USAT SD40-2

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Overview:

A nice loco, using the standard 3 axle USAT motor block.

Wiring for DCC:

USAT locos are "fun" because of all the different voltages and types of lights and weird commons, etc. R.J. DeBerg and I did this conversion by long distance, me with pictures and manuals, and him with the loco, from San Diego to Cumming, Georgia!

The picture below shows what you will find inside, the main board on the left (that has the switches on it) and the smoke regulator/control board on the right.

The picture below gives a closer look. I believe the three terminal regulator on the left is for the smoke power.

The picture below has an even closer view, and these two pictures will help locate the wires and connectors described next:

There are several connectors on the "upper" side of the board, will start at the upper left side, at #1, going to the right

Connector #1: upper left side (rear headlight and number boards)

3 pin connector with white, yellow, green wires, reading left to right

green - common - connect to decoder common (NCE D408 blue wire) (was positive in the USAT wiring)

yellow - number boards - two screw in incandescent bubs in parallel - connect to 133 ohm resistor then to ground (we used 2 diodes connected anode to anode, cathodes go to the rails, since NCE decoder does not have a ground. The makes the number boards always on

white - rear headlight- incandescent bulbs in parallel - connect to 200 ohm resistor then to decoder rear headlight (NCE D408 yellow wire)

note: voltage to these lights is between 3.5 and 4 volts

Connector #2: second from left (front headlight, number boards, strobe/beacon light on roof, cab light)

3 pin connector with white, red, black wires, reading from left to right

white - common - connect to decoder common (NCE D408 blue wire) (was positive in original USAT wiring)

black - front headlight - 5.5v- 70 ma - from 20v - connect to 200 ohm resistor, 1 watt then to decoder front headlight (NCE D408 white wire)

red - number boards, beacon, cab light - 6.2v - connect to 100 ohm resistor then to ground (we used 2 diodes connected anode to anode, cathodes go to the rails, since NCE decoder does not have a ground. The makes the number boards always on 

Connector #3: third from left - silkscreen on board says LED4 (rear classification lights)

2 pin connector with black and white wire

These go to red LEDs, 50 ma, running at track voltage,  There is a dropping resistor and a diode in the stock wiring, leave them

Connector #4: third from right side - silkscreen on board says LED (front classification lights)

2 pin connector going to the front class LEDs

"Connector" #5: second from right side

There is no connector here, the wires to power the smoke unit are soldered to the main board

Connector #6, at upper right of board has "S" silkscreen above it

This is power output to optional sound system, controlled by sound switch on main board.

There are small circuit boards that have the lights and the wires. The pictures below show the board in the back of the loco.

More shots:

The following two pictures help you see the wiring of the boards, and you can see the common that connects to the outer shell of the sockets and to the red wires of the headlight lamps.

The following picture shows the board that is in the front. I need R.J. to write up what he had to do to get to this board, it is "buried" in the cab.

And just for fun, here's a picture of the conversion in process, you can see the NCE D408 board near the rails, and a Phoenix sound board mounted in the end of the chassis by it's backup batteries. The amount of current drawn by the incandescent lamps calls for high wattage resistors!

More wiring notes:

Rear truck is motor2 and pick2

The way you can tell which connector from the truck is the pickups and which is the motor, is the track pickups have 4 wires going into the connector, and the motor has 2. (The pickups are 4 wires because one wire is the "skate" and the other wire is to the bearings in the sideframes that also pick up power from the axle tip, and thus the wheel.

Best thing to do is disconnect the existing JST's and wire up some new ones, or you can unsolder the wires from the board. Either way, make sure you leave the original wires and connectors on the trucks to remove easily for maintenance or disassembly.

DCC wiring:

The right hand rail pickups go to the decoder red wire.
The left hand rail pickups go to the decoder black wire.
The motor leads go to the orange and gray wires. (hook them up and reverse them if the loco runs backwards.)

There is a way to get the motor "right" the first time, put power to the motor from DC, when it runs forwards, the wire that has positive voltage from your supply is the "motor +" lead, connect to the orange wire. Most times it just as well to try it and see if you got it the first time.

Be sure to do this on rollers, in case you wire one truck backwards.

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