KB888: Installation of Digitrax Decoders in HO Scale Atlas RS1, RS3, RSD4/5

This article was last updated on April 18, 2012, 1:53 p.m. | Print Article | Leave Feedback

Many thanks to Bob Dattore for this application note!

This app note details using a DN142 decoder for the installation.  This decoder is no longer in production but the following decoders would work as well:  DN135D, DN163, DZ123, DZ125, DZ143.  Any decoder that is small enough to fit could be used.

The Problem:  Atlas RS1's (and RS3's & RSD4/5's) are hard to install Digitrax decoders in. There is no easy drop-in or light board replacement with back-emf capability.

I have installed several Atlas Dual Mode decoders in these popular hard to fit narrow, low hood diesels with ease, by replacing the original light board with the Atlas Dual Mode decoder. The results are always very satisfactory.

But, for a while, I wanted to try out scalable speed stabilization (back-emf).  Of course, the Atlas decoder does not support back-emf, nor do the so-called drop in Digitrax DH150A/K decoders.  The DH142 series is too big to mess with for these diesels. Besides, I wanted a light board of some kind, not just a tangle of wires and shrink tubing.

The original Atlas installed RS1 light boards are useless for modification for DCC conversion.  But, I discovered that the Atlas RS11 light board, Atlas sells it as part #850120, has a medium DCC socket and shorting plug. It's the exact same fit as the original RS1 light board. Atlas also uses this nice RS11 light board in their U-series diesels.

The Solution: Here's what I did to end up with a neat, easy installation that works like a charm:

1. Take the RS11 light board and de-solder the two factory installed diodes from it.

2. Solder a short piece of wire through each diode mounting hole, top to bottom, like a simple feed through, and cut the wires flush with the top and bottom surface of the light board.Be sure there is nothing protruding above the surface of the light board. Use Xuron rail nippers if you have them for a nice, smooth flush cut.

3. Remove the factory installed DCC jumper plug. Carefully clamp the light board in a small vise, component side down. Remove as much solder as possible from the bottom of the light board where the eight DCC sockets protrude.

4. Clamp the light board component side up. De-solder each of the eight DCC sockets from the top of the light board by heating a socket with the soldering iron and carefully wiggling it up and out of the light board with needle-nose pliers.

5. Place a DN142 decoder, with the wires pointed toward the holes where the DCC socket was, and center it on the light board where the diodes were removed from. Tightly tape the decoder to the board with two narrow strips of clear packaging tape or Kapton tape, one on each side of the motor lead connection tabs. Do not block the motor tabs with tape.

6.  Feed each decoder lead down through each appropriate hole in the light board where the old DCC socket was. The Digitrax decoder manual has a pin out and color code on Page 12 for which hole gets which decoder wire. Notice that there is a white arrow on the Atlas light board that points to Pin or Hole 1, orange. Hole 3 will remain unused.

7. You'll end up with two unused decoder wires, green and violet. Bend them out of the way for now.

8. From below the surface of the light board, cut each decoder wire so only one inch protrudes below its hole. Carefully strip the insulation off each of the 1" wires, from the bottom side of the board. Do not pull hard on the wires.

9. From the top of the light board, beginning with Hole 1, orange, position the wire so that a tiny bit of bare wire just protrudes above the surface of the hole in the light board and solder it in place.

10. Each soldered wire must act as a feed through from the top to the bottom surface of the light board. Cut the protruding bare wire off flush with the bottom surface of the light board.

11. Proceed to Pin 2, etc.

12. Finally, cut off the stripped ends of the green and violet decoder wires so they can't short circuit to anything. Coil them up and lay them flat on the surface of the light board. Tape them there for later use.

13.  Attach your lamps, truck pickup and motor wires with the black retainer clips and you're finished! And, you have a nice, neat installation you can be proud of, too.

14. Your diesel shell should snap nicely back in place. If there is any interference from the decoder, recheck your installation. There must be no wires, no bumpy pieces of tape, etc. protruding above or laying across the top surface of the decoder as it is taped to the top of the light board or the shell will not snap in place.

15. If you still have interference between the decoder and the inside of the shell, get your trusty Dremel tool out and, at the lowest speed, very lightly polish the inside of the shell where the decoder touches it, near the exhaust stack. That should solve the problem.

16. Now, if I could only figure out that FX lighting stuff . . . . .

Technical Note:  If using DH142, DN142, DN141K2, DN149K2 series decoder, when F1 is set up for FX, F2 must also be set up as FX (It can’t be used as standard on/off function.)  If you want to use a combination of FX & standard on/off functions, please use F1 as the standard on/off function and F2 as the FX function.

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