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Here's another decoder install I did recently, on a Pecos River Brass 4-6-4 Santa Fe that I got from a fellow forum member. This is a sharp looking engine, with a cool fully sprung drivetrain, so I was stoked about getting it up and running. It also posed some interesting challenges, which is half (at least... ) the fun.


This install was similar in many respects to the one I did on my 4-8-4 Greenbrier, so check that thread as well if you are interested. The basic components, block diagram, and 'buzz out' are described there, so I won't duplicate them here.

There were a few noteworthy differences on this one though, that might be of interest.

Motor start voltage and no-load current

One of the first tests I do on an engine once I get it all stripped down is a motor startup voltage and no-load current test. This can give a really good indication of any fundamental motor, geartrain, or linkage issues. (This is actually really easy to do just about any time you have the shell off, and is a good preventive check.)

I do this test using a benchtop DC power supply. Starting with the lowest voltage setting, I slowly ramp up the voltage and look for the point at which the motor just starts to turn. For a typical motor installed in the locomotive, I normally see startup at around 0.8 to 1.0V and 0.2-0.3A. If the voltage is much higher than that, something probably needs attention. This is also a good time to let it run just barely ticking over and watch for any irregularity in the motor speed or wheel speed. Any slight increase in drag will cause the motor to stall. If this happens at a consistent location, it usually indicates an issue in the drivetrain.

The key aspect of this check is that, unlike with the decoder installed, the voltage here is not feedback controlled. With the decoder installed and running normally, it will try to drive through increased load by boosting the motor voltage. While the engine may operate, this can mask issues that can lead to long (or short!) term mechanical and electrical reliability problems.

Then I ramp up to about 5-7V and look at the current. The motor should be humming along at a pretty good mid-speed pace at this point. With just the drivetrain friction, current should be in the 0.1 to 0.2A range. This is a good time to watch and listen carefully for any noise, vibration or rattling, which can indicate wear in gears, lack of lubrication, or minor binding in linkages and so on. You can apply some load at this point as well, just by grabbing a wheel a bit, and see how things respond.

I do these checks in both forward and reverse directions by reversing the polarity. If the forward and reverse direction voltages differ by more than a couple tenths, that's another indicator. If there are multiple motors, I test each motor individually and compare. Again, the values for both motors in both directions should be similar.

In the case of this engine, I was a bit appalled when I measured a starting voltage of over 2.5V and current of about 0.7A! Clearly something needed help, and it didn't take long to figure out - the old gearbox lubricant was completely congealed around all the gears. After a complete teardown, cleaning, and reassembly with proper lubricants, the starting voltage and current were right where they should be, and everything was running really smooth. Another example - when I did my Lionel docksider a while back, I discovered that the quartering on one axle was off by one spline on the shaft, because the motor was stalling out at a repeatable wheel position.

Decoder Mounting

There weren't any obvious mounting points for the decoder assembly on this one, so I fabricated some mounting bosses out of polystyrene. I bonded them in place in the boiler and then match drilled them to the decoder assembly:


Test fit of the decoder assembly:


Smoke Unit

There was no factory smoke fitted on this engine. But, it has a nice looking smokestack, so....

I started with a standard MTH diesel smoke unit, mostly because that's what I had sitting around at the time. I cut the legs off and filed the base smooth, then drilled and tapped some holes and added my own bracket. I replaced and terminated the wiring with 0.1" header pin connectors:


It's a bit of a secret handshake getting it in place, but after some test fitting and drilling some holes to pick up an existing bracket near the front of the boiler, here it is in place:


I fashioned a gasket out of a rubber grommet to seal between the top of the smoke unit and the smokestack. Here's a few from the top:


I designed and built a new helper pcb for this install, combining the rectifier, regulator, and power switch components onto a single board. Conceptually, it's the same as what I detailed in my smoke and couplers post - just a bit more compact. Programming is the same, and detailed in that post as well.

At the moment, I am running without the smoke, because I haven't figured out a great way to replace the ~1lb ballast mass that was there originally. It needs the mass more than the smoke for the time being.


When I bought this engine, I knew it had no electronics in it. And by 'no electronics', i mean nothing! No tether, connectors, brackets or other bits which I am frequently able to re-use for the decoder install. So one challenge was coming up with an elegant way to do the tether.

Using the same component layout as on my Greenbrier, I knew I would only need four wires going to the tender - 2 each for speaker and backup light. After a bit of preliminary looking, I decided to add a fifth in order to add a chassis ground connection between the engine and tender as well. So I needed to accommodate five wires total.

On the engine side, I used a prototyping board that I cut to size to fit where the old tether board went. With standard 0.1", 90 degree header pins installed, they lined up perfectly with the existing cutout in the engine shell. The inside wires are connectorized for easy hookup to the decoder, which is installed in the boiler.



On the tender side, I used all black wiring secured internally with a bit of a service loop to allow it to extend in and out of the tender somewhat. The wires go into a standard 5 conductor housing. This plugs in to the header pins on the engine. The finished result came out pretty nice.




I ended up with an assortment of different lighting types on this one. For the headlight, I used a Brickstuff LED as I frequently do. These are a nice color, very bright, and have a resistor built in so they can run directly off the 5V supply on the decoder. I held it in place in the boiler cover with a small grommet. I also installed one for a cab light.

One of the front marker lights was burned out, so I figured I'd replace both while I was at it. For these, I went with the Miniatronics 1.2mm green incandescent. These are 1.5V lamps, so I installed resistors in series with them so they could be wired to the 5V decoder supply. You could also skip the resistor and dial the PWM duty cycle way down on them, but that's a little riskier, and reduces the range of settings available.

The rear light is a 1.2mm 5V incandescent, and was working, so I left that one alone.

Power and Ground

One thing I was not impressed with on this engine was the power pickup and grounding scheme. Two of the three pairs of drivers have traction tires, and neither the front nor rear trucks had any electrical wipers on them, so I was concerned about good continuity to ground. It also has just two pickups under the engine, at about 5" spacing, which I expected would cause problems in some areas.

I started with the grounding. First, I added a ground wire between the boiler and the tender, described above. Following that, I added axle wipers made from 0.010" phosphor bronze sheet to the front and rear trucks.

Here is the front truck:


It's installed at an angle to put the brush as close to the center of the front axle as possible.

Here it is installed, with the ground wire tied to the chassis:


And here is the rear truck:


I didn't see an elegant way to get both axles, because of the suspension spring in the middle, so I just did the front one. Here it is, installed:


All that helped, but I was still getting dropouts in certain spots on the layout. I will probably add a couple pickups to the tender when I get around to it. In the meantime, I installed a supercap keepalive that I cooked up (more on that in a later post...), and that's working great.

All said and done, it's a great runner so far, and really nice to look at! The sound set, and the whistle in particular, are really in character.



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