A4 mallard sticking on points and re setting the decoder to 001

[Andy Train]

Hi

I have a A4 Mallard with the v finger tender.

It stops at my points at low speed and resets the decoder to 001

other locos cross without a problem 

l have to reset each time unless I keep it running at speed 

any suggestions 

thanks 

[96RAF]

Normally a short induced reset would put the decoder back to address 3 not 1.

 

Does it still do it without the tender - or is the decoder in the tender, if so does it do it without the loco.

 

Also which decoder as the early Hornby R8215 decoders were prone to short induced resets and are best replaced with a more modern decoder.

[Chrissaf]

If you mean the Accessory Decoder for the points resets to 001 then that can be seen as a symptom of transient 'short circuits'. If the Accessory Decoder is a R8247, then this resetting can end up with permanent damage to the decoder. I ended up having to replace one of mine after this issue. This 'short circuit' issue can also affect Loco Decoders.

 

Once the decoders start to exhibit this loss of DCC address issue, then they are permanently weakened and cannot in my experience be permanently fixed without replacement.

 

The issue of a transient 'short circuits' resetting decoders [whether Accessory Decoders or Loco Decoders] can be reduced by installing a 'snubber' across the track DCC.

 

A 'snubber' is a very simple circuit made up of two components. A 0.1µF ceramic capacitor rated at 100 volts in series with a 100 Ohm resistor rated at 2 Watts [3 Watts preferred]. The resistor only needs to dissipate about 0.5 Watts, but a higher rated resistor will run cooler to the touch and minimises the risk of a lesser rated resistor burning out. After fitting my 'snubber', I have never ever had a decoder lose its address.

 

Want to know more about the theory of 'snubbers' then follow this link.

https://sites.google.com/site/markgurries/home/dcc-general-best-practices/wiring-planing/snubbers-rc-filter

 

The author of the page above 'Mark Gurries', is a member of the NMRA, the organisation that developed the DCC standard specification.

 

With DCC, a brief transient 'short circuit' can generate what is called 'ringing'. Ringing is a form of runaway oscillation of the raw DCC Bi-polar signal. This ringing can generate track voltages up to in excess of 70 volts. This high voltage can therefore exceed the voltage ratings of components on the decoders and cause them to arc across internally. The components are damaged, but when tested under normal level voltages they appear to be working correctly. That is until the next track 'short circuit' occurs and the already weakened components briefly fail again. If these failures continue sporadically over a period of time, then there is potential for them to become permanently failed.

 

A 'snubber' will not fix a decoder that is already 'short circuit' affected. A 'snubber' will only offer protection to fully working undamaged decoders.

[96RAF]

I hadn’t considerred it may have been the acc decoder resetting, rather than the loco decoder.

 

The simplicity of a snubber, hanging on the end of my DCC bus final terminal board. Other bent wire combinations are available.

 

/media/tinymce_upload/13a2184a6e305c09a6093ec3d1f9d57e.JPG