Noisey GP-40

[Jim Banner]

After Ten Wheeler's problem with a noisy 4-4-0 http://www.bachmanntrains.com/home-usa/board/index.php/topic,1234.0.html
I thought I would investigate the noise in my Bachmann DCC-on-board GP-40.  As received, it growled at all throttle settings above a dead stop.  I did not find the growling all that disturbing as it was quieter than the side winders and other coffee grinders of a couple of decades ago.  But some of my cohorts were nervous about this locomotive which, to them, sounded on the point of self destruction.

Removing the shell revealed a cast metal frame with the motor sandwiched between upper and lower sections of frame.  The motor was slightly loose in its mountings because the upper section of the frame was slightly warped.  But it was not so loose that it could rotate and touch the plastic shell.  It appeared, however, that it could rotate enough to trap the decoder motor wires between the motor and the plastic shell, possibly transmitting motor noise to the shell via the trapped wires.

One solution might have been to unsolder the wiring, remove the decoder, remove the top section of the frame, and then try to straighten the latter without breaking it.  But being lazy, I tried something easier first.  I relocated the motor wires so that they ran between the bottom of the motor and the bottom section of the frame in the area between the brush holders and the farther end of the motor.  The plastic end of the motor is narrower than its metal case, so there is more room to run the wires vertically up past the motor to the decoder printed circuit board.  When the body was snapped back on, the locomotive was quiet, to the point that my (slightly deaf) ears can no longer hear it run.  I hope my model railroading buddies will find it the same.

This is a classic case of a noise (in this case, motor noise originating from Bachmann's non-silent decoder) being coupled to and amplified by the plastic shell of the locomotive.  If you can break the coupling (in this case by moving a couple of wires) then the sound is no longer amplified by the shell. 

[lanny]

Jim,

Thanks for another excellent 'how to do it' set of instructions for my "model RR ideas" folder. Really appreciate the time it takes you to share your modeling knowledge!

lanny nicolet

[chucknlead]

I guess I'm a little lazier than you Jim. I had the same problem and just HOT GLUED the motor to the frame as well as the wires. It quieted down a little. I think the real solution would be to replace the decoder with a Lenz Silver Series silent back emf unit. I love these and put them in almost all my locomotives. What a difference they make!!!!

The "DCC ON BOARD" line of loco's are extremely affordable and if you don't mind the coffee grinder sound, they are a great deal!

[Bojangle]

I have 2 Bachmann GP40, noticed a slight "whine", but I sorta liked the sound. Is the "coupling" you mentioned inductive or just harmonics?  I don't intend to work on them while in warranty, just curious about all this stuff.

[Jim Banner]

...  Is the "coupling" you mentioned inductive or just harmonics? ...

Neither.  It is mechanical coupling through the motor wires which were pressed against the outsides of the motor and the insides of the plastic shell. 

As a kid, did you ever hold a sewing needle in your teeth and put the sharp end in a record** that was spinning on a turn table**?  The sounds you heard were mechancially coupled from the record to you teeth by the needle and from your teeth to your ears by the bones in your head.  Same idea here.

** For you youngsters out there, "records" were flat disks of vinyl, or earlier, shellac, that had wiggly grooves in them and "turn tables" were what spun them under a needle mounted in ...  Oh heck, just look up "phonograph" and "gramophone" on Google.

[Stephen D. Richards]

...  Is the "coupling" you mentioned inductive or just harmonics? ...

Neither.  It is mechanical coupling through the motor wires which were pressed against the outsides of the motor and the insides of the plastic shell. 

As a kid, did you ever hold a sewing needle in your teeth and put the sharp end in a record** that was spinning on a turn table**?  The sounds you heard were mechancially coupled from the record to you teeth by the needle and from your teeth to your ears by the bones in your head.  Same idea here.

** For you youngsters out there, "records" were flat disks of vinyl, or earlier, shellac, that had wiggly grooves in them and "turn tables" were what spun them under a needle mounted in ...  Oh heck, just look up "phonograph" and "gramophone" on Google.

Jim,
I thought records were set  by sports persons!  lolol  Reference the noise...I glued some strategically placed packing (thin type) and that settled it down too!  Just an option!   Stephen