Table of Contents

Introduction

The following detailed description covers installation of a Digitrax DN93FX decoder in Life-Like F7A diesels, as actually performed by the author of this publication, and reflects the experiences encountered during those installations. Since multiple units (2) of the same type were converted from analog to digital, the descriptions presented reflect problem resolution and shortcuts developed.

This is one of the easier DCC conversions to carry out, and is recommended for a first or early-on installation.

The Digitrax DN93FX decoder was used because it was "on hand," and fit the Southern Digital retroframe. Several other decoders from Digitrax, Lenz, TCS, etc., could be used.

The most important factor to remember in performing an analog to digital conversion is to ensure both motor brushes and the decoder orange and gray wires are insulated from the frame. Any contact of the brushes and/or these wires with the frame may result in virtually instant destruction of the decoder.

The first step in the description which follows is to test the decoder for proper operation, following the instructions provided by the manufacturer. The purpose of this step is to ensure any non-operational or dead-on-arrival decoder can be repaired by the manufacturer under warranty.

 
Refer also to the instruction sheet provided by Southern Digital with the retroframe.

This conversion uses the Southern Digital LLF7N retroframe, which is actually a replacement for the rear weight in the locomotive, with a slot in which to place the decoder. This weight is made of a more dense material than the original weight, so there is no weight loss despite the slot for the decoder. The original rear weight from the locomotive can be modified to add the slot for the decoder; refer to the appropriate section below. There is no modification required to the front weight.

To install the decoder and modify the frame you will need the following tools:

You will be cutting a slot, front to rear, in the rear weight which is deep enough and wide enough to hold the chosen decoder. This slot will be centered from left to right in the weight.

When directed in the detailed installation instructions below, modify the rear weight of the F7A as follows:

• Using the actual decoder as a guide mark horizontal lines on the top of the weight, centered from left to right, to indicate the width of the slot to be cut.

• Again using the actual decoder mark the area on the ends of the weight to show the depth required for the cut. Be sure the depth of the cut leaves at least 1/8" of metal above the opening in the bottom of the weight.

• Place the weight in a vise. Use a Dremel #106 1/16" diameter rotary engraving cutter or equivalent in a motor tool to grind the necessary channel for the decoder (remember to wear eye protection). Smooth the sharp edges of the cut with fine sandpaper to ensure the insulation on the decoder and decoder wires cannot be pierced.

• Use 600 grit wet and dry sandpaper to polish the frame in the areas where metal was cut.

• Clean all of the grindings from the frame.

You can now continue to install the decoder per the detailed instructions which follow.

Set your browser's left and right margins to 0.5" (File Page Setup) and print out this document. As each step in the installation is completed place a "X" or a check-mark through the bullet. All references to the frame are based on the front being at the top or away from you.

In normal analog (DC) operation, the right motor brush is connected to the right rail and the left motor brush is connected to the black rail. In a reversal of normal color coding, Life-Like connects the left rail/brush with red wire and the right rail/brush with black wire.

• Begin by testing the Digitrax DN93FX decoder for proper operation per the instructions provided by Digitrax.

• Cut the decoder wires as follows, then strip 1/8" insulation from the ends:

• Note: The F7A does not have a rear headlight. It is suggested the yellow decoder wire be cut to 1/2", leaving a length that can be used should the decoder be later installed in another locomotive.

• Remove the F7A shell by spreading the bottom of each side of the shell carefully with your fingers and lifting upwards then forwards to clear the front coupler until free. Set aside.

• Remove both the front and rear weights from the locomotive.

• If you are replacing the Rapido couplers with other couplers such as Micro-Trains, Unimate or Kato couplers do this now.

• Using the flat hobby file, remove any casting "nubs" and parting lines from the Southern Digital rear weight.

• Place the new rear weight on the locomotive and check that the shell has sufficient clearance. Do this by replacing the shell on the frame, then removing again. File the rear weight as necessary to ensure clearance.

• All electrical connections are made to the motor brushes. The right motor brush is connected to both front and rear truck right pickups by black wires, and the left motor brush is connected to both front and rear truck left pickups by a red wire. The two red wires from the headlight connect one to each brush terminal.

• Carefully unsolder the existing wires from the motor brush terminals.

• Place the Southern Digital rear weight in place on the frame and place the DN93FX decoder in the slot with the wires facing towards the front of the locomotive.

• Solder the orange decoder wire to the right motor brush terminal, and solder the gray decoder wire to the left motor brush terminal.

• Solder the red decoder wire to the two (2) black wires from the F7 trucks. Insulate the joint.

• Solder the black decoder wire to the two (2) red wires from the F7 trucks. Insulate the joint.

• Solder the decoder blue wire to one of the red wires from the headlight, and insulate the joint. Solder the decoder white wire to the other red wire from the headlight, and insulate the joint.

• Using an ohmmeter, check for shorts between the two motor brushes and the frame and trucks. resolve any problems before continuing.

• Dress all wires away from the drive mechanism and test operation of the locomotive. Resolve any problems before continuing. (Note: dirty wheels can adversely affect locomotive performance.)

• Place the decoder in the slot over the rear weight and tape in place.

• Locate the headlight in its proper position and replace the front weight on the locomotive.

• Carefully dress all the wires so they do not interfere with either the mechanism or the replacement of the shell. Gather the wires above the motor together and tie with thread or small string. This will ensure they do not press down on the motor armature when the shell is replaced.

• Carefully place the shell on the frame and press down, being careful not to catch any of the wires, until it clicks in place.

• Test the operation of the locomotive on the railroad. It should operate just as if no modifications had been made. Resolve any problems.

• Place the locomotive on the DCC programming track and set the DCC Command Station to the programming mode.

• Program Configuration Variable "CV29" to "06" for 2-digit addressing or "38/x26" for 4-digit addressing then program the decoder to the desired address.

• Carry out a final check of the locomotive on the railroad.

• Record the decoder CV's and address, and the reporting marks of the locomotive.

The conversion is complete. Enjoy your DCC-equipped locomotive.

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Last updated: Saturday August 1, 2009

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