Digitrax DH121 decoder question?

Discussion in 'DCC & Electronics' started by trainsteve2435, Jan 23, 2007.

  1. Hello everyone, i have a Digitrax DH121 decoder that i installed in one of my locos. When i went to test it, as soon as i turn the control knob, it lunges and takes off. Basically it has no slow speed, just fast. What CV do i need to adjust in order to set the start speed? Thanks!:wave:
  2. Woodie

    Woodie Active Member


    Your decoder should NOT function like that, straight "out of the box" and installed, if you have not changed any settings, and wired it up correctly. Are you saying that the slightest bit of throttle, and it immediately jumps to top speed? Does it go any faster at all as you increase the throttle?

    Check to ensure the correct wiring. "red & black, too the track", and "orange and grey, move the motor on it's way"

    Even though it says it is "not available" for this decoder http://www.digitrax.com/v1/dh121.htm you can "reset" most decoders to factory setting by setting CV8 to 08. Try this on your programming track. This will clear any "accidental" CV changes you may have made, if any.

    refer Digitrax manual http://www.digitrax.com/v1/ftp/Decoder Manual.pdf page 58.

    This will also reset the decoder address to 03.

    You will need to reset the "loco ID" to your preferred (or just select loco 3 to ensure correct operation of the loco back on the main).

    Otherwise, try CV 2= 0, CV 6 = 128, CV 5 = 255 (to spread the start/mid/high voltage for the speed steps.)

    however if you have mucked with CVs (Specially CV 29 that contains the decoder address, amongst other things, like use "speed table or not" an you've set ALL speed steps to "top speed" (accidentally), then I 'd recommend a decoder reset.

    Let us know how you go.
  3. Well, i reset CV8 to 08 and it still runs wide open. The loco runs at full speed no matter how much you turn the throttle. When i reset CV8 to 08, that should have reset the loco number to 03 right? It didnt change. Seems to me like i might have messed with CV29 a good while back, but i cant remember for sure. How can i reset the decoder, or is that what CV8 does? Anyway, it used to run fine until i messed with it a good while back and i have lost all the paperwork that came with it. Like i said, when i turn the throttle up, it does nothing until it hits 06 on the throttle, then it runs wide open. Im puzzled. Any more suggestions? Thanks!:oops:
  4. Woodie

    Woodie Active Member

    Did you reset CV8 to 08 on the programming track?

    CV 8 contains the "manufacturer's ID", however setting it to "08" resets ALL CVs back to their factory defaults.

    Some decoders you need to "recycle power" (i.e.lift it off the track, and put it back on again, or turn your system off etc) for the reset to take place when power is again applied to the decoder. Not sure if Digitrax requires this. There's some links in my post above to some documentaiton and manuals for that decoder. Some decoders also will not let you "reset" them on "the main".

    If the "reset" is successful, you will only be able to address that loco as short address 3. It should not respond at all to the old address.

    You may have also accidentally "locked" the decoder to prevent "accidental" CV changes. (if this decoder supports that) and you'll need to check the manual on how to "unlock" it.

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