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Class M7 - Decoder fitting


Bulleidboy

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I've just fitted a Hornby M7 (30108 Weathered) with a TCS DP2X-UK decoder. Went straight in with no problems. When fitting decoders I've never removed the capacitors - is this a personal thing? Some say do, some say don't. This decoder is a plug n play - no harness to get in the way.

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The purpose of the capacitor is RF suppression when used on DC, the RF being generated by transients as the motor brushes pass across the sectors on the commutator.  It is then said that it is not needed for DCC as the decoder includes suppression circuitry. 

 

But what is is the actual effect and is it very different with DCC?  The capacitor is in parallel with the highly inductive load of the motor. A capacitor and inductor in parallel form a tuned circuit, a circuit which passes the tuning frequency and tends to short out the frequencies either side.  The design must be such that the tuning frequency is well below the unwanted RF, so shorting it out.  It must also pass the DC to the motor so the tuning frequency must be low.

 

The DCC signal is a pulse width modulated square wave at DCC frequency of 10kHz plus and because it is a square wave it includes harmonics (multiples) of the 10kHz. The inductive motor load responds only to the average DC value of this.  Adding the capacitor means  it will tend to short out more of the higher frequencies.  Clearly as locos continue to run with the capacitor included ,the effect must be marginal. But if you did some precise measuring ,you may find the loco goes slightly slower for a given speed step with the capacitor than it does without. 

 

This analysis ignores any more subtle effects that the different load on the motor output may have on the overall operation of the decoder in detecting and interpreting the DCC signal.

 

So after all of that, 2 possible conclusions. First, if it ain’t broke with the capacitor fitted, don’t fix it by removing the capacitor.  Second conclusion is that it is not needed and may not be helping so remove it to make sure it doesn’t cause problems. Your choice.

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For  information of those installing a standard harnessed decoder in an M7 there is this guide...

https://www.hornby.com/uk-en/downloads/view/download/item/146

 

This is in addition to the one listed in the usual installation guide here...

https://www.hornby.com/uk-en/hornby-dcc/decoder-installation-guides/r2506-br-late-0-4-4t-class-m7-decoder-installation

 

Rob

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Hi Rob

Both illustrations suggest removing the weights incorporated in the body. As you are probably aware one of the problems with small loco's is having sufficient weight to give traction when pulling rolling stock. The TCS DP2X-UK (there are probably others) is about the same size as the socket - it is a tight fit, and you have to be careful as to where the wires are when refitting the body. I found that by using a pair of tweezers to unravel the wires to the socket and motor, the decoder could be fitted and then the wires put back, one wire (black) almost goes under the decoder, but just runs between the side of the decoder and socket.  BB

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I have posted a picture - which is probably being authorised.

The TCS DP2X-UK is about the size of the socket, so it goes straight in and does not require any extra space, however it is a tight fit in the M7, you really need to make sure all of the wires are out of the way, taped down and not hanging down the side of the decoder - there is no room for even one wire between the side of the decoder and the loco body. The decoder is as wide as the gap in which it goes into when the body is replaced. 

I have used this decoder many times and all appear ok, I say this, as on some other forums, some users have found they gave eratic running. Most of them, if not all, I have purchased in the USA when holidaying, where they are considerably cheaper than the UK.

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I think I've managed it this time! Picture shows decoder in place. The posting above explains everything. The picture shows the wiring pulled aside - it needs to be put back on top of the decoder and motor - a piece of insulating tape held the wire originally - this needs to be replaced.If you click on the picture you get a very good close-up of the decoder.

 https://flic.kr/p/289iqMs

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