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Lou's Atlas HO Switcher Decoder Install

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  • Lou's Atlas HO Switcher Decoder Install

    Here's an updated application note for installing an N or HO gauge wired decoder into an Atlas 'S' series HO switcher.

    I prefer the load compensated Digitrax DN142 decoder because it has back emf and it easily fits up in the top of the cab. There's also a new decoder from NCE being sold exclusively by Tony's Train Exchange at which is designed for this switcher. It doesn't have back-emf load compensation.

    Special Supplies Needed:

    One small piece of .020" styrene sheet, approximately 2" x 2".

    Scotch clear package tape.

    #30 gauge insulated stranded wire (cutoff from decoder wires).

    1) Remove the Cab & Shell:

    Remove the side handrail end at the cab by gently pulling it out of its cab mating hole. Remove the rear grabrails by gently pulling them out of their frame mating holes. Release the cab side latches by carefully pulling the side's bottom out from the frame and sliding it upward about 1/16" and repeat for the other side. Slide the cab up and off the frame. Tilt the shell up at rear and remove from frame.

    2) Isolate the Motor from the Frame:

    Unsolder the four truck wires from the printed circuit board. Remove the single bolt in the top of the pc board. Remove the pc board. Lift the top metal weight up and off the motor. Turn the chassis upside down and remove the motor mounting bolt located in the bottom center of frame. Carefully turn the chassis upright while holding the motor in place. Note the position of the driveshafts and motor relative to the frame. Lift the motor out of its well. The driveshafts will also come out of the universal couplings. Note that the upper motor brush cap has a plastic insulator under it while the lower brush cap has a brass washer under it. You will remove the brass washer and install a new insulator under the lower brush cap to isolate it from the motor frame. You will solder a wire to it which will be run up the side of the motor and attach to the pc board according to the graphic.

    Using the purple upper brush cap plastic insulator as a pattern, cut a round hole in a piece of .020" styrene. Cut a square piece out of the styrene so the hole is in the center. Trim the styrene piece to the same size as the upper brush cap insulator. With the lower brush cap removed from the motor, solder a 6" length of #30 gauge insulated stranded wire to one flat on the edge of the brush cap. Do not solder the wire to the surface of the brush cap, only to the very edge. File off any excess solder. Install the new styrene insulator under the lower brush cap. Route the wire up the side of the motor for now. Be sure it is positioned so it can't be crushed between the lower brush cap and the frame. Place a very thin piece of insulating tape over the lower brush cap so it can't short circuit to the motor well in the frame bottom. Use good quality Scotch packaging tape. Don't use electrical tape because it's too thick and will prevent the motor from sitting down in the well. Reinstall the motor in the motor well while guiding the driveshafts back into their universal couplings. Check to be sure the 6" wire is routed up the side of the motor and is not crushed between the lower brush cap and the frame. Do not force anything. Turn the chassis over while holding the motor in place and reinstall the motor mounting bolt. Tighten it gently while holding the motor in place. Place the upper metal weight back into position on top of the motor while routing the 6" wire up through the space inside the cutout. The upper weight should touch the frame at its front and back. If it doesn't touch evenly, the motor is not fully seated in the well and you must correct the cause of the interference. Do not remove the factory installed insulating tape on the underside of the upper weight. Be sure the 6" wire is not crushed between the frame.

    3) Modify the PC Board:

    View the graphic diagram showing the Atlas pc board. Using the diagram as a guide, cut the traces of the pc board so the pc board looks like the diagram. This will create solder pads for attaching wires from a decoder and for lights. Carefully cut through the metal traces with an Xacto knife but not through the fiberglass pc board. Remove the metal trace material. Do NOT cut all the way through the pc board. The cuts must completely break the electrical circuit at each point indicated. Test the cuts with a continuity tester or ohmmeter to be certain there is no electrical path across any of the cuts. Route the 6" wire you soldered to the lower motor brush cap up alongside the motor and inside the top metal weight. Trim it to length and solder it to the pc board as indicated in the graphic. Use a small strip of packaging tape to hold the wire in place alongside the motor frame. Reinstall the single bolt through the pc board to mount it to the top metal weight.

    4) Install the DN142 Decoder

    Trim the decoder wires to length leaving just enough slack to locate the decoder up inside the cab. Solder each wire to the pc board at the locations indicated. The shell won't re-seat on the frame if the wires are too long or bunched up on the pc board. Note there are unused green and purple function leads on the DN142 decoder. Cut off the excess wire length but leave them long enough to use for future added features. Fix all wires in place with thin strips of packaging tape. Test the loco on a current limited programming track for proper function and correct any wiring errors or short circuits. Test with a continuity tester or ohmmeter to be sure there is no continuity between either the orange or gray decoder wires and any part of the loco frame.

    5) Finishing the Installation:

    Reinstall the shell by inserting the front tab in the frame notch and rotating shell down onto frame. Check to be sure it seats fully at the rear. Correct any interference. Place a small tab of double sided mounting tape up inside the roof of the cab. Position the decoder up inside the cab and press into place. Slide the cab back down onto the shell and frame until both side latches snap into frame notches. Be sure no wires are caught between the cab and shell. Reinstall the handrail ends.

    LED Installation:

    You may wish to replace the original headlamp with a white LED and also install a rear white LED in the cab. Use 3mm white LED's such as Miniatronics #12-300-02, and a 1K ohm. 1/4 watt resistor in series with each LED. Observe LED polarity according to the graphic. The blue decoder lead (positive) should go to the anode of each LED. The white (front) and yellow (rear) function leads (negative) should go to the cathodes of the LED's through the 1K ohm resistors. Tape the rear LED and resistor up inside the cab roof before installing the decoder.

    Edited by - nyc on 05/31/2002 16:27:12