Digitrax DH165A0 and an *old* Athearn Loco

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Zombie13

New Member
I got a DCC decoder for Christmas to go in my really old (like, 30+ years) Athearn SDP40 Loco. Electronically I can do the soldering and wiring needed, but I have questions first"
1) The docs list both sides having left track and right track pickups. Do both of them need to be wired or can I connect to RH TRK on the 'front' and LH TRK on the 'rear'? Then solder the motor to the orange/gray pins and the Front light (LED Replacement to F0F (both pins) is all I should need to do, right?

2) For the LED, do I need a resistor? The docs make it sound like I don't if the LED is rated for less than 15mA.

3) Does anyone know if there is a way to have this run in DC mode as well? (maybe does it by default?)

Thanks in advance.
Z.
 

NYC_George

Well-Known Member
I did one of these locos a long time ago. I've forgot what had to be done but I wouldn't do anything until I found out. Decoders are easy to blow with improper wiring. I would say the bulb, it's 1.5 volt right, would need a resistor but I'm not sure.
George
 

D&J RailRoad

Professor of HO
The decoder will have 12 volts output on the white wire forward headlight and yellow rear light with the blue wire as the return. Check the voltage with a volt ohm meter. I think you will need a 680 ohm resister for that.
The motor MUST be isolated from the frame or you may burn up the decoder.
 

kjd

Go make something!
Old Athearns use the frame as part of the circuit. Connect the two tabs from the trucks to the red wire. Connect the black wire to the frame. The orange wire goes to the top of the motor and the gray to the bottom. The blue wire goes to the long leg on the LEDs and the white wire to the short leg of the front LED and the yellow to the rear. The white and yellow should have a 1k resistor in series with the LED. Make sure the bottom of the motor is not making any electrical connection to the frame. That'said basically it.
 

NCRC5315

Member
The decoder will have 12 volts output on the white wire forward headlight and yellow rear light with the blue wire as the return. Check the voltage with a volt ohm meter. I think you will need a 680 ohm resister for that.
The motor MUST be isolated from the frame or you may burn up the decoder.
The DH165AO, is a drop in replacement, therefore there are no wires, just solder pads to add functions to. The light outputs are regulated, there is no need for resistor for either lamps or LED's. The outputs are sinking, therefore the "blue" is the 12volts, and the others are the "ground".
 

bnsf971

Gomez Addams
Staff member
The tabs sticking up on both trucks is the same polarity. To get power from the other rail, you will need to either drill and tap a hole in the frame, and solder that rail's wire to the machine screw you put in the hole, or solder the wire to the ground bar for the cab light.
 

trailrider

Well-Known Member
Old Athearns use the frame as part of the circuit. Connect the two tabs from the trucks to the red wire. Connect the black wire to the frame. The orange wire goes to the top of the motor and the gray to the bottom. The blue wire goes to the long leg on the LEDs and the white wire to the short leg of the front LED and the yellow to the rear. The white and yellow should have a 1k resistor in series with the LED. Make sure the bottom of the motor is not making any electrical connection to the frame. That'said basically it.

The older Athearn motors have a brass tab on the bottom side of the motor, which contacts the frame. You must pull the motor out and solder the gray wire to this contact. Before reinstalling the motor, place some thin insulating material between the motor pole and the frame. I've used a piece of Scotch tape! It works, as long as you insure any metal on the motor does NOT touch the frame. The black wire goes from the decoder to the frame. Those metal pieces coming from the trucks that rub on the metal strip that snaps on top of the motor, don't provide good contact. You need to solder wires from the strip to the metal parts that come from the trucks for reliable connection. Leave some slack to allow the trucks to pivot freely. The red wire can then be soldered anywhere on the strip or the truck tabs.
 



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