A picture tutorial on how to install a wired decoder into an Atlas GP7

Disassembly

This is not to dificult with Atlas models. Grab the body and fuel tank and pull the tank away from the shell. As the tank is attached to the frame it should pull the frame out of the shell. Sometimes the tank will pull off exposing the frame underneath. If this happens don't panic just grab the exposed frame and pull out of the shell. Sometimes the frame is a bit stiff but a bit of wiggling and jiggling will pull it free.

Here is a picture of the frame removed from the shell with the decoder sitting infront. The knife blade on the left hand side is pointing to where we will be installing the decoder. Yep, under the light board.

Now that we have the frame removed from the body we have to pull it apart.

But wait, before pulling it part we need to label some parts for reassembly and also to know where to solder wires.

You need to mark the front as well as the sides. It is also important to mark the top and bottom of the motor as we will need to know this for soldering the wires. Have a look at the attached picture. I also labeled the reaer after taking the picture.

It is recommended that you label the trucks too, so that they can be reassembled to the same end and in the same orientation.

I also at this point placed the decoder in the installation spot to see how it would fit.

So, lets start pulling this sucker to pieces.

First remove the lightboards from each end. I labeled mine but they are the same so it is optional. As each piece is removed I lay iy out in the box in the same location to make installation easier.

Next loosen off the screws at each end and remove. Don't worry the frame halves will not fall apart as is is held by the motor clips. Now that the screws are removed place them in the box and you will be able to part the frames enough to remove the trucks and place into the box.

Removing the motor

Now it is time to remove the motor.

The motor is held in the frame halves buy plastic lugs on the ends of the motor assembly. Best to look at the pictures. You can try and prise these out while putting slight pressure on the frame but I found it just as easy to insert a small screw driver between the frame halves (starting at the bottom) and while gently levering apart press down on the motor toward the frame that is furthest away from you. Once the bottom clips have come out of the frame do the same for the top clips.

Now that you have got this far it is time to fully remove the motor and drive shafts.

I found it easier to remove only either the top clips or bottom clips and swing the motor out slightly and then remove the shafts and place them in the box.

Here are some pictures of the shafts and all of the stuff in the box.

Ok Cool, so far. That was the easy part.

Modifying the frame

Have a look at the picture and you will notice the motor tab sitting outside of the black motor frame. These are there to make contact with the frame. The one on the bottom, circled in the pic makes contact with the left side of the fram and the one at the top the right side. You can just see it and the tab on the frame that it contacts

Now as motors touching frames in DCC is a bad thing we will have to do something about those tabs. We'' discuss these in greater detail later.

Now to fit the decoder we need to do a little bit of surgery to the frame. Very little is required and even less if you don't use the original light board.

Word of caution.
The frames are made out of a relatively soft material so take care when working with them not to bend them. Bending them is very bad. Be light fingered with the file and Dremel if you are using one. Also when using the Dremel the frame will get hot. I found that I would hold it close to where I was cutting and if it got hot I would wait for it to cool then continue.

Have a look at the pic. The tab on the frame where the top motor clip contacts the frame will be removed and I also removed a small section at the front of the frame so that I could mount the decoder slightly further back so there was more clearance at the front for the LED. You don't have to use the board and can have the LED taped to the top of the decoder instead but I thought it's there so may as well use it. I also found that I had to file down the side of the frame at the point where the decoder sits for more clearance. These are circled in the pic.

When the two frames are held together side to side you will notice that the right frame is slightly higher at the front than the left frame This is just to the left of the left circle. I just filed down the right frame until it was the same height as the left and found this gave me enough clearance.

To cut the tab off I mounted a Dremel with a cutting disc in a vise and then holding the frame gently cut it off.
For the groove at the front I cut as much as I could with the Dremel and then used a flat needle file to finish it off. At all times I held the frame. DO NOT clamp the frame in a vise.

We will use the hole created by filing off the tab to feed the motor files down through the frame.

That's all there is for frame mods.

Isolating the motor

So the first thing we have to do is get the motor ready for soldering the wires to it.

As I mentioned before there are two bronze tabs that extend from the motor brushes and touch the frame. What I found is that if you unclip the black motor fram and then put it back on using a twisting motion in the direction that the tabs are pointing you are able to roll the tabs underneath the black motor fram and thereby insulate them from the frame. This also leaves a bit of the tabs sticking out of the frame where the wires for the motor can be soldered to.

Have a look at the two circled bits in picture. Also note that there there are two black nubs that extend from the black frame towards the motor housing just to the left of the tabs. Trim these off as it makes makes it easier to get the tabs under the frame without distorting it.

