Fit a Zimo decoder with Digitrains sound to a Hornby J94


Installing DCC sound into a small tank locomotive like the Hornby J94 might seem a step too far, but it's possible with the Zimo MX648 decoder and ‘Sugar Cube’ speakers as Ian Mellors demonstrates.

I had previously fitted a Lenz decoder and a ‘Stay Alive’ capacitor pack into Harry - the main shunting locomotive on my Summat Colliery layout (November 2013 BRM), and so did not want to lose this extra functionality once sound was fitted. Thankfully the MX648 also allows for the fitment of an external capacitor, although it needs extra external components to control the in-rush current caused when the capacitor charges up.

The sound scheme was produced for Digitrains by Paul Chetter and features his ‘Multi-drive’ system whereby two distinct sets of exhaust notes are present on the decoder, one for a light engine and the other for one working a heavily-loaded train. You can switch between the two sets by pressing the F5 key.
Dismantling the model is straightforward. At this stage it's best to remove the front motor fixing plate by removing two more screws – now the motor terminals are exposed.
Unsolder the two bundles of wires leading from the pick-ups from the motor terminals. Make a note of which bundle leads to which motor terminal as you'll need to make sure that you connect the orange wire from the decoder to the bundle you connect to the red decoder wire, they're not colour coded for you!
Next, space needs to be made for the decoder and speaker. For a simple sound installation all that is needed is the removal of the ