N SCALE DECODER INSTALLATION GUIDE

ATLAS GP30 LOCOMOTIVE

DIGITRAX DZ121 OR DZ143 DECODER - FRAME MODIFICATION REQUIRED


INTRODUCTION

I planned to install DN146A decoders in these locos, but they turned out to be just a tad too long to fit in the top of the loco shell. So I used Digitrax DZ121 decoders. (The DZ143 decoder only appeared on the scene much later.)

I took a few pictures of the reassembled frame with the decoder in place, so if anything about the procedure is unclear, hopefully the pictures will clarify matters. The pictures are included at more or less appropriate spots in the text. (I did this installation and wrote the original text long before I acquired a digital camera!)

FRAME MODIFICATION

You need to do some frame milling to make room for the DZ121. I put the decoder below the rear light, and I milled out a chunk of frame of 8mm deep and 20 mm long to make room. This is done with a vertical cut straight down from the top of the frame, just to the rear of the slots into which the rear light board fits and in such a way that the part numbers on the two frame halves (17728L & 17728R) remain intact; and a horizontal cut just above the top of the rear bearing retainer.

I also cut a vertical 2 mm slot in the left frame half, 3 mm ahead of the hole in the side of the frame where the rear flywheel fits in. This is to make room for the wire to the bottom motor brush so that it cannot get pinched when the loco is assembled.

And I cut a groove, 5 mm wide and 1 mm deep, lengthwise along the centre of the top of the assembled frame halves between the lips of the light board slots, to accommodate the wiring that goes to the front light board.

I also milled out the four areas on the insides of the frame halves closest to the brush caps, just enough so that the brush caps cannot touch the frame sides when the loco is assembled. (I found that, even with the tabs on the brush caps removed and the motor wires soldered directly to the brush caps, the caps still touched the insides of the frame halves after assembly. I did try thin insulation first, but the motor fits very snugly in the motor cavity and the caps managed to pierce the insulation.)

Finally, I drilled a 3mm hole in the top of the assembled frames just to the rear of the position of the top motor brush, and I cut a little nick out of the right side of the top of the motor saddle, also just to the rear of the position of the top motor brush, for the motor wires to pass through. (The right side of the motor saddle is the side with the halfmoon openings that clip onto the motor.)

DECODER WIRE LENGTHS

Cut the gray decoder wire to a length of 70 mm measured from the end of the decoder's shrink wrap, and the orange wire to a length of 50 mm. The rest of the wires will all be soldered to spots on top of the assembled loco frame and it will be easy to determine where to cut them then. Just remember to provide for a little slack in all the wires for the occasion when the loco may have to be disassembled again.

LIGHT BOARD MODIFICATIONS

On both light boards, cut through the trace that's next to the number on the board (637-X001), 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 and the light board, and touch the solder spot on the board with a heated solder gun until the resistor leg pops out of the board.

DECODER INSTALLATION

Cut off the wipers on the brush caps so that just a stub remains. Then 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! Solder the grey and orange wires to the remaining wiper stubs, and then reinstall the brushes with the grey wire to the bottom of the motor and the orange to the top. (The bottom of the motor is marked by a little hole in the nickel-plated motor jacket.) Make doubly sure that the motor is properly isolated by installing it, in its saddle, first in one and then 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.

On each light board, bend the half-desoldered resistor around 180 degrees so that the resistor is positioned next to the LED. Then assemble the two frame halves, with the frame spacers, motor, drive worms and both light boards in place, and with the grey and orange wires passing through the hole that's drilled in the top of the frame halves. Leave the trucks and side 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 under the rear light board, with the wire end to the rear of the loco and the grey and orange wires folder over the top of the light board past the LED. You'll find that the rear light board holds the decoder firmly in position.

Carefully part some of the blue wire's insulation at a point close to the decoder (about 10 mm) 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 should the wire get separated from the DZ121! 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. 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 front light board's resistor. And insulate the solder joint.

Next, cut the black wire at a suitable length (remember slack!) and solder it to the solder spot on the rear light board where you had desoldered the one end of the rear light's resistor.

On the front light board, solder the red wire to the solder spot on the light board where you had desoldered the one end of the front light's resistor.

Next, solder the yellow wire to the rear light board's LED leg that's NOT connected to the resistor. Put simply, 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're installing a DZ143 and 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.)

Solder the white wire to the same spot on the front light board. And if you're installing a DZ143, 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.)

And that should be it!

LOCO REASSEMBLY

Loosen the two frame scews some, just enough so that the trucks can be slipped into position. Install the side wipers, making sure that they pass below the small protrusions on the frame's bottom edge that's slightly offset below each flywheel. Then fit the fuel tank - it holds 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

While you program your loco with its shell off, it's a good opportunity to check the lights to see that they're working - they will flash each time the decoder acknowledges to the programmer, and the motor will also turn slightly. After programming your loco on the programming track/PR1 setup or whatever you use, it's time for the final safety check.

Take the loco to your layout, and while leaving the loco OFF the tracks, dial up its address on your throttle, and make sure that the speed is at 00. (Zero, nil nil, double-oh, nothing else!) Then put the loco on the track and watch its lights. If they both start flashing at you, remove the loco from the tracks and go look for the short that you missed! Whatever you do, leave the throttle at 00 while the loco is flashing its lights at you, since if you should turn the throttle clockwise in the slightest, the decoder will send power to the motor, and there will be some very pretty lights of brief duration on the decoder and you will actually see $17 worth of smoke waving goodbye at you!

If the decoder's lights don't flash at you, however, there should be no short and you can test-run the loco. And you're all done! Replace the shell and add the loco to your fleet of decodered machines.

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 GP40
Bachmann GE C40-8 & C40-8W
Kato EMD SD40
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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Created in July 2001. Last updated on 11 September 2003.