Athearn HO SD45T-2 Directional Lighting Problem

    I purchased three Athearn SD45T-2 locomotives from my local hobby shops because they are so Southern Pacific prototypical.
    The directional headlight lighting in these units does not operate the same on each unit going forward and backward, or from unit to unit. On the first two SP units the rear headlight pair come on before the locomotives start to creep backwards, which is great. However, the front headlights need more track voltage to illuminate, at which time the locomotives are already traveling forward. On the third unit, SSW9372, the locomotive requires considerably more voltage applied to the tracks than the other two units to get the front headlights to illuminate (by then the SSW locomotive is racing around the tracks). The rear headlight illuminates at a fairly low voltage, but not before the locomotive has started to move.     Has anyone else noticed this operation or is it unique to my units? This is certainly not the way my Atlas units work. They light before the locomotive creeps, in either direction.     I am considering removing the bulb wires from the circuit board and wiring all six bulbs in series with electrical pick-up and return through the trucks. Having front and rear headlights on all the time is better than what I have now. Anyone have an opinion on whether that would work? I don't know the electrical layout of the circuit board and am wondering if removing these wires would mess up any other functions on the board? I haven't looked inside yet, but are there good places on the trucks to pick-up and return the voltage, like on a standard Athearn truck?     Thank you very much for you comments, hopefully helpful.
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My "bluebox" Athearns (GP 38 GP40 and F7) all come with just a 12 volt bulb wired across the rails. In this case, you need quite a bit of track voltage before the bulbs light, and that much track voltage makes the locomotive move at a pretty good clip. I converted my Athearns to the "constant lighting" circuit. This uses a 1.5 volt bulb across a pair of silicon diodes and in series with the motor.
N-rail-----1.5VBulb--------motor--------S-rail |-->|--->|------|      |--|<---|<------|
4 diodes
1.5 volts is enough to light the bulb to full brilliance but not enough to move the train. When track voltage is high the diodes conduct and limit the voltage across the lamp to 1.4-1.5 volts so the lamp burns at constant brightness irregardless of track voltage. A diode has a constant 0.7 volts across it when forward biased. Two of them in series give 1.4 volts, just right to light a 1.5 volt bulb. A second pair in the opposite direction lights the lamp when the locomotive is moving in reverse. I mounted the diodes and the 1.5 volt bulb on a piece of vector board and them glued the vector board to the ceiling of the cab. This lets the bulb shine out the headlamp but not out the cab windows. I no longer do directional lighting. I like the headlamps at both ends of the locomotive to light up so I can see if the locomotive is getting power no matter which way it is facing. I have seen plenty of prototype locomotives with the rear headlight on in daylight, putting a spot of light on the end of the first car.
David Starr
Byron Lane wrote:

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Hi David, Thanks for the detailed response. That's excatly how I want to set up my lighting to work also. I'm still digesting your circuit description and how it would work for headlights on both ends. One question I have is why are both sets of diodes in series with the motor? Seems like that would limit voltage to the motor. The four bulbs in the SD45T-2 are 1.5v. I guess I will go play around with a older engine until I get it right.
On Tue, 13 Sep 2005 01:12:08 GMT, "David J. Starr"

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Byron Lane wrote:

It does. The constant lighting circuit robs 1.4 volts from the motor, reducing the top speed somewhat. But the top speed of HO locomotives is usually 200 scale miles per hour, so limiting top speed to something less is a benefit. Suppose you crank up the track voltage from zero to 1.4 volts. The pair of diodes in series drops 1.4 volts, so the motor sees zero volts and hence does not turn, and the train stays put in the station. But the 1.4 volts dropped across the diodes lights the lamps, so the train stands still with the lights on. If you raise the track power to the full 12 volts the power pack is capable off, the diodes still drop 1.4 volts, so the lamps stay lit at the same brilliance and now the motor gets 10.6 volts, which is plenty. Diodes are non linear devices, and unlike resistors, they show 0.7 volts across them no matter how much current is flowing thru them. This "diode drop" stays constant while diode current varies from just a whisper, to enough to melt the diode. The GP 38's want a bulb in the long hood and a bulb in the cab ceiling. You can connect both bulbs in parallel (across the diodes) and they both light. Tip. You can buy a "full wave bridge" at radio shack that contains the necessary four diodes in a single package for something like a buck. Individual diodes can be robbed out of old electronic equipment.
David Starr
The four

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