Modelling what might have been in southeast BC and northwest Wasington

Atlas/Kato GP7 LED headlights

Many years ago Atlas sold a variety of locomotives in the so-called “Yellow Boxes”. A number of these were made by Kato for Atlas, and have proven to be some of the best running locomotives of all time. They are still worth picking up at swap meets. In this series there were two numbers for Canadian Pacific in the block lettering scheme, and many of us have acquired these over the years. They of course need to be converted to DCC, but fortunately a number of the DCC vendors make drop-in boards that snap on to the mounting points perfectly. The lighting, however, is another story.

The original design used a single incandescent bulb sitting up above the motor, so that the light would shine in both directions through clear plastic light guides, and out the end headlights and number boards. With DCC, we can now control each light independently, so we need two LEDs. The first photo shows the new DCC decoder mounted, but still with the original light guides in the shell.

Chassis with new NCE decoder and the shell with the original clear plastic light guides before shortening.

The decoders were installed in a flock of these engines a couple of years ago, but we were not sure how to do the lights, so they have been running around without headlights for some time. On Saturday, I helped John start this process, to see where it might lead. It turned out to be much simpler than anyone had suspected.

Soldering the LEDs to the decoder board is easy, as there are two tabs for the connections. The big issue is how to limit the current through the LEDs. Different brands of decoders may or may not have on-board resistors for just this purpose. You will have to consult your decoder’s instructions. In this case, we used the NCE DA-SR board which has a tiny, surface-mounted 1K resistor for each light circuit. The only trick is that there is a tiny printed circuit trace that shorts out the resistor so that the decoder can be used with 12V light bulbs. This needs to be cut cleanly to allow the resistor to be in the circuit to the LED and do its job. Failing to do this step will mean that the LED is subjected to the full 12V and will go “poof” very quickly! You also have to get the polarity of the LED correct, otherwise it will not work. The Common “C” connection on all DCC decoders is positive, while the numbered function connection is negative. In this case, we have “C” and “1” for the front headlight, and “C” and “2” for the rear one. You may have to test the LED to determine which lead is which. If one lead is longer than the other, that is usually the positive side and it is connected to the “C” terminal. Here is a link to an article on how to test the LEDs.

Chassis with new LEDs soldered to the decoder board.

After the LEDS were in place and tested for correct operation, the next step was to shorten the plastic light guides. These were “eyeballed” and cut with a razor saw. I have measured the lengths for future reference, and found them to be 1.415″ overall. It is a good idea to polish the cut ends for better light absorption. I filed them flat, sanded with 600 grit, and then put one drop of Testors liquid plastic cement on them while they sat vertical in a small vise. This forms a slight lens on the end and serves to make the plastic clearer. I found that Testors worked the best, as it is a much slower drying cement than many of the others, which allowed time for it to soften the plastic and make it clear.

Shell with the light guide trimmed to length.

There is one other change needed to make room for the new decoder board. In the photo above you can see the circular remains of two posts in the shell, just to the right of the cab area. These supported the ends of the light guides, but need to be trimmed down to clear the board. I used a small circular cutter in my Dremel to simply cut them off, using the slowest speed. You may want to add some drops of canopy glue to hold the plastic guides in place, as they no longer have the support posts and are just held in with friction.

Cutter used in Dremel to shorted the support posts.

Here is the final result during a test fit to make sure that everything clears.

The final result.

So, after all of the procrastination because no one was quite sure how to proceed with these, it has turned out to be a rather simple process, especially with the NCE decoders and their onboard resistors for the LEDs. Thanks, John!

So, now we have how many more to do?

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