Why does this old Spectrum loco run away? It's not the decoder.

Model Railroader Electronics and DCC
1

Ok, this is a cheap old pre-owned split-frame Spectrum GP30. But it is in pristine shape and a very quiet runner for a Spectrum.

But ever since the first decoder install (NCE D13SR) it has run away forward when simply set on a powered track, throttle on brake. If I move the throttle to forward and then move up a notch, the initial fast runaway sometimes ceases and it responds to the throttle setting, but back to fast runaway when the throttle is turned down to zero.

I can bring it to a halt by switching the throttle to reverse. Then the forward setting will work properly until the loco is once again removed from the track and then set down again on a powered track. The situation returns to runaway at zero froward throttle setting and/or on brake.

Now here’s the odd part. It is not the decoder. I just replaced the NCE with an old Bachmann Lenz decoder and to my infinite surprise, the darned thing ran away exactly in the same way

What I have done:

Cleared all loco settings in my Zephyr

Set the NCE to DCC only setting

Tried both decoders at various addresses

checked for odd wiring/continuity with my meter.

Could it possibly be in some glitch in the motor? This was never a DCC loco so it has no Bachmann cap.s or diodes.

If it didn’t run so smoothly I would break it up for parts.

Thanks for any experience with this.

2

Seems to me you have an intermittant short between one of the motor leads and the frame.

David B

3

Thanks, David. I’ll bet you are right. I was so focussed on the decoder as the problem (these NCEs will run away if “stop on DC” is not written even on DCC) that I forgot to think clearly. I’ll bet the motor is not isolated completely. That’s what it seems like now because it must be picking up current that is bypassing the decoder. I must have pushed aside the bit of kapton tape or something when I pushed the motor back in place.

4

I agree. It sounds like an intermittent short between the motor and the chassis. Double and triple check all connections, insulate any places that look suspisious and try it again. If it runs away again try turning off the decoders analog function so it responds to DCC only and see what that does.

5

Thanks. Problem solved.

While expecting the decoder to be the problem for reasons I noted earlier, it was just the “oft mentioned” pinched wire. How silly of me not to look for it first. [D)] I would have expected a pinch to just break and not run. Or perhaps it was the brush solder points touching the frame, but they seemed clear.

Anyway, just a rewiring and wrapping around with kapton tape and it is solved. [swg]

6

Good. Now on to the next task at hand.

7

+1 for me!

David B