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Bachmann Spectrum 2-6-6-2 shorting issue


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I'm hoping someone can help me here as my Google Fu appears not to be much help in this case.


I've got a Bachmann Spectrum 2-6-6-2 USRA steam loco into which I've hard wired a Digitrax DZ126T I bought second hand many moons ago from the classifieds on this forum.


Having not used one of these decoders before I held off on installing it, but I finally have a loco that needs a hard wired decoder, so out came the soldering iron.


I followed the instructions that come with the loco on how to connect it up, but when I put it on the track it shorts out and runs away.


I've checked my soldering at it's all looking fine - I've wrapped the decoder in kapton tape to prevent shorting, as well as the wires where they are soldered together, for the same reason.


I've taken the shell off and pulled the wires apart and spread them out some to see if it was something shorting between them when they were squashed up inside the shell but it still happens.


I've tried a factory reset on the decoder and this seems to work - I.e. my NCE Powercab does detect the decoder and does let me set the CV to the "reset" value, but nothing happens afterwards and it still shorts out.


I'm wondering if it'd be wise to strip the loco down completely and ensure the motor wires are isolated from the frame where they connect to the motor itself, as the issue seems to worsen if I press the loco down onto the track.


Any other suggestions? Has anyone done one of these locos before and if so did you encounter anything which might cause the issue I'm having?




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Did you test the loco before installation - and for isolation with a meter before wirng in the decoder.  Did you test the decoder beforehand?

Although a decoder may need to be 'hard-wired' it can still be tested outside a loco, either using a dedicated tester such as the ESU, or a loco/motor ... with connections using readily available croc-clip test leads.

Also, did you measure the current taken by the loco, in dc - and check tha the chosen decoder can supply sufficient. (or even, the dcc controller! 8-) )


Finally, but perhaps first question: WHERE is the 'short;' being reported ?  If it 'shorts' (as reported by the controller shutting down) then it CANNOT be 'running away' because these are contradictory statements !!

A loco could also be being commanded to go at full speed - using a number that you have previously used on your controller, but not the one you are testing with. Have you reprogrammed the second-hand decoder since reciving it??

Was your intial testing on a 'programming output' (which limits the current available)- did the loco appear to accept reprogramming - did  the motor show a short burst of power in response to programming??

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That has a PCB with soldering pads in the tender, doesn't it? My first step would be to desolder the wires and test the decoder. It is possible that the drive circuit is blown in a way that is reading as a short but providing voltage to the motor. I've done that before. Digitrax decoders are light on the error checking, so it may actually program correctly even with blown drivers.


If the decoder tests fine, then I'd start digging into the loco.


Just looked at the diagram - are you sure the connector between the loco and the tender is the right way around? If you get it wrong it looks like it will connect the track feeds directly to the motor.



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Okay, I'll try to answer the questions as best as I can...


This is the loco I have - I have the 2005 version where the decoder goes just in front of the cab, not in the tender:




I did test the loco before I installed the decoder, it was running fine on DC.

I did not test it for isolation as I have no meter to test with.

I did not test the decoder beforehand.

I did not measure the current for the loco beforehand


I get the fault buzzing noise from the loco, sometimes it runs away, other times it just sits there.

The Powercab intermittently detects the manufacturer and decoder type when trying to program the decoder via the programming track.

The Powercab intermittently lets me program the decoder but the settings do not appear to be retained for long - I.e. if I remove the loco from the track it and place it back on again, it may or may not retain it's modified address settings.

The decoder intermittently accepts new programming - i.e. the motor jerks to life for a split second a few times as you'd expect.

The Powercab has intermittently displayed a "SHORT DETECTED" message on the display.

I have attempted a factory reset of the decoder but this seems not to work.

I have never used the decoder prior to putting it in this loco.


I'll take a photo of the loco tonight and put it up on here.


Thanks guys. :)

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You seem to be doing most of the basics - test it on the programming track output first, if it doesn't read/write, something is wrong.  Only mistake is to test-run when the programming track behaviour is not 100%;  if the programming track does not behave completely correctly something is wrong and needs fixing before proceeding.

The question is what is wrong ?    I suspect it is either a faulty decoder (but was that fault caused by the loco?) or an intermittent electrical fault (such as motor brush sometimes shorting to pickup, or possibly a low-resistance path inside the motor). 
A multimeter is pretty cheap,  I suggest getting one to check for continuity around a loco before installing decoder.   For example, this from Maplin (there are cheaper options from other sources) will do fine.  You don't need expensive high-precision meters to check continuity and to give rough figures for resistance or voltage.


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That was my first thought too Adrian, as I've had a similar issue when first fitting one in a Kato U25B, but I wanted to check with the experts here as to any other possibilities - if I'm going to strip the loco down I might as well try multiple possible fixes, right? :)


Nigel - thanks for the link, I'll see about getting one after I get paid. :)

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