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Discussion Starter #1
I just received another Bachmann Spectrum 4-6-0 in the mail this weekend (used), and I'm running into a problem with the motor. Under DCC it appears to short the track and the controller resets the power until I turn off the throttle. Since I already have one of these same locos, it gives me some ability to swap things around. So far I have:
  • Connected the new tender to the old loco -- works fine so the problem is not in the tender.
  • Connected the old tender to the new loco -- shorts the track again, so the problem must be inside the new loco.
  • Disconnected the tender and used a couple wires to apply DC power directly to the motor pins -- runs without hesitation, however the motor speed is slower on the new loco than what I observe in the same conditions on the old loco
  • Used a multimeter to check for shorts between all the pins -- none were found
  • Measured resistance across the motors -- Old motor is about 58ohms, the new motor is only 7 ohms.
So my initial guess would be that one of the coils in the motor may be shorted, despite it running smoothly under DC power. However I'm wondering if what I'm seeing might indicate a common issue with brushes or something else? It's been nearly 40 years since I had to troubleshoot a DC motor so I'm open to suggestions. I'm reluctant to take the loco apart since it's a Spectrum and there are a LOT of detail parts to work around, but I have this feeling I'm heading that way anyway.
 

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What I would do and it sounds odd but I would hook up the whole works put it on a chunk of track that’s isolated from the layout and I would measure resistance between the rails. In theory you shouldn’t have continuity through there unless the decoder has power. If I’m understanding this correctly it does this as soon as you put track power on? Or am I wrong and it’s only when you go and try to throttle up the locomotive?
 

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Discussion Starter #3
It was only when I tried to throttle up the loco. It could sit on the tracks and sound would start playing as soon as I turned on the track power, but any attempt to move the loco resulted in a short.

I decided to go ahead and tear down the loco. It was every bit as painful as I feared. However once I got to the motor, I again applied power from the DC power pack. Wow it was not happy! Lots of sparks inside, but each time I applied power it got a little better, so I went ahead and let it come up to speed, switched direction a few times, and suddenyl everything cleared up and the motor started running smoothly at a much higher speed. I have this feeling it may have been sitting for some time before being sold, and probably had some dirt in the motor. Once it cleared up I set it back together with the tender and DCC hooked up, and it ran just fine under DCC control.

While I had it apart I swapped out the horrible cool-white LED for a warm white, checked it on the tracks again, then put the loco back together. It seems to be working just fine in either direction now, so I think I'm good to go. Which is a relief because Bachmann does not have any replacement motors in stock, and being a discontinued model for several years I am betting they have no plans to get any more motors. Guess we'll see how it goes, but for now the loco is happy again.
 

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Discussion Starter #5
Thanks! Of course I'm assuming this problem won't creep up again in the motor, but for now it's running pretty decent. Now I have to look into another problem... I'm seeing a bit of a bind in the rods at slow speeds causing a hiccup at a certain spot. I'll need to get a closer look at both sides to try and figure out where that problem is coming from, but nothing is bent or noticeably out of whack, so hopefully that's just a simple matter.
 

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I think that’s just a bachmann thing. You might have a set of drivers that are just out of quarter. My 2-10-2 and my bachmann 2-10-4 are the same way for some reason.
 
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