Genesis SD75 decoder instalation (NCE DA-SR)

Model Railroader General Discussion (Model Railroader)
1

Hi

I’m about to install the decoders in the locos and wondering if the motor needs to be isolated from the frame? I’ve read the instructions and they don’t say this needs to be done, so I’m just double checking from those who have done it. Also do the lights and ditch lights need drop resistors(what value is needed)?

Any help would be appreciated

Regards, Richard

2

I haven’t decodered any Genesis SD75’s but I’ve done a Genesis SD70 and a couple of Genesis F3’s.

Nothing against the DA-SR’s (I’ve used them myself, in other locos) but for a Genesis I’d recommend using a TCS A6X instead.

The Genesis bulbs seem to be finnicky and have a tendency to burn out if you don’t “over-resistor” them, but that makes them too dim.

However, the A6X has a 1.5v regulator circuit that works great with these bulbs and does not require the use of resistors.

Now, to answer your questions: The motor is already isolated, so you don’t have to do anything more there.

And yes, if you use a DA-SR, you will need resistors, one for each of the bulbs. 1000 or 1200 ohms should be about right (see above).

But again, I’d think long and hard about using the A6X for this particular model of loco.

HTH,

Steve

3

I concur with Steve, reconsider using the TCS A6X as you will have the easiest way to install, having 1.5 volt bulbs. Also, the decoders voltage regulator eliminates the concern for track voltage sensitivity affecting light output or resistor value calculations necessary for optimum light vs bulb life.

4

Thanks for the quick response. Unfortunately I already had purchased the DA-SR (went with Tony’s recomendations), I’ll try one and see if I’m happy with it.

Regards, Richard

5

To answer your original question, you do not have to isolate the motor because it already is completely isolated.

With the DA-SR decoder you must use resistors in all of the light bulb circuits or they will just make a bright flash and never light again. With the TCS A6X, the resistors are built into the decoder so separate resistors are not necessary.

My advice would be to get the A6X and save the DA-SR for use in something else, because soldering the resistors to the circuit board can be tricky if you don’t know anything about electronics and select the wrong resistors.

6

Not quite true. The A6X has a voltage regulator, which is not the same as having resistors.

Resistor values are dependant on the specific voltage drop and current requirements of the load (bulb, LED, etc.).

On the other hand, a voltage regulator is an active circuit (in this case, probably an SMD IC) that holds the voltage at a given level regardless of the current in the circuit. That’s what makes the A6X so good for those finnicky Genesis bulbs.

The newer DA-SR’s (Rev. D) have resistors on them that you can “activate” by cutting traces on the board. However, as the instructions warn, they are the correct value for LED’s only and should NOT be used for bulbs.

Richard,

The DA-SR isn’t an expensive decoder. Keep it for another loco, or sell it to someone. I use them in non-DCC-ready Athearn locos, attaching them to the top of the motor with double-sided tape.

Steve