Tap Creek

The Chicago & North Western Tap Creek Branch


Step 1 - Disassembly

Disassemble the RDC as described on this Kato web page:


When you are nearly done the frame will look as below, when seen from the above. The front end is to the right.

This reveals a small “pit” in the frame, which is where we will fit the decoder.

To finalize the disassembly do also remove the motor and the black plastic pieces at the each end of the frame, as shown in the picture below.

Continue by removing the remaining plastic piece in front of the m otor.

Step 2 - Modifying the frame

The next step is the tedious part. You need to mill the frame in order to create enough space for the decoder and the wiring. The area enclosed by the red lines in the picture below needs to be removed. You also need to create “channels” for the wiring, as indicated by the blue lines. The channel at the top of the picture (on the left hand side of the frame) needs to be wide enough for 2 wires, while the bottom channel needs to hold only one wire. Actually, the part of the channels that go between the motor and the decoder “pit” will have to hold one more wire each. The gap in the frame, created at the very front (to the right in the picture), shall be large enough for 7 wires.

I used a Dremel motor tool to do this job. I used both a cutting disc and a milling bit. My job did not turn out very neat, but in the end it will not be visible, so who cares? The finished work is shown in the picture below.

You have to frequently check your progress by test fitting the decoder and the wires. You might also want to cut away some of the decoder’s plastic wrapping to make it a little shorter. There is some “excess” wrapping at one end (the one where the wires do not emerge) that can be removed.

Step 3 - Decoder placement

The decoder will sit right above the front drive shaft. To prevent it from interfering with the motor axle and the drive shaft I inserted two pieces of .080” (2 mm) square styrene rod at the rear end of the “decoder pit”. I used CA to glue one piece at each side of the pit. The rear end of the decoder will rest on these styrene pieces, keeping it free from the motor axle and the drive shaft.

The picture below shows the frame with the motor reinstalled, and with the styrene pieces visible in the corners of the decoder pit.

Step 4 - Electrical pickup

I did not want to solder any decoder wires to the electrical pick up strips on the underside of the interior detail piece, since that would make assembly, and any future disassembly, extremely difficult. Instead I wanted a way to make the pickup strips to automatically make contact with the red and black decoder wires when the interior detail piece was snapped in place.

The idea was to solder the red and black decoder wires to bronze contact strips, and attach these strips to the frame in such a way that they would make electrical contact with the strips on the interior detail piece. Of course, these contact strips could not be directly attached to the frame, since that would create a short. Instead I sat out to construct two identical “holders” or “pockets” from small pieces of styrene.


Each “holder” is made from 3 pieces of .010” x .125”  (0.3mm x 3.2mm) styrene strips. The strips must be as long as the decoder pit is deep. To start, make them too long and cut them to length later. The pieces are glued together as shown in the picture to the right. 

The “channel” created in the middle has the same width as the contact strips that came loose when the interior detail piece was separated from the frame. A fourth piece of styrene is glued at one end of the channel. It serves as a stop, preventing a contact strip lying in the channel to go to far down the channel (where it would eventually make contact with the frame and create a short).



A piece of .080” (2 mm) square styrene rod is then glues across the channel, serving as a lid and giving the “holder” a bit of rigidity.







Finally the “holder” is trimmed and cut to size so that it will fit, channel opening upwards, at the inside of the decoder pit. The trimmed holder is shown to the right (sorry for the blurry picture.)




To make the contact strips I reused parts of the original LED contact strips. Cut two of the strips as shown the picture. We will use the short pieces.



Now bend the short pieces to the form shown to the right.


Glue the styrene holders to the inside of the decoder pit. Using a sharp knife trim them to length (they should be level with the frame). Insert the contact strips, making sure there is no way they can make contact with the frame. The “bend” should be facing inward, towards the centre line of the frame.


The description continues on the next page...

DZ143 Decoder in RDC

When I did this installation I owned two RDC:s, one RDC1 and one RDC2. In both cases I wanted the decoder to be hidden and not visible through the windows. In the case of the RDC2 this could be accomplished by putting the decoder in the baggage compartment. For the RDC1 I needed to find a way to fit the decoder somewhere in the frame, under the interior detail piece. For both cars I also wanted the wiring to go somewhere out of view. Since the RDC1 was trickier to do, I have chosen to show that installation here. Once you understand how I did that one, you will easily find a way to install a decoder into any other type RDC.

One thing was clear from the very start - an “as small as possible” decoder would be needed. Because I also wanted to install a Kato interior lighting kit I needed a total of 3 function outputs

· Front head light and the associated red rear light

· Rear head light and the associated red front light

· Interior lighting


If you want to be able to control the white and red lights separately, which is nice if you alternate between running your RDC:s as single units and running them as a consist, you would need a total of 5 function outputs. Trading that flexibility for decoder size, I decided to use the Digitrax DZ143 which has 4 function outputs.

Decoder Installation (DCC)
Digitrax DZ143 in an N scale Kato RDC