PRR NW2 - S-CAB
04/29/2015

Preliminary Work

This model used to have the Soundtraxx Tsunami TSU-1000 Digital Sound Decoder for EMD 567 prime movers (product ID 827101) installed in it (factory installation). That worked great, and I was happy with it. However, I got tired of the engine stalling when the track and/or wheels weren't perfectly clean. So, I decided to install the Stanton S-CAB DCC system, and to have it be battery-powered. You can read all about this system in the article I wrote about it. The first step in any installation is to remove the body shell. I have a separate article on how to do that. I then stripped the engine of all electronics, and installed these two "rails" of wires to connect the power pick-up of the two trucks.
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In the back of the engine, the main power pick-up wires are firmly held in place, but in the front they floated loose. I used liquid electrical tape to attach the two wires to the front truck bridge.
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In the back, I exposed the two wires so that the system can tap into the power pick-up from the wheels. The most important thing to remember with any DCC installation is that the motor must be completely isolated from the electrical pick-up done by the wheels. If there is any contact between these two during normal operation, some or all of your DCC components in the locomotive may be permanently damaged (and often not covered by manufacturer warranty).
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Installing the Battery

The first thing to worry about is the battery, because it is the largest component to install. As luck would have it, the battery fits width-wise in the shell, as is shown here. The outside material of the battery is somewhat flexible and can be compressed to get it to be the exact width the inside of the shell is, i.e. 30mm (for small measurements I find it easier to use the metric system than to use "1.181 inches").
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I decided that the battery should lay flat inside the shell. However, through observation, I realized the speaker in the engine is in the way. There is no way to work around that. The bottom of the shell has some thin piece of foam attached to it that covers the front cavity into which the speaker fires. I removed that, which is why you see the white glue residue. Once that was removed, there is a plastic piece (barely visible under the speaker in the photo below) that can be lifted up (no glue used). This then allows the removal of the speaker. There is another plastic piece at the bottom of the assembly that can also be slid up to be removed.
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Since the battery has to be all the way up front against the two body shell mounting posts, the plastic protrusions on the inside of the shell that held the speaker in place have to be removed. S-Helper Service's shells are made of some tough plastic, so it took quite a while to carefully remove those without damaging the shell. This photo shows all but the very bottom material removed, but in the end I wound up having to remove that as well.
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Just a quick test to make sure the battery fits as intended. It does, without bowing out the body shell.
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To mount the battery and to avoid interfering with the mechanism below, I wanted to install a "shelf". What I discovered was that there are two holes in the top of the truck mounting bridge. These are already tapped (from the factory installation of their original circuit board). It was then just a matter of cutting a piece of 0.040" thick styrene, drilling two holes, and mounting the "shelf" using the two screws (which I found again in my parts box). The styrene shelf is 28mm wide to fit in the shell and leave a bit of spacing just in case I didn't quite center it. I used the truck mounting bridge as my guide for centering it. The length of the shelf was determined by measuring the interior of the shell up to where the motor capacitor is. The shelf sits "below" the top of the motor, to make sure there is enough clearance to the top of the shell (discovered the hard way).
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I am going to glue the battery to the shelf, but the screw heads stick out above the styrene, so I glued a second piece of styrene on top of the first one so that I would have a solid surface onto which to glue the battery.
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This is actually my second installation; the first one failed to fit things properly. I did that one using 5-minute epoxy glue, which did not hold up well. For this second installation I spread some hot glue (using a hot-glue gun, of course) on the styrene, and quickly pressed the battery on to the glue. It sets in a few seconds, and it turns out to be a very solid installation, yet somewhat removable should the battery need to be replaced in the future.
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Here is a side view of the battery installation. I put the shell back on to make sure it fit properly, and it did.
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Installing the BPS Board

