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Problem with Digitrax shd166D decoder

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  • Member since
    February 2018
  • 16 posts
Problem with Digitrax shd166D decoder
Posted by Fred4hp on Wednesday, February 5, 2020 9:16 AM

I converted an old Blue Box Athern to DCC with a Digitrax SHD166D decoder. The sounds are good but the low speed throttle is giving me a problem. The locomotive speeds up for a second and low speeds before settling back down. Repeats ever 5-10 seconds. I have checked all the wiring and Digitraz CV values and I am stumped at what is causing this. A Bachman Sound ON Borad loco does not have this problem on the system. I have also tested the Athern on a DC system and the speed is varying there too. Any help or ideas would be very appreciated.

  • Member since
    February 2002
  • From: Reading, PA
  • 27,321 posts
Posted by rrinker on Wednesday, February 5, 2020 9:40 AM

 That it does it on DC pretty much rules out the decoder, when running on DC, the decoder doesn't do anything with the motor power except pass it on. The number 1 reason for a loco mysteriously changing speed or randomly stopping on Digitrax is having the same loco selected on more than one throttle, but since the same thing happens on DC, that is not the case here.

 Any number of mechanical issues, all pretty easy to fix on those locos, could exist. I would recommend removing the decoder and switchign the loco back to plain DC to sort this out, as it will make things easier - for example, you cannot just use your DC power pack at the workbench and connect the wires to the motor to power it with the decoder attached, you WILL destroy the decoder. 

This site has a good explanation of how to clean and tune up Athearn BB locos:

http://www.mcor-nmra.org/Publications/Articles/Athearn_TuneUp.php

That's the beauty of the BB loco - they were relatively inexpensive, maybe a little rough out of the box, but with some simple work they purr quite nicely. There's a reason others copied their basic design. Make it run smoothly on DC, then put your decoder back in. If it runs well on DC, odds are it will continue to run well on DCC, but if it doesn't run well on DC, adding DCC won't fix it.

                                           --Randy


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

  • Member since
    February 2002
  • From: Mpls/St.Paul
  • 11,413 posts
Posted by wjstix on Wednesday, February 5, 2020 10:06 AM

I always believe in trying the simplest thing first, so - although I agree with Randy that if it's doing it in DC it's very unlikely to be the decoder - I'd still try changing the ID number of the decoder, or clearing out my DCC system of all consists, and try the engine again. What you describe happening is exactly what happens when you have the same engine assigned to two throttles, or as the trailing unit in a consist. If that doesn't change it, you might try putting in another decoder to see if that works better. It's possible there's something in the decoder that's causing the trouble.

If it is still doing it after trying those steps, it is most likey a mechanical issue of some sort.

Stix
  • Member since
    February 2018
  • 16 posts
Posted by Fred4hp on Wednesday, February 5, 2020 1:11 PM

I removed the decoder and returned it to DC. I checked all the soldering and it looked OK. I noticed that it ran slightly irregularly on slow speed on DC but OK at higher speeds. This is an old Athern Blue Box EMD1500. I tried two old Blue Box Atherns on DC, a GP38-2 and an SD 40 and they ran great on slow speed. I am begining to think the 1500 has a smaller motor and maybe some mechanical issues that only are evident at slow speeds. Anyone else noticed this with the older Atherns, larger units run smoother?

  • Member since
    May 2010
  • 6,390 posts
Posted by mbinsewi on Wednesday, February 5, 2020 1:28 PM

Yes and no, question for you, have you ever taken a BB loco apart and cleaned all the connections?

Remember the frame is part of the electrical system, and where the frame sits on the trucks needs to be free of all grime and oil.

Also check the wheels.  Every BB loco I redo, and convert to DCC, I install new NS wheel sets, the complete sets that includes the gear and brass blocks.

If you have done these before, which it appears you have, you know all the key points to check.

The Athearn drive train straight forward and easy to work on.

If you haven't yet, please check out the link that Randy has in his post.  It's anexcellent overview, step by step, on the complete servicing of an Athearn BB loco, no matter if it's GP or SD or SW.

Another item, if it's the older gray colored motor, it's drawing lots of amps, and rplacing it with the newer brass or yellow colored motor will improve performance.

Mike.

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