My new decoder-is it working correctly?

[Anonymous]

I just installed my first DCC decoder. It is an MRC D13SR hard wired to an Athearn F7A BB loco. Wow, couldn't believe how simple and quick that is to do. I have never soldered wires so small. Anyway, I don't have my DCC unit yet, so after installing the decoder, I put it on my DC layout (per instructions) and gave it a nudge. Everything works, but no matter which way I turn the throttle, it moves forward-no reverse. I think this is correct because now the decoder is controlling it, and without a DCC command, it will only run forward, right? Before I screw up something, I thought I would ask all you experts. Also the headlight is very dim. Is this normal also?
Tim

[rrinker]

I guess you mean an NCE D13SR? I pulled up the instructiosn for this and I didn;t see any mention of DC operation, other than a "brake on DC' feature. So I still do not know from that documetn if this decoder supports autmatic analog conversion. Decoders that do work just fine on DC - there is no different, the work just as they did on DC before having the decoder installed, with the exception of the light bulbs. Decoders that do automatic analog conversion usually have a CV that allows setting which functions are on when running on DC power - for example,y ou would program is so that F0F is on in DC so the headlight would come on. The manusl for the D13SR also makes no mention of a CV to set this. Leading me to believe that this decoder does not operate on DC, hence what you are seeing.

--Randy

[Anonymous]

Randu,
I'm sorry, you are correct, it's an NCE decoder. The instructions say to test it on a DC track first. Per the manual:
"We recommend that the first full power testing be done on regular DC."
I presume this is to make sure the polarity is correct, as they make the comment about switching the wiring if the engine runs in reverse. As I said, I don't have my DCC yet, so programming and further operation is impossible at this time. What I was thinking is that since DCC works with track voltage and polarity constant, it is the decoder that actually makes the changes in direction. I have the decoder installed per the instructions, but I didn't know what to expect on it's maiden voyage.

[Anonymous]

I've never used NCE so they could be different. With my Digitrax DZ123 equipped N-scale locos, running them on DC tracks, the direction switch DOES work. I can drive it in either direction with a buzz pack. Don't remember the light bulb though.

[rrinker]

Tim,

You are correct that the decoder handles the direction, when running under DCC. But if it does analog conversion, which means it can run under DC, what happens as that the processor in the decoder closes both legs of the H-bridge motor drive circuit when it doesn't detect a DCC signal. This is the equivalent of having a switch in each leg of the motor, and closing it - the motor is effectively connected directly to the track, this direction when running under DC is controlled exactly the same as with a normal DC locomotive, that is the polarity on the rails determins which way the engine moves.
If you have access to DCC to program it, try doing a factory reset. On that decoder I believe I saw in the instructions that a factory reset is done by programming CV30 with the value '2'. A lot of odd decoder behaviors cna be fixed by doing a factory reset, there could be odd settings left over from factory testing or something.

--Randy

[cmurray]

Jim Scorse is the man to ask at NCE. He could tell you anything you need to know about NCE decoders.

[e-mail]

[Anonymous]

Well, for those that don't know, you need to isolate the headlight on an Athearn unit also. I removed the wiring from the headlight, and the engine ran fine. I e-mailed NCE, and that is what they said also. Nice guy (Larry), very quick response-less than an hour. Gave me quick info on what to do.
Thanks for the responses!!
Tim

[rrinker]

Well,t hat would explaint he dm headlight, but it SHOULDN't cause a problem with the motor drive. What you had, if you left the light as is and only connected the white wire to it, was "half wave" lighting. This is an accepted method of wiring lights to decoders, TCS even included half-wave in their table of resistors for various lighting solutions in their manuals. When you use 12v bulbs, you just get a dim light, when you use 1.5v bulbs, or LEDs, you need a different resistor value than if you just hooked the bulb or LED to the blue wire and the functon wire.
The main reason for half-wave lighting is for steam locos where it can be impossible to run a second wire up through the boiler. Or in other oddball setups like the Proto2000 SW9. The NCE SW9-SR decoder for that loco is set up to use half wave for the rear light, simply because it is a monumental task to take the cab and cab interior apart to replace the bulb, and the required resistor for full power would be too big and get too hot, possibly melting the shell. For whatever reason, if I just plugged in the decoder in both my my Stewart locos, they would run with half wave lighting. I guess so the resistor tht's in them for DC peration will also be the correct value for DCC. Well, I bypassed that on mine and put in a correct resistor, no half wave lighting for me if I can avoid it.

--Randy

[Anonymous]

Randy,
I had both the white and blue hooked up, per instructions. Because the blue wire is labelled positive in the diagram, that's how I rigged it-blue for positive and white to negative. According to NCE, that is fine IF the lighting is isolated. Mine wasn't. I still don't know which wire I am supposed to connect, though. Do I hook up the blue positive or the white negative? Doesn' the positive blue wire SEND power, then the lighting grounds through the frame? If I isolate it, it needs both, but if I install the light the way Athearn has it, I would think I should use the blue wire, right? Isn't the white wire a ground? Or am I backwards-do I hook up the white negative wire as you say?

[rrinker]

What they told you is correct. You have to either isolate BOTH terminals of the light and hook up the blue wire and the white wire (assuming front light here - white is normally F0F for forward light, although with programming that can be changed in many decoders). Or, you leave one side of the light connected to the frame and connect the other isolated terminal to the white wire.
You are correct that the blue is the positive for functions. This is the common for ALL functions. The actual function lines like the white and yellow wires are in electronic terms "current sinks". Think of it like a kitchen sink - the 'water' is there, supplied by the pipe (blue wire) and the drain lets it out (the white wire). (lest you think I'm clever, I stole that explanation from Bruce Chubb's original C/MRI articles from the 80s. I'm an engineer, but not a very good teacher, as my wife is fond of saying. She IS a teacher).
Hooking up both wires to an isolated bulb is regular wiring, 'full wave' if you will (not technically correct but if the other type is half wave..well...). For half-wave lighting, one side of the bulb is connected to the frame (or a track pickup - but only ONE rail - in the case of an Athearn BB loco, one and the same with the frame). The other terminal of the bulb must go to one of the function leads. If it went to the blue wire, it wouldn't work. I needs to be connected to the white or other function lead so that you can control it. Only one half of the DCC waveform will be positive in reference to the function lead, so the bulb effectively sees only half the voltage. Half the DCC waveform. Thus, half-wave.
If you connect the BLUE wire to the free terminal of the light bulb, you have positive on that terminal. During one-half of the DCC waveform, the voltage on the track (thus the other terminal of the bulb) will be negative in reference to the blue wire, and again the bulb will light, with half voltage. But it will be uncontrollable. No function key turns on and off the blue wire.
Clear as mud?

--Randy