In my first attempt at installing the components, I had this mental picture that there is all this space vertically above the motor. I had built a nice styrene shelf that had the BPS on it (which is the board shown in the next photo). It turns out that there is very little space above the motor and the inside-top of the shell! Plan B called for wrapping the metal of the motor with electrical tape and directly placing the BPS circuit board on top of it. The issue with the BPS is the two inductors that are mounted on one side of the board. The other issue is that the way to turn on the power in the engine is to waive a wand over the sensor that is just to the left of the electrical tape in the photo below. This needs to be within 1/2" of the wand, so it needs to be just inside the shell of the locomotive. So, the BPS board has the inductors facing down and pushed as far toward the battery as they can. The two-wire plug from the battery can then be plugged into the board, with the wiring pressed against the side, just under the board. The long sides of the motor just fit inside the shell, so there is no space other than for a couple of layers of electrical tape.
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Here is a side view of the BPS board installed. The electrical tape does not cover any open spaces of the motor itself, so it should maintain most of its cooling.
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Installing the Receiver/Decoder Boards

As you can see from the previous photos, we are running out of space in the hood section of the engine. My original idea of placing the two circuit boards on top of each other in that area was quickly replaced with a dose of reality when I discovered what little space is really available, even in S-scale. The cab interior and a substantial weight take up the entire cab's 3D space. There is no space for the decoder/receiver board combo. It is too long to fit under the roof. So, the interior is going to have to go. It was actually quite difficult to get the cab off of the rest of the body shell, but it is possible. I inserted the blade of a small screwdriver, one on each side, and then forcefully pushed the cab down off of the body. The metal pieces that are used for powering the lights (the yellow, brown, and blue wires), will get in the way, so you have to bend those straight.
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This is what is left after the cab has been removed. The weight is glued to the body and not removable without damaging the body shell.
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I then tried fitting this shell back on the chassis. This is what I was left with. There is a cavity where the wires come out! There's hope!
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To be able to test fit the decoder/receiver board, I knew the crew cab interior piece had to come out of the cab. I used the blade of a screwdriver to slightly push out the cab sides, which made the limegreen interior piece fall out easily.
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After studying the opening into the body, I realized that there was some space above the motor mount and under the BPS circuit board. Here I am test fitting the decoder/receiver board. However, the yellow wrap around the two boards is very wide; much wider than the two circuit boards (which are 3/4" wide, exactly). The circuit boards by themselves would fit, but the yellow wrap is too wide for the narrow body section in front of the cab of the NW2. It has to go, but I am sure that if I cut the wrapper, I'll void the warranty (update: the person who does these decoder conversions said it is OK to remove the wrapper; just be careful with the two loose boards afterwards). So, the first thing I wanted to do is do a test install of the decoder/receiver and see if the system as it stands now actually works.
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I soldered the two gray wires from the BPS board to the wires that connect to the wheel pick-up (so that the board can recharge the battery from the rail, if there is power present). For safety sake, I covered the solder joints with liquid electrical tape. I then connected the red and black wires from the decoder to the BPS. Finally, I soldered the orange and gray wires from the decoder to the motor. I ignored the wires for the lights for now.
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External Reference:
And off to the layout I went. The track is not powered. I swiped the magnet wand over the sensor at the top of the BPS board, turned on the S-CAB throttle, and moved the speed control. The chassis started running smoothly, in both directions. Yippee! What a neat sight to see the engine running without rail power! To prove that this system works, I took a quick video and posted it (the chassis is running on my desk!!!).
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I then carefully removed the yellow wrapper around the decoder/receiver combo. They are held together by that wrapper. I put a wrap of electrical tape around the combo to replace the yellow wrapper's purpose. I then cut a piece of 3/4" wide styrene that I inserted under the decoder and above the rear truck mounting bridge. This keeps anything out of the spinning flywheels. After routing the wires, I then put another wrap of electrical tape around these parts to hold everything in place.
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This is what the set-up looks like now.
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Next, it is time to connect the headlights. This photo shows the white wire, which is connected to the front headlight LED, soldered to a 1K ohm resistor, which in turn is soldered to the decoder's white wire. If you are doing this install converting a factory-install, then you will have to remember to add the resistor, because the factory had that resistor built-in on the circuit board. The blue wire you see is the common (or return) wire for all LEDs. The yellow wire is for the back-up/rear LED light. It, too, of course, must have its own 1K ohm resistor.
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I then snapped the cab back onto the body, put the engine on the track and tested it again paying attention to the LEDs. They looked good and they matched the direction of travel of the engine (an important check to see if you connected the correct wires).
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Turning Power Off (initial version)

