N SCALE DECODER INSTALLATION GUIDE

KATO SD40 LOCOMOTIVE

DIGITRAX DZ143 DECODER - FRAME MODIFICATION REQUIRED


INTRODUCTION

This installation was the first I've done since the new series decoders were released by Digitrax in 2002, and I used a DZ143

DISASSEMBLY

The complete shell lifts off the frame easily with a little jiggling of the long hood. Remove the shell, fuel tank, headlight hood and side wipers, and then undo the two screws that hold the frame halves together. With the screws removed, the trucks can be removed easily by just pulling them out. Also remove the light boards.

With the frame lying on it's right side, prize the two halves apart carefully and lift off the left half. Mark the top of the motor with a permanent marker, to make sure you'll know which side is which when you get to soldering wires to it, and to reassembly. Then remove the motor and the drive trains, as well as the bearing retainers on the left frame half. Finally, remove the motor from the motor saddle.

FRAME MODIFICATION

Like I did with the Atlas GP30 and GP40 and the Kato U30C, I put the decoder below the rear light board, and I milled out the rear end of the frame to make room. Refer to the picture below, where the areas to be milled out are marked out on the frame halves.

I also cut a groove, about 5 mm wide and between 1 and 2 mm deep, lengthwise along the centre of the top of the assembled frame halves. It runs between the lips of the light board slots, to accommodate the wiring that goes to the front light board and the motor. On the next picture the area that has to be milled out is marked out.

The next picture shows one set of frame halves with the areas that were milled out. The recesses in the insides of the two frame halves were milled to make sure that the brush caps cannot touch the frame halves and cause a short. On one frame half I also milled out a slot to accommodate the grey wire to the bottom motor brush. The undercut space for the decoder was milled out in such a way that the light board slots remained intact.

The next picture shows the groove cut in the top of the frame to route wires to the motor and the front light. I also made a hole in the top of the assembled frames just above the brush cap position for both motor wires to pass through.

After milling, brush off and, if necessary, wash the frame halves thoroughly to remove all milling shavings.

LIGHT BOARD MODIFICATIONS

On both light boards, cut through the trace that's next to the number on the board, halfway between the LED's solder spot (on the LED leg opposite the one that's connected to the resistor) and the contact pad that fits into the loco frame. Just a brief touch with a Dremel or Proxxon or similar tool's cutting disk will do the trick. On the opposite side desolder the end of the resistor that's closest to the contact pad. It's easy if you insert the tip of a small screwdriver between the resistor's wire and the light board, and touch the solder spot on the board with a heated solder gun until the resistor's leg pops out of the board. On both light boards, bend the half-desoldered resistor around 180 degrees so that the resistor is positioned next to the LED. Refer to the next picture.

DECODER INSTALLATION

Unless you're very sure of your soldering abilities, do not attempt to solder to the brush caps directly without having removed them from the motor. If you apply heat for too long, (a second comes close to be too long!) the heated caps will melt the surrounding plastic parts of the motor and the motor may not work so good after that. Remove the brush caps carefully - remember that there's a brush and a spring inside that will certainly try to go into orbit if you're not careful! Remove the wipers and discard them into your spare parts box.

With the motor placed into its cavity in one of the frame halves, determine the length of the grey and orange wires required plus a little extra for slack, cut them to length and then solder them to the brush caps. Reinstall the brush caps with the grey wire to the bottom of the motor and the orange to the top. Cover the brush caps with pieces of insulation tape, as shown.

Make doubly sure that the motor is properly isolated by installing it, in its saddle, first in one and then in the other of the two frame halves and checking for contact between the brush caps and the frames. Your meter MUST show NO contact! If it does, go look for the contact and fix it before proceeding.

The grey motor wire gets routed around the side of the motor in the slot that was cut out for it, to be out of the way so it can't get pinched when the shell is replaced. Assemble the two frame halves, with the bearing retainers, frame spacers, motor, drive worms and trucks in place, and with the grey and orange wires passing through the hole in the top of the frame halves. Leave the side truck wipers aside for now. Tighten the frame screws, and check again for shorts. If there is a short, open up again and find and fix it before proceeding.

The decoder can then be placed in position, with the wire end to the rear of the loco and the grey and orange wires folded over the top of the decoder.

Replace the light boards. The rear light board will help to keep the decoder in position. Carefully part some of the blue wire's insulation at a point close to the decoder (about 10 to 15 mm away) so that the cleared spot will be able to reach the free end of the resistor on the rear light board, while still leaving some slack. I used my fingernails, but be very careful since it will cost you $17 plus shipping (or $23 plus shipping if you're in South Africa, like me) should the wire get separated from the decoder. Then solder that cleared spot on the blue wire to the free end of the resistor on the rear light board. Insulate the solder joint. Refer to the next picture for this and the rest of the the wiring.

Then string the rest of the blue wire along the top of the frame, cut it at a suitable length (remember to leave a little slack - about 10 mm will be more than sufficient) and solder its end to the free end of the resistor on the front light board.

Next, cut the yellow wire to a suitable length, remembering to leave some spare for slack, and solder it to the rear light board's LED leg that's NOT connected to the resistor. In other words, solder the wire to the solder spot on the rear light board that's right next to the cut that was made in the circuit board's trace. If you do not need the green and violet leads for other functions like ditch lights, you can also solder the green function lead to this same spot. (See below.)

Then cut the black wire at a suitable length (remember slack!) and solder it to to the spot on the rear light board where you had desoldered the one end of the resistor. Next, the red wire goes to the spot on the front light board where you had desoldered the one end of that resistor. Finally, solder the white wire to the front light board's LED leg that's NOT connected to the resistor. In other words, solder the wire to the solder spot on the front light board that's just ahead of the cut that was made in the circuit board's trace.

And you can also solder the violet function lead to this same spot. This way it is possible to program directional blinky light functions of your choice to the F1 (green) and F2 (violet) leads. Then, with F0 off and F1 and F2 on, the loco will have the blinky lights of your choice, and when F0 is turned on (without having to turn off F1 and F2), the lights will be on steady. (On DT100 throttles F2, being intended for the horn if sound is used, is on only while pressed. To keep it latched on, press F2 and while holding it down, also press Run/Stop. Pressing F2 again will unlatch it again.)

Tuck the wires into the groove cut in the top of the frame. And that should be it!

LOCO REASSEMBLY

Install the side truck wipers. Then fit the fuel tank - it helps to hold these wipers in position. And install the headlight cover, making sure that all wires pass on the sides of the LEDs so that the light will not be obstructed. Leave the shell off till later.

PROGRAMMING

Do a final check for shorts, program your loco on the programming track or PR1 setup or whatever you use, and test-run your loco. And you're all done. Replace the shell, add the loco to your fleet of decodered machines and have fun!

If anything about the procedure is perhaps still unclear, or if you used this guide and it worked for you, please drop me a note.

Good luck, and keep them on track!


INDEX

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The following pages contain decoder installation guides for some other N scale diesel locomotives:

Atlas Plug & Play Locomotives
Atlas EMD GP30
Atlas EMD GP40
Bachmann GE C40-8 & C40-8W
Kato GE U30C
Life-Like EMD GP38-2
Life-Like EMD SD7 & SD9
Life-Like EMD SW9 & SW1200

And the following pages contain guides to servicing or performance enhancement of some N Scale diesel locomotives:

Atlas & Kato Diesel Truck Servicing
Life-Like SD7 & SD9 Power Pickup

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The Kgalagadi Transfrontier Park
The Story of a Kalahari Telephone Pole
The Chessie System in N Scale
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Created on 5 September 2003. Last updated on 11 September 2003.