This old Kato mechanism is one of my favorites and I think I've done more of these types of installations than any other. The same mechanism is used for both the SD40 and SD45 models. The more recent releases take a board type decoder but for the older releases you will need to get the frame milled. The
Aztec Mfg. part number for a frame milled to take a small decoder is TM3002.
The decoder I am going to use in this one is the Zimo MX621. It is similar in size to the TCS M1 and comes in a little plastic box.
Little boxes like this are perfect for storing spare motor brushes, brush springs, brush caps, etc.
The Aztec milled frame comes with clear instructions for the basic installation. It is mentioned in the instructions about using the blue wire for decoders with transponders. This is because for the transponder function to work, the blue wire has to be connected as a return for one of the function wires.
As this decoder does have a blue wire, I am going to follow the option of using the blue wire as the return path for the yellow function wire on rear light board.
This photo shows the completed decoder assembly.
I have replaced the stock yellow LEDs on both boards with bright white LEDs. On the rear board, I also replaced the stock 270 ohm resistor with a 750 ohm resistor. This is because with the blue wire, the signal going to the LED is going to make the LED brighter and I wanted to reduce it to be close to the same brightness as the front LED.
The motor assembly goes back together in a very similar way as it does with most installations. What is different about this installation is that the decoder is in the fuel tank and all of the wires go up around the motor to the top of the frame.
This photo shows how everything is laid out just prior to re-installing the motor assembly in the frame.
This photo shows what it should look like on the other side of the right side of the frame after the motor assembly is placed into it.
The wires should lay side by side and not be twisted over each other. It is also very important that none of them have gotten caught or pinched between the motor and the frame.
The rest of the reassembly of this model is very straight forward. These are normally very quiet running engines but I have noticed ones that have sat around for years get a little noisy. This is almost always cured with a small drop of Labelle 102 or other plastic compatible gear lube on the spots shown in this photo.
In conclusion, this would be a great model to try as your first wired decoder DCC install. Other currently available decoders that would fit in the pocket milled in the frame would include the Train Control Systems M1, M4, or Z2, or the Digitrax DZ125 or DZ143.
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