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Short circuit problems when fitting Bachmann decoder to Hornby Kenilworth Castle loco


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I have an older Hornby Kenilworth Castle loco that is not DCC ready that I have tried to fit a Bachmann 8-pin decoder to.  I snipped off the decoder plug, ready to hardwire the decoder to the loco chassis.  There is a weight that goes into the firebox that needed some modification to allow the decoder to sit on top, in the boiler area.  
 

After unsoldering all the wires and binning the capacitor and other resistors that connect the track pick up wires to the motor for ordinary DC operation, I soldered the red wire on the decoder to the rear metal upright behind the motor, the grey wire to one of the motor terminals, the orange wire to the other motor terminal and the black wire to the black wire that protrudes from the chassis.  I placed the loco on my programming track and programmed it to match the cabside number.   I  over the loco up and down a length of flexi and all was fine.  I reassembled the loco and placed it back on the track and went to run it up and down the same length of track.  It didn’t move.  I tried to read the loco address and it came back with a cv programming error.  I packed it away in its box.

 

The next day, I dismantled it again and was expecting my solder join to have come undone but all were secured properly.  I placed the loco on the track but was then getting a short circuit message from the DCC system (Roco Z21).  I removed it from the track and cleared the short.  I rechecked that I had soldered the decoder wires correctly and then placed it back on the track.  Again I got a short circuit message.  I packed it away again.

 

The following day, I went and bought a multimeter so that I could try and find the short.  It appeared that both of the track feed wires were electrically connected to the motor terminals despite them not physically being connected to each other.  I dismantled the loco, removing the post that the red wire was soldered to (pickup for this would appear to be the chassis itself) and being able to see how the black wire was connected the pick up strip under the chassis.  I then reassembled the chassis and after checking the connections, found that the previously mentioned short still existed.

 

I did an online search to try and get some help on how to fix the problem but none of the guides referred to the problem that I was having.  Last week I had fitted a similar decoder to a Hornby Builth Castle and that had worked fine with the decoder wired the same as I had done with Kenilworth Castle.  What I did learn is that Hornby’s 2007 era Castle (and the Hogwarts Castle loco) had a live chassis that meant that when you disconnected the track feed wires from the motor connections required for DC operation, the pickups were still connected to the motor via the brushes.  I believe that current is collected through the insulated wheel’s axles to the brushes via the pickup strip rubbing on the top of the axles.

 

Feeling that this now makes it fit into the “too hard for me category”, I have returned the loco to DC operation and put it onto eBay.  However, in the back of my brain there is a voice challenging me to find a way to convert it to DCC if I use some Heath Robinson methods to overcome the issues.  Has anyone else experienced the same problem and if so, how did you overcome it?

 

Thanks for any help or suggestions.  I have attached a photo showing the decoder wired as per the normal method.  I also noticed that the plastic covering on the decoder seems to have “melted” somehow which leads me to suspect that I have ‘fried’ my decoder......
 

Regards,

 

Steve

C96FB144-1899-4AE0-BE5F-96E3584E12C7.jpeg

Edited by 6029 King Stephen
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After dismantling the loco and checking all connections, I disconnected the decoder and fitted a new one, wired up the same as in the photos.  This worked perfectly and so I concluded that the short was between the black and grey wire connections on the decoder.

 

Top marks to Kernow Model Railways in Camborne who gave me a no-quibble exchange of the faulty decoder for a replacement one.

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