The next thing that needs to be done is to file a small grove down the left side of the motor for the grey wire to fit in. You can just make it out under the grey wire in the picture below.

We won't do any soldering yet as we need to modify the front light board and led first, you'll see why later.

Light Board Modification

You still with me, I hope so.

OK, lets crack on. Lets modify the front light board and LED.

As the LED is mounted under the front lightboard and the decoder is mounted under that I decided to file down the led to a squareish shape and also file down the fromt of the LED back as close to the cathode and anode as posible (they are the shiny things inside theclear plastic of the LED and what produce the light). I also unsoldered the LED to be able to file down the board side of the LED and also as I have shortened it I could then rebend the legs of the LED so it was mounted cloder to the board and also move it more towards the front of the board, thereby giving a better clearance for the decoder.

Also I cut the traces just after the pads that mount in the frame. You don't want power coming from the frame to the light board.

As the decoder is mounted under the board we needed a way to get the wires out and to the motor, Frame and rear board. To do this uses a flat needle file on its side and file a slot in the board as per the pics.

You can see where the trace has been cut. (this also has to be done on the rear board)
You can also see the slot for the wires.

Notice how the LED now sits closer to the board and the legs are longer, so the front of the LED is pretty well in the same position as before but the rear of the LED is now further forward.

Also note that the height of the LED from the board is now about the same as the resistor.

Cutting and Soldering the Wires

Now lets get some wires to the decoder.

First off all you should do some test fittings of the decoder under the light board to make sure you have filed enough material from the frame.

Also, I nearly forgot. There is some flash on the metal frames near where the end of the lightboard fit in the frame

I removed this flash so that the wires would be able to be kept down in the frame.

BTW I mounted the decoder with the wires to the front and the solder points where the wires are soldered to the decoder facing upwards towards the lightboard.

Once you are happy with this place the motor loosely in the right frame and run the grey and orange wires from the decoder through the lightboard along the top of the frame and then down roughly where the tab was removed and then down to the motor terminals. Grey to the bottom and orange to the top. This will give you an idea of where to cut the wires.

Cut the wires and trim back a little of the casing and then tin the exposed wire.
Remove the motor from the frame half and also remove the black plastic motor mount.
My bronze tabs were a little oily so I cleaned them off with some paper towel and then gently filed, just to cleanup, the bits I was going to solder.

CAUTION: Be very careful when soldering the wires to the tabs on the motor. To much heat and the grey plastic will melt any bye bye motor. You can remove the brass caps that hold in the brushes, which will also remove the tabs, if you are not comfortable soldering while they are in the motor. Be carefull when removing the brush caps as there are tiny springs inside and you don't want to loose them

Tin the tabs with a bit of solder just on the bend and solder the grey wire to the bottom tab and the orange one to the top. Once this is down slip the plastic motor mount back over the motor making sure the tabs are trapped under the black plastic as described earlier and shown below.

Finishing off

So far so good.

At this point I reassembled the chasis. Making sure everythin went back in from where it came. Except for the trucks I left these off till last.

I mounted the decoder on the frame with double sided tape. Make sure that the tape goes all the way from the front to the rear and also around the notch we cut out. The tape I use is called KwikGrip Double Sided tape. It is very thin when the backing is removed. It is also made of thin plastic so insulates as well.

I also insulated the underside of the lightboard from the decoder.
Once you have stuck down the decoder insert the lightboard making sure the wires come through the slot and don't get trapped underneath.

Insert the rear light board too.

Then start hooking up the wiring as per the pics.

The blue wire goes to the resistor next to where the tracks were cut, make sure you don't bridge this gap with solder.

The white wire get soldered directly to the front LED on the left leg.

The yellow wire is soldered to the rear LED right leg

and finally another blue wire is run to the rear resistor.

To install the trucks loosen the screws at each end and prise apart the bottom of the frame till they drop in.

Make sure that the pickups connect properly.

Oh you maybe wondering what the yellow bit of wire on the front board is for. Well the track from the resistor to the LED was damaged when I ws removing the LED so I bridged the connection with that piece of wire.

Set it up on your programming track and check to see you can read the CV's correctly and program a new adddress into the loco. If all is well and there are no shorts take it out for a test run and then tape down the wires to the top of the frame and reinstall the body and fuel tank.

Enjoy your new DCC model.

 Note that TCS has now brought out a decoder, CN-GP, which is in two parts and is almost a drop in installation.

I would like to thank Robert Ray for his article on installing a decoder in the same locomotive which gave me the confidence to attempt this one. You can see his article at :- http://raybob.boche.net/dcc/page704.htm he does some thing slightly differently namely, no board and the way it is wired to the motor so worth a look.