DCC in Tight Spaces

By James Taylor

As a long time user of DCC and in prototype terms proponent of the odd, small and unusual I have had occasion to install mobile decoders in some tight spaces. Here are my thoughts on some recent “tight space” installs:

Small but straightforward

The first installation was in a Busch HOf Deutz diesel. The prototype is a tiny 2ft gauge loco used in industry throughout the mid C20th in places like brickworks, sewage farms and gravel pits. The Busch locos are 3.5mm / ft (HO) scale running on Z gauge (6.5mm) track – the mechanisms can also be used with 4mm (OO) scale bodies to represent 18” gauge. As a result, the models are tiny, with the top of the bonnet being no more than about 15mm off the rails   They are designed to work on 3v DC, and have motors originally designed for the mobile phone industry. To overcome the obvious lack of weight and therefore absence of adhesion and electrical contact the Busch system uses a powerful neodymium magnet and steel strip in the track – which means they will even run upside down!

Installing DCC is actually relatively straightforward as there are no lights or accessories to wire in – its just the red and black wires soldered to the pick ups (retain the smoothing capacitor) and the orange and grey wires to the motor. The problem is the size; most N and Z scale decoders would need to be in the cab. However, I selected the CT Elektronik DCX76z decoder available from Coastal DCC – at around 6mm square its one of the world’s smallest decoders, but has excellent motor handling qualities even for tiny coreless motors. Selecting this tiny decoder is the right choice to make DCC installation easy even in this tiny space.


Sound in a Shay

The Bachmann HO scale 80-ton 3 truck Shay has been around for about 10 years – it’s a great model, a vast improvement over the MDC model which had been the alternative to expensive hand built brass units since the 1960s. For an HO scale loco its still pretty small – again these are industrial locos, mainly used in the logging industry with a low centre of gravity. Although Shays are geared engines Bachmann have gone for an all die-cast construction to help adhesion. The only appreciable space inside the loco is the water tank “tender” over the third truck, however, the thickness of the die-cast construction has really reduced what at first appears quite a useable space. For US steam outline sounds I prefer Soundtraxx products – at the time of the install the Tsunami Micro TS-750 was the smallest available, the new Econami is both smaller and more powerful and would now be the decoder of choice (plus its 40% cheaper)! With a modern small can motor and high ratio gearbox the TS750 will easily handle the current draw required from this Shay.

The second part of a sound install is the selection of the speaker. The best sounding micro speaker bar none is the Zimo Sugar Cube – this is a tiny, twin membrane speaker with a pre-installed enclosure which makes it about the size of a …. you guessed it a sugar cube! Unfortunately, the enclosure makes it just too tall for this install. I found that a 16 x 25mm ESU Lok Sound speaker and enclosure would fit with a bit of gentle filling – its not as good sounding as the sugar cube and the enclosure needs careful sealing with Araldite to get the best out of it. Whilst writing about speakers an essential tip is to ensure that the front and rear of the speaker are properly separated to give a much clearer sound with greater bass response – a proprietary enclosure is one answer, but gluing the speaker into a home made baffle can also work in certain circumstances – more on that in a minute.

The Bachmann circuit board includes a standard NMRA 8 pin plug, and some components to handle DC directional lighting and RF interference from the motor. It also includes the sockets for connection to the remainder of the loco and a cast tube containing the grain of wheat bulb for the back-up light. I decided to retain the board but removed the NMRA plug and diodes – instead hard wiring the decoder to the circuit board where the plug was and making alterations to a couple of the board tracks to accommodate the new wiring scheme. I cut a small notch in one corner or the board to accommodate the 220µF “stay alive” capacitor.

The TS750 is well known for running hot – not just warm like a lot of decoders – there is some debate about what causes this but the most likely culprit is the amplifier circuit when the sound is turned up. In this limited space install there is no room for the brass or aluminium heat sink which Soundtraxx suggest – but I hoped that the die-cast metal shell would do that job for me. Unfortunately, there is also no ventilation in the Shay’s tender either – good for tightening up the bass response from the speaker, but not good from a heat perspective. With the master volume set to about 80% I tested the loco on stands running hard in either direction – after about 45 minutes the decoder went into thermal shut down. Reducing the volume has extended the test run time considerably – but I need to consider getting some method of heat transfer or ventilation in there. However, in actual use the loco will not be at continuous ¾ throttle for 45 minutes and overheating is therefore less likely.


All the whistles and bells

The final install I wanted to share was a sound and smoke install in a Hornby OO M7 0-4-4T. The latest version of the Hornby M7 has an 8 pin NMRA socket and space for a small “N scale” decoder above the gear box towards the front of the boiler; Hornby suggest removing the weight from one of the side tanks to fit a larger decoder such as their own. The disadvantage for a sound install is that there is precious little space inside the front portion of the loco for a speaker – maybe a Zimo sugar cube retrofitted with one of the new curved top enclosures might fit in the smoke box. Instead I opted to install both speaker and decoder in the bunker. I opted for a Zimo MX645R loaded with a You Choos M7 sound scheme. The Zimo is a match for the Soundtraxx units and has a number of distinctly British sound schemes available from several different suppliers. I firstly removed all of the Hornby wiring as it was now in the wrong place. To keep the cab clear of unsightly wires I drilled a hole in the bottom of the bunker and routed the wiring under the footplate. The speaker chosen was another from the excellent Zimo range – their 15 x 25mm unit without an enclosure. I secured it to a plastic-card baffle and then glued this into the top of the bunker. I used Araldite for this to ensure there was an air-tight seal to use the entire bunker as the sound chamber – generally the bigger the enclosure the better. A coal load white-glued to a piece of semi-rigid grey foam ensures that the install is not visible.

I also installed a Seuthe smoke unit in this loco – I replaced the Hornby plastic chimney with a white-metal casting. Initially I had the Seuthe unit a tight fit in the cast chimney and nearly to the top. The smoke looked fantastic on first test, until I realised that the smoke unit was transferring way too much heat to the chimney casting and seriously softening the top of the smoke box – fortunately I caught it before irreparable damage was done. I adjusted the installation by milling away the front part of the chassis block to set the smoke unit lower and give an air gap between the chimney casting. This has solved the heat transfer problem, but the smoke production is no longer anywhere near as impressive.

I won’t be bothering with smoke in my E4 or Q1 anytime soon, but do highly recommend the Zimo decoders and speakers.