DCC people please help!

[Anonymous]

I have 2 identical SD70MACs from Genesis. Both run great on DC and one runs even better on DCC. But the other one just sort of lurches along while under DCC control. What CVs would i change to fix this? I was thinking speed tables but im not sure.

Thanks

[cacole]

This sounds more like dirty wheels than a DCC problem. What specifically do you mean by "lurches along?" -- this does not appear to be a CV problem. Clean the wheels on both locomotives and let us know the results.

[Anonymous]

wheels are clean, i tried that a while ago. by "lurches" I mean its like it cant decide on a speed step. Its always speeding up and slowing down within a range of probably 10 scale MPH. Its not a gradual change either, thats why I say "lurches"

[cacole]

The decoder's default speed table should suffice, provided you haven't created a custom speed table in one decoder and didn't enter an identical table in the other. Since you have identical locomotives, and assuming you have identical decoders, you probably need to adjust the speed step. You indicated that you had been running these locos on an EasyDCC system, and someone may have set the decoder's speed step to 14 instead of 28/128. Reprogram both decoders to identical speed step values (CV29) and see if this helps.

[jwr_1986]

If your decoder is equipped with some form of speed compensation such as BEMF you might want to check that. If programmed to strongly this can cause such a lurch. Especially when in a consist.

Jesse

[Anonymous]

Before making a bunch of changes, can you swap decoders and see if that makes a difference? That would show you if it was the decoder or something else

[darth9x9]

If you can't easily swap the decoders as Twee4469 suggested, reset the problem loco's decoder to the factory defaults. That will ensure you start from a clean slate.

[Anonymous]

Thanks for the tips, i will try them tonight. I asked at my LHS yesterday and the guy there said it might be that the dither is set too strongly. Does anybody know what CV controls the dither?

[Anonymous]

Dither is a manufacturer-optional feature, so different vendors control it with different CVs. What brand are the decoders?

[darth9x9]

Ahhhh, you didn't mention they were TCS decoders....in that case.....

Dithers are used at low speeds to help overcome the "striction" of locomotive mechanics. It provides better and smoother low-speed performance. It is similar to a start voltage but not the same. Dithers give an additional higher-voltage pulse to the normal motor output voltage. The effect is similar to adding pulse power on conventional DC power packs. Dithers are especially helpful on locomotives that have high starting friction.
There are two parameters that control the operation of a Dither in a TCS Decoder. Program the frequency of the additional pulse by putting a value in CV56. A value of 1 will give the highest frequency of 30 additional pulses per second. The higher the number (ranging from 1 to 255), the lower the frequency. I recommend to try starting with a value of 5. The practical range for CV56 is 1 to 10.

CV57 controls the amplitude of the Dither's pulses; again, valid values range from 1 to 255. (1 being the lowest voltage and 255 being the highest voltage) I recommend starting with a value of 25 or 30 for CV57. The practical range for CV57 are values of 5 through 50.
When giving CV57 a value of 0, the Dither is disabled. Dithers will be automatically cut off and not used above 21% of full speed. It can be used in addition to start voltage, kick start, and/or by itself. Factory default for CV57 is 0. CV57 will also be reset to 0 when using the Factory Reset feature.

The Factory Reset feature can be enabled by setting CV30 or CV8 to a value of 2, then turning the power off and back on.
Try experimenting with Dithers to achieve better low-speed operation. The effect will narrow the window between what point the locomotive starts to move and at what point it will stay running.

As for resetting a TCS decoder, to reset CV's:

1. On the programming track, write a value of 2 into CV30 or CV8.
2. The decoder must now go through a power down, power up cycle.
3. The reset will take effect upon power up.

Note: Writing to CV8 doesn't actually change the value in CV8, because it is a read-only register. When writing to CV8, it will write to CV30. This enables a factory reset with command stations that use physical register programming, like the MRC 2000 Command and the Atlas Commander.

CV's that will reset to factory default: 1, 2, 3, 4, 5, 6, 17, 18, 19, 29, 30, 49, 50, 53, 54, 55, 56, 57, and 61. The user-loadable speed table is not affected by the reset.

I hope this helps.

[darth9x9]

QUOTE: Originally posted by jsalemi

Dither is a manufacturer-optional feature, so different vendors control it with different CVs. What brand are the decoders?

Actually as far as I know, Dither is an exclusive TCS feature that provides outstanding slow speed control. I have not seen Dither as a feature of any other manufacturers' decoder. If I am wrong, please tell me so.

[Anonymous]

Dither is the TCS name for it, but other vendors have similar functions that they call a different name. I believe NCEs decoders have a similar function, and some Digitrax decoders have a "motor frequency" function that does something like dither as well. Can't say for sure right now what the other vendors call it, as I'm away from home and my decoder documentation.

It's possible the dealer just referred to it as "dither" without meaning a specific brand's name for it, which is why I asked.

[johncolley]

I had the same lurching problem on a Stewart FT with Digitrax DH163D's and doing the CV56 to 1 solved the problem. Also going to 128 speed step made a big difference. I am using the new Prodigy Advance and as someone told me," the toys are there, play with 'em and learn to use 'em." I would strongly suggest whatever decoders you have, go online and download whatever free mobile decoder manuals are available.