Chris Walsh describes and illustrates how he selects, fits and sets up DCC decoders.

Fitting DCC decoders to locomotives is not a complicated business. Even where there is no socket provided in the locomotive - all you need is to be able to solder and be adept in the use of Blu-tack. Almost all the locos on John Bryce's Thurlby Railway were originally kit-built or scratch-built for DC operation. Each one has its own peculiarities but the great thing about O gauge is that there is usually plenty of space to install decoders and – where necessary - loudspeakers. In this first part, I’ll look at selecting and fitting decoders.

Fig 1. Connections for motor stall current check

There are many decoder manufacturers and, because of NMRA standards, any decoder should operate with any manufacturer's DCC system. When selecting a decoder, as with other things in life, size is not important. What is important is the maximum amount of current the decoder is able to supply to your locomotive's motor when the loco is under full load. You’ll need a DC supply and a meter capable of reading current in amps. To check the maximum current drawn by the motor (known as the stall current), connect your meter in series with the motor so that the same current passes through the meter as does through the motor. This is best done using crocodile clips as you will need your hands free for the next stage. See Figure 1

Next, hold the loco's wheels and turn up the controller to maximum speed. Hold the wheels for only a few seconds – just long enough to read the meter. Holding the motor in its stalled condition for a long time will cause the motor to overheat with disastrous consequences.

The stall current on the Thurlby locos ranged from a BR Std. Class 2 at 0.25amps to an LMS Stanier which was taking 2.5 amps. Most were drawing between 1.0 and 1.4 amps. Locos with twin motors, in our case, some Heljan diesels, will draw twice the current and so will need a more powerful decoder.

Having found the maximum current, your next decision will be whether or not you want to have sound on your locos - and which particular manufacturer's decoder to go for. This is a personal choice but my favourites are the ESU V4.0 and Zimo MX645 for sound. Higher current motors will need to be fitted with the XL version of the ESU V4.0 decoder.

For non-sound decoders, I use the Digitrax DH126D or Zimo MX632. The DH126D has no frills and a maximum current capability of 1.5 amps. The Zimo boasts 1.6 amps capability with 8 function outputs – ideal if you have lots of different functions for the decoder to perform. There are, of course, alternatives and your DCC supplier should be able to recommend suitable decoders for you.

Most steam locos will require motor drive only, although you could use some of the function outputs for things like lights and fire-glow. Sound decoders are more expensive but the careful use of sound can add greatly to the atmosphere of your railway. So we'll look at the both types of installation.

In both installations, you have to make sure that there is no direct connection between the track pick-ups and the motor. This is sometimes difficult in a loco with a split chassis or a live chassis. There are ways around this but, for this article, I'll assume ours are standard locos with fully insulated wheels and an isolated motor. Where’s best to fit the decoder?

O gauge locos generally have lots of space for fitting decoders. Here are some possibilities… Steam Locos:
1 - Between the frames.
2 - In the firebox with the motor.
3 - Tucked into the boiler before you install the speaker.
4 - In the tender.
Diesel Locos:
1 - Almost anywhere within the body where it cannot be seen.
2 - Under the chassis between the bogies.
In all these cases, make sure the decoder cannot make contact with any metal parts and there is plenty of space around it for air to circulate. I try to avoid mounting the decoder too close to the motor if possible. It’s a personal hang-up of mine but all the arcing that goes on around the motor could induce surge currents in the decoder if it’s too close – and destroy it. So, best to keep them apart.

All the decoders on the Thurlby Railway are held in place with Blu Tack so they can easily be removed - and it makes them ever so easy to fit.

If your loco already has a decoder socket fitted, all you have to do is choose a version of the decoder with the correct plug, plug it in (making sure the plug is the right way round – pin 1 is usually marked) and the job is done.

But what if the loco was lovingly built a few years ago for DC and you never thought you would consider running it on DCC? John Bryce's Thurlby Railway had thirty locos. Twenty six of them were built for DC operation and so we became quite experienced at these ‘full’ conversions. The first stage was to disconnect the leads from the motor. Usually, pick-ups along each side are linked together to improve contact with the track. Removing the leads from the motor left all that linking intact. Some DC RTR locos have a capacitor fitted across the motor terminals and this should be removed.

The DCC decoder fits, electrically, between the pick-ups and the motor and there are 4 leads on the decoder for this. Luckily, they are colour coded and each manufacturer's decoder follows the same colours. The easiest way to remember the colour connections is by this awful rhyme…
Red and Black to the track
Orange and Grey go the motorway
So, those leads we removed from the motor now go to the red and black leads of the decoder. The DCC track voltage is a form of AC and so it makes no difference which way around these are connected. The orange and grey leads from the decoder go to the motor. This is a pulsed DC and so it does matter which way around these are connected. As with DC, get the motor connections wrong and the loco will go in the wrong direction. So, it's worthwhile experimenting with this on your DCC layout whilst the chassis is out of the body.

There is a way you can correct this problem using the CV settings - as you'll see in Part 2 of this article - but it's easier to get it right at the installation stage – especially if you are not happy about changing CVs.

We could have soldered the decoder leads straight to the pick-ups and motor but decided it would be really useful if we could easily convert the locos back to DC again. That way, we could work on them, if necessary, away from the layout using an old DC controller.

Also, there is a further benefit. Sometimes it is difficult to know whether a fault lies in the decoder or the motor. For testing on the layout, we have a spare decoder with its plug and socket fitted so we can easily swap decoders and see how the loco runs. The spare decoder doesn't need sound and we use a Digitrax DH126 because it is not expensive and has a 1.5 amp current rating – enough for most of our locos except the twin-motor Heljan diesels.