Everything is now functional. However, there is one more part to install, the off switch. I had it hanging in the cab area. At first I was thinking about removing the cab's door and replacing it with a miniature scale hinge and then just swing open the door to gain access to the push button mounted in the cab. I was not really looking forward to doing major surgery on this beautiful engine. Then, all of a sudden I had the idea of using the exhaust pipes. They are hollow and open into the body shell. The photo below shows them indicated. My idea is to remove the push button from the orange wires of the BPS circuit, and solder them to two brass wipers. These are then going to be mounted under the two smoke stacks. An external tool can make electrical contact to trigger the BPS to shut down the power to the battery, just like what the push button does. I cut and formed a strip of brass for each smoke stack.
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I unsoldered the orange wires from the push button, and soldered the strips of brass to the wires. Not shown in the photo below, I actually had to add a length of wire to each to be able to comfortably reach the smoke stack area while the shell is off of the chassis (which makes interior maintenance easier). Together with the LED headlights, the shell is now "permanently" attached to the chassis. You could use miniature plugs, if you wanted to.
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I used 5-minute epoxy to glue the brass strips to the underside of the shell, with each wiper directly under a smoke stack opening.
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Since those wipers are going to be right above the BPS circuit board, I didn't want to take any risks and applied a liberal amount of liquid electrical tape over the brass strips.
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Here's the handy tool I formed from some brass wire. The wire's diameter is just a bit smaller than the holes in the smoke stack. The handle makes it easier to use the tool.
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I just insert the brass tool to connect the two brass wipers under the smoke stacks to turn off the power from the battery. Note that you only have to do this at the end of the operating session, so this isn't going to be done too often. I put the engine on the track and tested the final set-up. Everything worked great. I turn the battery power on with the wand on the pencil, by placing it right behind the front smoke stack. I can then run the engine. When I am done running, I briefly insert this handy tool, and the engine is off! I love it! All with no external modifications to the engine.
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Turning Power Off (revised version)

External Reference:
Inspired by Neil's HO-scale installation example, where he used a second reed switch to turn the battery power off, I went back into my engine and retrofitted my installation. I disconnected the two orange wires from the brass strips that I had glued under the shell's exhaust stacks (as shown above), and soldered them to the second reed switch. I placed this second reed switch away from the one on the BPS board to avoid interference. As you can see in the photo below, the one in the center of the engine turns the battery power on, and the one near the front of the engine turns it off. A quick test proved that it worked. I highly recommend taking this approach to controlling the battery power in your engine, regardless of in which scale you model.
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Interior Cab Light

While I had the engine open, I wanted to go ahead and take the time to connect the model's interior cab LED to one of the additional function output leads of the NCE decoder. It, too, needs a resistor, so I used a 1K ohm 1/4W resistor, which I soldered directly to the function output on the back side of the board. There is a separate pre-drilled hole in the back of the board for the blue-wire common, but the one blue wire I had already installed for the headlight LEDs is also connected to that common, so you don't need a second blue wire to implement the cab lighting. As a matter of fact, S-Helper Service had already connected the cab interior LED to the common blue wire inside the connector on the underside of the cab roof, so there is really nothing to do here. The brown wire connected to the other lead of the cab LED needs to be soldered to the resistor. Once done, a quick test also proved that the connection worked. On the S-CAB controller you press "FG1" button followed by a press of the "1" button to turn the light on. Press those same buttons again and the light goes off. If you solder the brown wire to the other function output on the board, you will need to use the "2" button instead.
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Here is a close-up of the resistor connected to the pre-drilled hole of the decoder board. I wrapped it around the board so that I could more easily solder the brown wire to the resistor.
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External Reference:
Here is a top-down view of the final installation of all of the electronics. Note that I had put liquid electrical tape around the leads of the new reed switch (the front one), just in case there might be any contact with the headlight LED's wires. I also put a new wrap of electrical tape around the solder joint of the brown wire, which will then hold the cab light resistor in place. Re-installing the shell requires pushing the decoder board side into the shell until it clears the cab's weight. The shell can then be pushed down and be positioned such that the four screw holes line up.
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The cab interior LED is hard to photograph, but you can see the engine sitting on a foam pad, with the cab LED turned on.
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Testing & Results

