Installing a Digitrax Decoder in a Kato E-7

It’s a routine drop-in installation, with just one catch.

For N scalers, Kato’s new 20th Century Limited set is this season’s hot toy. There’s so much to love about Kato’s streamliner sets. They’re beautiful, well-engineered, highly authentic, and a pleasure to operate. With the 20th Century Limited, Kato has finally released a set based upon a train that ran through my town. Of course I ordered one. It arrived a few weeks later than it was supposed to, but I’ve endured far longer waits than that.

I also placed an order for a pair of Digitrax DN163K0a decoders, which fit Kato E-8s and other passenger engines. It seemed reasonable to assume that the brand-new E-7 would have a new body shell over Kato’s existing chassis. Once the set arrived, I gave it an hour’s runtime on DC, and prepared to do the installations the following Saturday. I expected a straightforward install—I’ve done E-8s before.

My assumption about the E-7 chassis was correct, mostly. The tiny gray plastic clip for the motor leads is still useless. The long contact strips still need to be removed and wrapped in Kapton tape. Some additional plastic constraints have been added around the strips, which make removal a more delicate operation, but should ensure that they’ll stay put once reinstalled. The biggest surprise was the light pipe to the headlight and number boards: it was much more elaborate, and larger, than the one in an E-8.

For the E-7, Kato updated the light board. Instead of the traditional discrete 3mm LED, a side-emitting surface-mount component sits on the front end of the board. This kind of LED doesn’t blast out as much light, so a light-pipe redesign was necessary to shepherd as many photons as possible through the headlight. The result looks great, but occupies more of the precious space inside the shell.

Digitrax decoders come with a 3mm warm-white LED, with leads left long, attached to the board. Have I talked about Digitrax’s LEDs before? They’re awesome. The color is a perfect simulation of incandescent, not the blue or amber tint you often see, and they’re bright. Very bright. I first encountered these LEDs years ago, when I put decoders in my C-Liners. If there was room in those engines for batteries, they could double as flashlights.

Because the DN163K0a is made to fit several different engines, it’s sometimes necessary to remove the LED, shorten the leads a little, and reattach it. Based on the rear edge of the light pipe, I made a best guess as to how much to shorten it by, re-soldered it, and placed the body shell back on the engine.

It wasn’t obvious, but on close inspection, I could tell that the front of the shell was not seated all the way on the chassis. It was riding high by maybe 0.020–0.030 inches or so. Why does it matter? If the shell sits too high on the chassis, so does the front coupler, which is body mounted. The coupler on the nose of the trailing unit forms part of the most-stressed coupling in the entire train. All the power is on one side, the entire load is on the other. If this coupling is not perfect, the train will break in two constantly. Nobody wants to spend that kind of money on a train that won’t stay in one piece for a single lap around the layout. I certainly don’t.

I looked around the house for some kind of putty I could use to check internal clearances, but came up empty-handed. Instead, I blobbed some acrylic paint on the LED, and temporarily replaced the shell. Sure enough, some of that paint had transferred to the rear edge of the light shield. This area forms a little black box to surround the LED. The cavity is just large enough for a 3mm LED, but the LED needs to be precisely positioned. I’d actually cut the leads too short.

The light pipe and light shield in a Kato E-7. The LED must be positioned to fit inside the space outlined in red.

Luckily, the DN163K0a comes with two LEDs, for those engines equipped with a rear light. I trimmed the spare as precisely as possible, soldered it in, and this time, everything fit.

A Digitrax DN163K0a decoder with the LED properly positioned to fit in a Kato E-7 shell.

After I’d tested the engine, repeated the installation on its mate, and admired their front-end lighting for a while, it was time for some configuration work. On every decoder I’ve ever installed, the default setting for headlights is to turn on only when the engine is moving in that direction. I think it’s a holdover from the days when we’d wire a diode in series with the headlight on a DC engine to provide directional lighting. In reality, headlights don’t blink on and off when an engine is engaged in back-and-forth movement at a terminal. For my double-ended engines, I configure the front light to respond to F0 as normal, and the rear light to F1. Neither responds to changes in direction; they stay lit until they’re turned off by the engineer. It only recently occurred to me that the same could be done for a consisted pair of A units. I set up the lower-numbered one for F0, even when consisted. The higher-numbered engine of the pair responds to F1. I did this on the Pennsy E-8s, and was pleased with the result.

Kato E-7 chassis. In the background, with the stock lightboard. Foreground, with a Digitrax DN163K0a decoder installed.

I cloned the settings of the E-8s, changed the addresses, and wrote them into the E-7s with DecoderPro. Now I have easy one-touch control of both headlights. Never again will the postal workers in the RPO have to suffer the constant glare of the trailing unit’s light.