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Decoder fitting - Live Chassis


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Hello,

 

Having been fitting decoders to all sorts of locos for about 15 years or so I've found one that has stumped me. 

 

I'm fitting a decoder for a friend's 4mm kit built J39 (unknown kit but fairly old I reckon).  This one is powered from pickups on the wheels on one side and via the chassis on the other.  Fairly straight forward - or so it should be, and my range of chip fitting experience includes the old split chassis locos so usually I know what I'm doing (usually!)  When connected to DC it runs quite nicely, however when the decoder is fitted it will not run.   NB - to save a ton of questions: I've serviced it so all contacts etc are spick and span, it runs well on DC, the chip has been tested on my tester, the decoder address is confirmed as set to #3.

 

Following the procedure ie disconnected the motor terminals from the track feeds - attached orange & grey to the motor terminals, black to the wheel wipers side and the last one, the red, I attached to the chassis, connect to the DCC controller - Nothing!  Now if I disconnect the chassis connection and power that one lead directly with a croc clip attached from the track - then it runs nicely.  So the issue seems that I can't get power to the chip via the chassis connection - although, as stated, when on DC power this works fine.  Hmmm..  

 

As far as I can tell I've done it by the book and the only thing I can think of now is that the motor is not isolated from the chassis.  If so, then I'm stumped as to how I might do that as the axles running through the pinion gear are live from the chassis.  

 

So, am I attempting the impossible - or just been plain old stupid?           

 

Grateful for any knowledge that can be thrown my way from more experienced decoder fitters.  Cheers ..

 

Alan

 

Edit - the chip is a Lenz standard V2..

 

The J39 - the brown wire on the right connects with the wheel wipers .. tuther one to the chassis.

20200828_104346.jpg.7569bf900398d7bfd85c9fc0e5b97aa5.jpg

 

Edited by Alan Kettlewell
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4 minutes ago, giz said:

Is the motor mounting bracket in contact with the right hand brush holder? It looks like it is in the photo.

 

Hi giz,

 

Could you elaborate a bit ie where do you mean?  Although this is a just a photo taken at some point during proceedings intended to show the chassis I'm working on - it's not necessarily representing the issue when under test as the wires have been off and on as I've tried all sorts of things.

 

Thanks.

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34 minutes ago, Alan Kettlewell said:

 

Hi giz,

 

Could you elaborate a bit ie where do you mean?  Although this is a just a photo taken at some point during proceedings intended to show the chassis I'm working on - it's not necessarily representing the issue when under test as the wires have been off and on as I've tried all sorts of things.

 

Thanks.

The brass bracket that hold the motor in place, which is screwed to the chassis so presumably live as well, appears to be touching the right hand brush holder and thus it is not isolated from the chassis.

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1 hour ago, giz said:

The brass bracket that hold the motor in place, which is screwed to the chassis so presumably live as well, appears to be touching the right hand brush holder and thus it is not isolated from the chassis.

 

Ah yes I see, thanks.  It's very close but not touching - but just in case, I've put a bit of black tape over it.

 

Cheers ..  Alan

 

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1 hour ago, Mike Buckner said:

with a multimeter set to Ohms, check if there is connectivity between the chassis and one or other of the motor terminals

 

It seems there is.  On the left terminal I get a reading of  around 20 to 28 ohms.  

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This is a killer for DCC decoders.  If either of the decoder's motor leads comes into contact with the current on the track, it's usually curtains for the decoder.

So you need to definitively isolate both motor terminals from the track.

And you may have a dead decoder.

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Did you not test read the CVs in programming mode? If that fails then there is plainly something wrong without exposing the decoder to the full DCC. Is one of the brush holders live to the motor casing which in turn is contacting the chassis.

Edited by Butler Henderson
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Thanks for the replies. 

 

Yes of course I've used the programming track to set up - my system is the Roco Z21 (which is set up on my H0 layout down the shed) and I also use a Roco Multimaus system attached to a yard of track on my workbench up in the house - although with the Multimaus system you can only write to a chip but not read.   In addition I use the ESU decoder tester.

 

A strange thing is that there was no apparent failure on the Z21 programming track.  I can get a response when sending read and write commends to the chip when fitted (the usual small movement of the motor) but when it comes to driving there's no response at all.  And the chip has not blown as I can still get it to run the motor when I disconnect the (red) decoder lead from the chassis and attach it to a croc clip from the track power (as mentioned in the opening post).  It also runs ok when attached to the ESU tester.

 

However, let's struggle no further as it looks like a motor terminal is in contact with the live chassis and the solution is to do some surgery to isolate it/them from the chassis - which is probably not worth it for this old loco.  The friend who I'm doing this for agrees and has decided to abandon the attempt and to put it back up for sale as a DC loco.

 

Thanks again for all the helpful replies.

 

Cheers ... Alan

 

 

 

  

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