So, we purchased some 2-pin plugs and sockets from Rapid Electronics to make the connections to the decoder. The ones we chose were part number 22-0520 for the plugs (about £1 for 10) and 19-0090 for the sockets (about £4 for 50). Whilst there, we also picked up some small bore heat-shrink sleeving to protect the connections.

We protected each of the solder connections to the plugs and sockets with the heat-shrink sleeving to make sure there was no chance of short-circuits. Also, the point on the wire where the solder flow ends is always the weakest and the sleeving helps to support it mechanically. We decided, as a convention, to fit the sockets to the wires from the loco’s pick-ups. So, its corresponding plug was connected to the decoder’s red and black wires.

The orange and grey leads from the decoder were soldered to a socket and its corresponding plug was soldered to the motor leads.

There are two extra leads on a sound decoder which will need to go to the speaker. The sound signal is an AC waveform and so it doesn't matter which way round they are connected if you are using a single speaker. What does matter is that the speaker has the correct impedance (ohms or Ω) as the decoder. Impedance is the speaker’s resistance to the AC sound voltage. Too high a speaker's impedance will give low sound and too low an impedance will destroy the decoder.

Figure 2 shows the connectors fitted to the loco. Notice that the plug and socket would fit together if we wanted to run the loco on DC.

The small speakers we use in models of all scales are not able to reproduce the range of sounds made by a full-sized steam or diesel engine. We can get close to it by making sure the speaker has the largest possible surface area. So fit the biggest speaker possible and always use a sound chamber. Ideally, the chamber should be airtight. Sealing up any holes for connecting wires, etc., with black-tack (a stickier version of Blu Tack) will enhance the sound quality.

Figure 3. The decoder ready for installation.

In steam locos, the ESU 28mm round speaker usually fits neatly into the boiler, leaving plenty of space in the firebox for the decoder. Blu Tack is ideal for holding the speaker in place as it allows you to easily remove it if necessary.

I have a twin-car DMU built from an Easibuild kit. The prototype has an engine in each car and, to simulate the sound as the loco passes by, I mounted a speaker in each of the cars. The decoder is an ESU Loksound v4.0 which is designed to take a 4 ohm speaker. To err on the safe side, I connected the speakers in series (as in Figure 4) to increase the impedance to 8 ohms. Connecting them in parallel would have reduced the impedance to just 2 ohms and would put too much load on the decoder’s sound output stage. The overall sound level is slightly lower than with a single speaker but you can easily set it to the level you need by adjusting the CVs.

One thing you do have to watch out for is that both speakers work together and not against each other. Speakers work by pushing and pulling air to make sound waves. Some speakers are marked with + and - signs on their terminals. So, if you are wiring them in series, you connect the + (positive) terminal on one speaker to the – (negative) one on the other. That way, they push and pull the air at the same time.

However, not all speakers have their terminals marked. But all is not lost, the following simple test should sort it out.

Connect the speakers to the decoder, keeping them about 8 inches apart for this test. Next, fire up the loco with its sound and drive it past you. Alternatively, you could keep the loco still and move your head past the speakers. As the loco passes, the sound level should appear constant. If it drops, then swap the connections to just one of the speakers and try again.

Fig 4. Twin speakers Impedance and Connections

Fig 5. Decoder Switch Operation

Adding lighting to locos and coaches helps bring the railway to life. These days, LEDs can create a range of colours which can replicate all the colours we need for railway modelling.

In steam locos, a couple of LEDs in the firebox can be made to flicker very effectively and coach lighting can be switched on and off at will. One of my coaches has a separate LED in the toilet which is connected to its own output on the decoder. That way, I can switch it on and off independently - but that’s just my perverted sense of humour.

If, like me, you are an analogue man at heart, you will expect the positive supply to be switched. Unfortunately, these decoders switch the ‘return’ side of the circuit. So, if you look at Figure 5, you will see that the decoder’s DC outputs are set up so they connect to the negative supply and there is a common positive which is the blue wire from the decoder.

The common positive is about 10 to 12 volts DC above the negative. Whilst the common negative lead is shown here, it is not easily found on the decoder. That’s because the aux outputs connect to the negative lead within the decoder itself.

There’s an LED connected, with its limiting resistor, between the common positive and the AUX 1. Since the aux output is switched OFF, there is no current flow and so the LED doesn’t light. When AUX 1 is switched ON, the circuit from +ve to -ve is now complete and the LED lights up. The connections to lights on my locos use more of the Rapid small plugs and sockets that I mentioned earlier. They sell 3, 4 and 5-pin versions so you can prevent the lighting connections being confused with the motor connections. The only problem is that they can be reversed. So, to make it obvious, I colour-code the connectors on one side only and, as a further check, remove one of the pins.

Figure 6. Lighting connector modified and ready for use

Figure 6 shows the (almost) fool proof set-up for what was a 3-pin connector.

Which key on your controller operates AUX 1 is decided by the decoder itself and its settings. That brings us neatly to the second part of this article, ‘Setting up the Decoder’, which will appear in a future edition of the Gazette.

Editor’s Note. John Bryce’s Thurlby Railway was originally built and wired as an analogue system and Chris described the conversion to DCC operation of the layout wiring in the Gazette, volume 19, issue 12, August 2016

Sadly, John has since passed away and the layout is in the course of being dismantled. However Chris and I decided that it was appropriate to continue the description of the conversion and I plan to publish part two of this article in the February 2018 Gazette.