Wednesday, February 1, 2012

TCS CN-GP in an Atlas GP7

Between 1995 and 1997 Atlas did several releases of N Scale GP7's and GP9's based upon this mechanism.  The TCS CN-GP decoder is a perfect choice for these. 

The frame requires some very minor modifications that I do using a Dermal rotary tool with cutoff wheel.  The small ridges between the light board clips at each end will need to be removed and also a notch needs to be made on the left side frame at the point shown in this photo.
On some GP9 releases the frames have interlocking tabs.  If you have one of these just snap off the upper tab.  This will create the space needed for the motor wires.

In this photo all of the frame modifications are done.  Also shown is the plastic motor saddle with a  notch on the top.  The notches on the frame and the saddle are needed for the motor wires.

Remove the brush caps from the motor and remove or cut the contact tabs.  Then remove the orange and gray wires from the decoder and solder them to the brush caps.  File a slot big enough for 1 wire on the left side of the motor.  With the orange wire going to the bottom, re-install the brush caps with wires and motor saddle.

Re-assemble the locomotive mechanism with the orange and gray wires going through the hole on the top of the left side frame and the plastic motor saddle.

Install the CN-GP decoder boards on the frame and connect the yellow, black, and blue wires as directed in the decoder instruction sheet.
Cut the orange and gray wires to the correct length and re-solder to the front board.  All wires should lay flat and not overlap on the top of the frame.

Re-install the trucks, contact strips, and fuel tank.

These Atlas GP7's were among the first N scale engines to have lighted number boards with the numbers already in them.  Because the LED they used was so dim, this feature was not really noticeable.

With the bright white LED's that come with the CN-GP decoder, this feature can now be fully appreciated as seen in the photo above.  You may even find it necessary to mask for paint the inside of the shell at the ends if the light shines through the shell.


  1. I sure wish I would have found your post before I started on my GP7! I tried to mill down my frame to fit in a Digitrax DN135D decoder. It was going great up till the last 15 seconds of milling, then one side of the frame fell apart in 4 pieces!

    So I ordered a new frame from Atlas, and they sent me a frame half that was bent and cracked, ready to break. So I sent it back and am still waiting for my replacement.

    Once I am ready with new frame in hand, I will definitely go with the TCS CN-GP decoder setup! I'm sure it'll be much easier than what I was originally trying to do. ha ha Would have saved me time, money, and a lot of hassle!

    Thanks for sharing this info, it's hard to keep up with everything nowadays, things come and go and change so fast, it's crazy. Thanks!

    Bookmarked your sight for future reference and "hassle free" decoder installs! ha ha

    1. Glad you are finding the information on the blog useful and welcome aboard. The CN-GP is a great decoder which will work on a number of locos. If you don't get satisfaction from Atlas on the frame, contact me at my Email, as I may be able to help you with that.

  2. Replies
    1. Your welcome Justin, and thanks for checking out my DCC blog.

    2. Few! Got everything together without cooking anything! Thanks to your guide I was able to mill the frame where needed. The last time I installed a decoder it was a cn and one of those tiny hairs from a wire shorted another pole and it was bye bye smoke and all. So I was very careful this time. I got it running, tested on DC and then moved to programming track, everything ran smooth accept the lights were reversed to the travel direction! Found the solution in TCS's online guide pg 9:

      "First, put a value of 7 in CV 29; this causes everything (the motor and the lights) to run opposite from their original orientation.
      Now the motor is running in the proper direction and in order to change the lights put a value of 16 into CV 49 and a value of 0 into CV 50."
      It's a generic card so it depends on how the loco manufacturer wired it originally, so it's easier to do these fixes as needed than rewiring things backward as I've read some people suggest.
      So now I got it nabbed.

      Now for fine tuning speed, CV 2, 5& 6 because its a bit jumpy and jerky on acceleration. Thanks again for your guide, it was very helpful in this confusing project.

    3. That's great Justin, glad it worked out.

      My first installations were in 1993 on 3 Atlas GP30/35's. Took 5 Digitrax DN82 decoders to get those going. Couple of tips that I might suggest related to the problems you mentioned.

      Tin the wires before soldering them. Twist the strands, apply flux and solder. This keeps those tiny strands from straying.

      Test on DC. Keep a DC engine as a reference, the one with the decoder in it should move in the same direction as the DC one. If it does not, then reverse the wires to the motor. On a TCS CN or CN-GP installation this can be done at the decoder board.

      Good luck in your future installs.

  3. Any comments on a low-nose GP7 of the same period?

    1. I've not come across this. I'm assuming that this is a model you are modifying from a high nose version ?

      In that case the CN-GP would not work. I would suggest starting with a frame that has been milled to make space in the long hood and use a wired decoder.

      As there would be no LED boards may need to drill & tap a couple of holes to attach the red and black decoder wires to the frame. Then wire both LED's and their resistors and secure in place with adhesive or tape.

      Another possibility may be to use the new TCS MT1500 decoder and install it on the long hood end.