Rapido Model Rail Skirtless J70 Steam Tram

J70 Steam Tram Sound Project

It will be no major surprise to those that follow my blog that the release of Model Rail Magazine and Rapido Trains’ J70 Steam Tram is something that I am very excited about.

The main motive power on Brewery Pit is based around these little steam trams that originally operated on the Wisbech and Upwell branchline with some of the trams finding their way to Ipswich and Yarmouth docks in the later part of their lives.

I have built two kits of these steam trams so far. One resin cast J70 from a Silverfox kit and a brass version of the earlier Y6 0-4-0 from a now-defunct Connoisseur kit.

I have always had a dream of scratch building a ‘skirtless’ J70 steam tram so all of the valve gear could be on display. I even started a forum topic on RMweb about this, but it seemed that I would likely have to create a small chassis myself as there was no propriety chassis with such a small wheelbase available.

Model Rail/Rapido Trains J70

To my complete surprise Model Rail then announced that they planned to commission Rapido Trains to create a J70 with open and closed front door options and ‘skirted’, ‘skirtless’, and ‘partially skirted’ versions.

I was lucky enough to run an early prototype of the Rapido Trains J70 back in December while exhibiting Brewery Pit at the National Festival of Railway Modelling and I found it to be a lovely smooth runner, which easily navigated the tram lines on my layout.

I have now ordered three of these little tram critters, each with a slightly different skirt arrangement. That takes my total steam tram locomotive stock up to x5. I keep considering getting another skirted version, but I need to stop listening to that wanting (and expensive) voice in my head.

Excessive Steam trams

Excessive Steam trams

DCC Equipment

Mind you, the model is hardly expensive in terms of a OO gauge model in 2019. At £127, with a range of extras in the box and a booklet, it really is a bargain for such a fine quality item. I also discovered that the model comes pre-installed with a sugar cube speaker, thus reducing the cost of installing DCC sound.

I asked the advice of my friend John Gay of JMRC what can be squeezed into the limited area of the chassis in terms of digital circuitry for sound and he advised I focus on Zimo decoders, so I set myself the task of attempting to not only fit sound to one of these tiny steam trams but to also fit a stay alive capacitor in the limited space.

I purchased a Zimo MX6496N 6-pin sound decoder and purchased a J70 sound pack from Digitrains which came pre-installed on the sound chip, a sugar cube speaker (not realising that the model included one already installed) and a tiny SACC16 stay alive charging circuit.

I decided to fit the sound decoder to the ‘skirtless’ version of the J70 as I thought it would be great to see the valve gear in motion as it chuffs in sequence with the wheels. I noticed a certain clunky movement to this tram while performing running-in duties on the rolling road, and decided to swap out the chassis for the one in my partially skirted model (which ran a little smoother). Now don’t get me wrong, both are excellent runners, but I wanted my first sound fitted steam model to be as smooth as possible.

DCC Equipment for the J70

DCC Equipment for the J70

Chassis Removal

Removing the chassis is very simple, but care needs to be taken with how you hold the chassis as you don’t want to damage or interfere with the valve gear which is designed to fit in an incredibly tight space.

Four tiny screws easily release the chassis from the main body of the loco with the full chassis lifting out of a slot within the diecast moulding of the trams skirts, revealing beneath that diecast boiler is hollowed out to fit the coreless motor and speaker inside.

On the chassis, the little speaker is taped down above the worm gear on a small square bracket with red and black wires prepared for soldering onto the sound decoder. There is a six-pin socket at and circuit board at the front of the chassis that will take the Zimo decoder.

Chassis of the J70

Chassis of the J70

DCC Sound and ‘Stay Alive’ Charging Circuit

The Zimo decoder does not come pre-fitted with a harness to attach the stay alive capacitor, so it requires soldering directly onto the positive ground and negative outputs. In order to do this and allow the Zimo sound decoder to easily fit in the gap between the 6-pin harness and the speaker, I had to first shorter the 6 pins so they fit flush in the harness and then I cut away some of the plastic insulation from the decoder to allow me to access the outputs for the stay alive charging circuit.

The SACC16 charging circuit is a very interesting design, which allows for the installation of various extra capacitors and tantalums (strange yellow bricks which act as a small capacitor). The pack comes with one tantalum and x1 14v capacitor, but there is no way the larger 14v capacitor will fit inside the hollowed out boiler of the steam tram. For now, I decided to fit the tantalum and I might fit an extra tantalum when I install sound in my next tram. As the decoder and SACC16 are so tiny to solder reliably, I decided to utilise some solder paste that I purchased a few months ago, which can simply be rubbed onto the connection area with a cocktail stick and is a mix of flux and solder particles; all you have to do is touch the soldering iron on the paste and it suddenly becomes molten metal for those few precious seconds until it goes nice and hard.

SACC16 Charging Circuit

SACC16 Charging Circuit

I tested the sound decoder on my decoder tester which includes its own speaker. I do this to ensure that the decoder is working fine before I attempt to install it within the locomotive. I then tested the decoder on the steam tram chassis with both of the sugar cube speakers (the pre-installed one and the one that I purchased). Unfortunately for me, the temporary wired speaker arrangement created a short circuit, which burnt out my expensive sound decoder, leaving me in dismay for one evening, but luckily I was told that Digitrains operate a decoder repair service for £18 and sure enough, I had a replacement decoder sent out to me for the easter bank holiday.

It soon became apparent that the pre-installed speaker operates perfectly fine and sounds a little better than the purchased sugar cube in my sonic opinion. In addition, it isn’t possible to install the more oblong-shaped sugar cube speaker into the boiler as the boiler is clearly pre-moulded to accommodate the pre-installed speaker.

I found that if both the SACC16 and the sound decoder are taped down and sit lower than the speaker, near the front of the loco, then the whole package of electronics and motors will easily fit back inside the under-frame of the tram.

J70 Chassis with Zimo Decoder and SACC16 Stay Alive Charging Circuit

J70 Chassis with Zimo Decoder and SACC16 Stay Alive Charging Circuit

Here is a video of the steam tram sound FX in action. Apparently, the sound files included on the decoder include x3 different engine sound packages including heavy loaded engine, light engine and an extra broken cylinder version (which needs to be unlocked with CV values). The sound is very effective and realistic, but I will reduce the volume on the bell as it sounds a bit loud to my ears.

Windows and Details

Next, I pushed out the moulded windows with a cotton bud and replaced them with the etched windows provided in the detail pack. These require the windows to be cut to shape from acetate or plasticard. This is quite a fiddly operation and I found that it was easiest to cut the perspex to the correct shape and then paste the edges with non-frosting superglue and then drop the window frame down onto the plasticard and move it into position.

J70 Window Frames

J70 Window Frames

Completed J70 Windows

Completed J70 Windows

I then glued the windows in place in an open position, as I imagine it was blumming hot in these little sheds!

I also included a little lamp, which is actually unpainted white metal, which I have dabbed a bit of the non-frosting superglue to so it looks like a bulb. I would have really liked to wire the lamp up, but it is complicated to see how it can be done without drilling through the diecast under-frame, but I still may need to find a way to do that in order to fix my preferred couplings to the model, but that will have to wait until next time.

DCC J70 with Sound on Brewery Pit

DCC J70 with Sound on Brewery Pit

Grasslands out!

1 thought on “J70 Steam Tram Sound Project

  1. Johnny Haynes

    Hi Tom, superb post, your brace of J70’s makes quite a considerable contrast to the 5 Peaks we occasionally run around the loft. Toby rules! jh


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