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A high-pitched hum?

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  • Member since
    May 2008
  • 14 posts
A high-pitched hum?
Posted by pschuman21 on Tuesday, September 2, 2008 3:07 PM
I recently took out a factory installed decoder in a Atlas GP38 and replaced it with a Digitrax one that is made for Atlas locos. I wanted the new white LED highlights instead of the factory yellow decoder one. The installation went smothly and tested the loco (before putting the shell back on) the loco was quiet and ran well. After I put the shell back on a low hum came from the loco and the engine noise was louder. What's worse is that when I dim the headlights the hum gets louder. Can anyone help me out? Has this happened to anyone else? Should I just put the factory decoder back in? Any solution would be a great help, Thanks. 
  • Member since
    April 2004
  • From: Ontario Canada
  • 3,425 posts
Posted by Mark R. on Tuesday, September 2, 2008 3:31 PM

Sounds like an older variety of decoder. The pulse width modulation of earlier decoders worked at a lower frequency that you can actually hear .... well, most of us ! Wink [;)]

Later decoders, commonly known by terms like "silent drive" operate at a higher frequency that we can't hear .... although your dog may be somewhat annoyed by them. Placing the shell on the chassis is akin to placing that high frequency "buzz" in a speaker box - it just amplifies it.

Some (maybe all) decoders exploit this frequency to dim the headlights. By lowering the frequency of the modulation, it in effect dims the headlight. In all reality, the modulation is actually making the headlight blink - it just blinks so fast we don't see it. When dimmed, the headlight is just blinking a little slower causing it to appear dimmer. I've had a couple decoders that had the modulation on the headlight output so far out of whack, you actually COULD see the headlight blinking !!!

So, in a nut-shell, nothing is "wrong" with the decoder .... it just comes down to how tolerant or sensitive you are to the noise. Myself - it just screams at me and can't deal with it while some of my other operators can't even hear it !

Mark.

¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ

  • Member since
    May 2008
  • 14 posts
Posted by pschuman21 on Thursday, September 4, 2008 9:37 PM
I put in a Digitrax DN163A0 Nscale Moble Decoder. So should I just put the factory decoder back or just put up with the noise? In the instructions it says to put tape (that comes with the decoder) across the Laft Frame but I don't quite get that. Is this the problem and i'm just being dumb? This the first time i've worked on decoders, so thanks for the help.
  • Member since
    April 2004
  • From: Ontario Canada
  • 3,425 posts
Posted by Mark R. on Thursday, September 4, 2008 9:57 PM

Well, according to the manufacturers specs on that decoder, it's equipped with SuperSonic motor drive for silent operation. That said, my original thoughts are now moot.  

At this point, and having read your OP again, the naked chassis was smooth and quiet and it became noisy upon installing the shell. That would tell me SOMEthing on the chassis is making contact with the interior of the shell. Double check your installation and pay very close attention as you re-install the shell for ANYthing that may be tight or binding.

Mark.

¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ

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