Short Circuit problem with Hornby 4 Function decoder

[6up]

Hi all,

 

After fitting a Hornby 4 function decoder to an old Loco (0-6-0 from the 80's) I kept getting a short circuit when I put it onto the track.

 

After thinking it must be some sort of insulation problem, perhaps to do with the metal body of the engine, I spent much time trying to insulate as much of the inner workings from each other as possible but to no avail.

 

I eventually de-soldered the decoder from the motor itself and tested the motor with a DC controller to check it still worked (It ran fine before as an analogue engine) which it did, so I re-soldered the orange and grey wires from the decoder to the motor, put the red and black wires to the track, and get the same short circuit problem.

 

Can anyone point me in the right direction as to what I'm doing wrong?

 

I realized from the start I may ruin both decoder and engine but was happy to take that risk as I want to use this as a learning process, but would like to know if anyone could hazard a guess as to what the problem may be.

 

I cant see any wires from the other unused wires from the decoder shorting each other out, though I am going to try to de-solder the remaining wires from the decoder later to make sure.

 

Many Thanks all,

 

Kindest Regards

 

6up

[Izzy]

Basic point I know, but you must ensure that the motor/brushes are completely electrically isolated from the chassis. Many designs in the past used a live chassis for current passage to the motor and you need to make sure this isn't the case here, and isolate the relevent brushes if it is.

 

Izzy

[Ray H]

Can you be a bit more precise about the loco as this may help someone to indicate any peculiarities with that particular model?

[6up]

Thanks both, the loco is an old Thomas tank engine based on the Hornby E2 i believe.

 

At the moment I'm trying it with just the motor, separate from the rest of the chassis and its still overloading the Hornby Select controller.  I've de-soldered the remaining wires from the decoder and so I'm fairly confident its not those wires shorting out.

 

The only thing I can think is that I've damaged the decoder in some way and that is causing it? But would a damaged decoder lead to the problem I'm having or would it just not work at all.

 

I did removed, what I believe to be a capacitor from the circuit, I wonder if this needs to go back into the circuit somewhere?

 

I've attached a picture of the loco chassis I found on the web and also one of how I'm testing it at the moment.

 

Many Thanks

 

6up

[Izzy]

I'd look very carefully at the wire soldered joints on the decoder. Make sure that you haven't breached any connections between them. Do you have any other motor you can try the decoder with? It may be that in first wiring it up, with perhaps the DCC input connected to the motor output because of the live chassis you have blown it. I don't believe Hornby decoders have overload protection like many others do, though I might be wrong. The capacitor often needs removing on locos for decoders to work properly - it varies a lot, but in leaving it off the decoder should still work.

 

Izzy

[RFS]

The output of the Hornby R8249 decoder is only 500ma. It's possible this is not enough for this older model and is causing an overload.

[Phil S]

Do you have a multimeter? If not, why not?  They can cost less than a decoder !  Only a very basic model is required to be able to test continuity, or the lack of:   I strongly recommend you buy one NOW before proceeding further with any decoder installations !! They are very sumple to use, and can save or avoid many problems.

 

What, as others have said, is to ensure there is NO ELECTRICAL CONNECTION between EITHER RAIL ( whether through pickup or chassis ) and EITHER terminal of the motor. (Test WITHOUT the decoder fitted... result should be OPEN CIRCUIT = infinite resistance)

 

Check, at the same time, that the resisitance between the wheels on the left to the wheels on the right, is also infinite .... that nothing is shorting the rails ... such as a single stand of wire / steel wool / track pin etc attracted by the loco magnet. 

 

(I had a decoder failure once, during testing, when a piece of metal was attracted, from the layout, to the motor ... and found the exposed motor terminals in it way, on the modern Hornby Bogie mechanism: if you acquire such a model, I recommend applying insulating tape or piece of placticard glued over the exposed contacts !!! - prevention is better than cure)

 

Refit the decoder: Red to right rail, black to left rail pickups:  

(IF these are swapped, then you may find the loco runs backwards, instead of forwards - but this can be corrected by CV29 odd or even)

 

Check also, after refitting the decoder, that there is ZERO RESISTANCE between the wheels on the 'Right' side and the Red lead, and the wheels on the left side and the black lead, and recheck that, there is no connection (infinite resistance) between the Red/black wires and the grey/orange motor wires.

 

It might be useful to see if there is any electrical connection between the metal case parts of the motor, and the chassis or track pickups .... this used to be 'standard' as it made assembly cheaper !  (Check the interference suppression capacitor (possibly marked '104') is not shorting to the chassis or motor case, and be aware this might only happen once the body is refitted !  - if in doubt, remove this capacitor ), 

 

This capacitor is 'legally required' to be fitted by the manufacturer, to avoid TV/radio interference etc., as occurs when used, without a decoder, on an analogue 'dc' power supply.  It is not usually required once a decoder is fitted, as it is the decoder which is in contact with the track pickups, where the problem lies.  Some Bachmann models have 3 capacitors fitted - one of which may not be readily visible without removing the motor!!

 

Thankfully the motor you have is not the old  extremely power-hungry X04/X03 open frame motor of old, and should therefore be within the capability of your Decoder, when in use (500mA max)

[GoingUnderground]

In your lower photo of the motor on its own, it looks like the grey lead from the decoder( at the top of the photo ) is making contact with the underside of the motor which looks to be metal, implying that the brush on that side is connected to the underside of the motor, When the motor is installed, it will be sitting on the live chassis thereby making electrical contact between the chassis and the lower motor brush. So I think you may have blown the decoder by inadvertently connecting one side of the decoder motor output to one of the track inputs. 

[Grovenor]

Just try the decoder all on its own, not connected to the motor, hold the red and black wires, one on each rail. If you still get a short the decoder is damaged.

Regards

Keith