External Reference:
I use the headlight to determine if the battery power is really off, and then I push the throttle up to make sure that the motor doesn't respond to that command. This is my test to make sure the battery power is truly off.

Another thing I have discovered is that one of my cabin cars (caboose), which has a super-capacitor in it to provide flicker-free lights, will actually leak power back to the rail, which the BPS circuit in the NW2 detects and turns itself back on! I usually wait about 5-10 minutes for the super-capacitor to fully discharge before turning off the NW2's battery.

I tweaked the CVs of the decoder and got the engine to run nice and slow, while keeping its top speed reasonable for an NW2. For a demo of its slow speed performance, follow the video link above, right.

As a stress-test, I ran the engine back and forth on my layout, which has about a 20-foot mainline. It had the headlight on, but it was not pulling any cars. It was tedious, but I wanted to find out just how long the battery would last. The night before I had charged the battery overnight so I was sure that it was full. I turned off the Digitrax system completely. No power on the rails. Then I ran the engine purely on battery power back and forth. I varied the speed every so many runs, including maximum speed sometimes. The engine shut down just a few minutes short of THREE HOURS! Wow!

My next test was to see how this engine behaved on our Digitrax-powered club layout (the Houston S Gaugers). We had a one-day show. As my S-CAB article mentions, Neil indicated that the engine's drain on the battery is faster than the battery can be recharged. However, I figured that with the layout providing power and my home layout test showed three hours, I was hoping to get at least four hours of running out the engine. I started it with a 15-car train when the show opened at 10am. At 4:30pm when we started to take our layout down, I turned off my engine. It had run the entire day! It never stopped (which is a sign that the battery is drained), and it kept on running when one of the other guys caused a short on the layout that shut the Digitrax system down. Several people came by the layout throughout the day to see if the "engine with the battery" was still running! These two test results were way better than I had expected. I would have been happy if I could get a one-hour ride out of the engine on my home layout and maybe a two- to three-hour run on the club layout. This was fantastic. It convinced me that the system works, and that I need to convert all of my engines.

Fitting Cab Interior

I finally took some time to figure out if I could get the original cab interior back into the cab. Here's a photo of the original part.
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And this is my situation. Part of the decoder/receiver combo board sticks into the cab where that interior used to sit. However, there is space to the left and right of the boards.
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I cut up the interior part, liberating the two crew members. The console parts were removed from the center section. I then used Aleene's Tacky Glue to glue two of the parts directly to the electronic circuit board. To make all this electronics look less like what it actually is should you glance into the cab, I used Floquil "Burlington Northern Green", which was the closest match I had to the cab interior color, and painted the electrical tape and the top surface of the board and its external wires (sorry about the out-of-focus photo).
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Here's a close-up of the cab. I glued the two sections holding the crew members to the interior of the cab using Aleene's Tacky Glue (again, sorry about the out-of-focus photo).
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After very carefully navigating all the wiring and the boards, I was able to get the body back onto the engine's frame. In this photo I turned on the cab's interior LED, so the crew and the console components are visible. They are not in the correct position, but it is good enough for a casual glance inside. At least it makes it look "busy" in there.
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Here's another view of the cab interior.
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