts away from the metal grindings. Smooth the sharp edges with fine sandpaper to ensure the insulation on the wires cannot be pierced.

• Use 600 grit wet and dry sandpaper to polish the entire frame, especially in the areas where metal was cut.

• Clean all of the grindings from the frame.

• Disassemble the frame, placing the screws and nuts in a safe place.

You can now install the decoder per the detailed instructions which follow, just as if you had purchased a new frame.

Set your browser's left and right margins to 0.5" (File Page Setup) and print out this document. As each step in the installation is completed place a "X" or a check-mark through the bullet. All references to the frame are based on the front being at the top or away from you.

In normal analog (DC) operation, the right half of the frame connects to the right rail and the bottom motor brush, and the left half of the frame connects to the left rail and the top motor brush.

• Begin by testing the Digitrax DZ120 decoder for proper operation per the instructions provided by Digitrax.

• Cut the decoder wires as follows, then strip 1/8" insulation from the ends:

  Red	1-9/16"
  Black	1-1/2"
  Orange	1-7/8"
  Gray	1-3/8"
  White	1-5/8"
  Yellow	1-3/8"
  Green	Note

  Note: The green wire is not used. Cut the green wire to ½"; do not strip any insulation from this wire.

• Remove the U25B shell, by prying out on the bottom of the cab carefully with a hobby knife (both sides) until the cab is freed, being very careful to pull the handrails out from their holes in the cab. Then remove the hood casting and, finally, the handrails and walkways. Remove the fuel tank from the frame. Place aside where they will not get damaged. They are not used again until the final step.

• Remove the shroud around the front lamp by sliding it towards the front of the frame.

• Remove both the front and rear lamp boards by pulling the front board towards the front and the rear board towards the rear of the frame.

• Place the mechanism on its side, screwhead side up, and remove the screws. Once the screws are completely loose, hold the frame up and press down on the screws so the nut on the other side will pop out. Using a flat blade screwdriver, gently pry the two halves of the OEM frame apart, carefully lining up parts on your work surface in the same order as they are removed and noting what each piece is by looking at the diagram Atlas provided with the locomotive.
  
  If the two halves of the frame do not come apart easily then locate the four holes around the motor in the center. This is where the motor saddle locks the two halves together. Gently lift and push the tabs to release the saddle clips.

• Before removing the motor, make note of which motor brush contacts the right side of the frame, and place a mark on the motor (piece of tape, dab of paint, etc.) to indicate the top. Note that some Kato motors are already marked with a white stripe, either on top or on one side near the top.
  
  If you are modifying the Atlas frame yourself, do so now following the instructions in the section above.

• Before installing the decoder, re-assemble the mechanism using the Track Master frame in place of the OEM frame. Install the trucks, etc. - everything except the body parts. Put the assembly on a test track and make sure the locomotive runs as it should, checking both directions, etc. When operating correctly, disassemble it again as described above.

• Pry the brush holders up from the motor housing and remove the contact strips. Replace the brush holders.

• Solder the orange wire to the bottom brush retaining cap on the motor, and the gray wire to the top retaining cap. The soldering of the wires to the brush holders must be compact to ensure enough spacing to be free of both frame halves.
  
  Note: always remove the motor brush caps before soldering to them. Then lightly sand them to get a good clean surface, and apply liquid flux. Finally, solder the wires to the brush cap.
  
  Do not attempt to solder to the motor brush caps while they are still in place in the motor, as too much heat applied to the brush caps during such soldering may melt the plastic brush housing and permanently damage your motor.
  
  After soldering the wires carefully replace the brush, spring and brush cap into the motor.

• Carefully place insulating tape over the brush retaining caps to ensure they cannot short against the frame.

• Lay the light board out in front of you, parts side down as shown below. With a sharp hobby knife, make a cut in the left hand circuit trace as shown, so that the circuit is not complete to the left half of the frame.

  

• Solder the Red and Black wires from the decoder to the lamp circuit board, as shown below; this is the prime power input to the decoder from the track.

  

• Solder the white wire to the front light board as shown below.

  

• There is not sufficient clearance over the decoder to use the rear headlight assembly. The light board must be removed.
  
  Carefully unsolder the LED and resistor from the circuit board, and solder the resistor directly to the same wire from the LED that it was soldered to on the circuit board; this is important for polarity reasons.

• Solder a wire from the resistor to the black wire on the forward light board (the same spot as the black wire from the decoder). Solder the yellow wire from the decoder to the remaining terminal on the LED. The final wiring diagram is shown above.

• The decoder and motor are now ready to be installed into the Track Master frame. With the left mainframe laid out in front of you (short hood to your right), install the motor with the orange wire on the bottom and gray wire on the top. Route the wires to the top of the frame in the wiring channel provided in the left mainframe. Inspect the brush holders and wires to be sure they are not touching the frame.

• Install the front and rear worm assemblies.

• Install the front and rear insulating bushings in the frame.

• Position the green motor spacers so the nubs line vertically against the frame, i.e. top and bottom of the frame. Ensure these are placed in the little channels on the frame. This step is very important to smooth mechanical operation of the mechanism.

• Place the right mainframe on to the left mainframe, being careful that the orange and gray wires remain in the wiring channel. Install the front and rear nuts and screws, but do not tighten.

• Install the truck assemblies, and tighten the screws.

• Mount the front headlight board to the frame in the normal manner.

• Snap the front lamp shroud back in place.

• Using an ohmmeter, check that the frames are properly insulated from each other, and are not shorted.

• Test the locomotive on the layout for both analog and digital operation of the mechanism to ensure there is nothing binding and electrical connections are correct. Resolve any problems and ensure the mechanism runs perfectly before continuing.
  
  Note: if the locomotive runs in the wrong direction, the motor may be mounted in the locomotive upside down. Remove the brush retaining caps and turn the motor over. Replace the brush retaining caps and test again.

• Position the decoder and rear light in place over the frame, and, using electrical tape, tape in place. Dress all wires over the top of the frame so that they will not interfere with the body as it is placed back on the frame. Adjust and secure with tape as necessary.

• Replace the fuel tank on the frame. Place the handrails and walkways on the frame, then replace the hood. Check to ensure all wires are positioned out of the way, and that they do not cause the hood to bow out anywhere. Finally replace the cab, and put the handrails in their proper holes in the cab. Installation is complete.

• Perform a final test on the railroad to ensure proper operation. It should operate just as if no modifications had been made. Resolve any problems.

• Place the locomotive on the DCC programming track and set the DCC Command Station to the programming mode.

• Program Configuration Variable "CV29" to "06" (for 2-digit addressing or "26" for a decoder with 4-digit addressing) then program the deocder to the desired address.

• Carry out a final check of the locomotive on the railroad.

• Record the decoder CV's and address, and the reporting marks of the locomotive.

The conversion is complete. Enjoy your DCC-equipped locomotive.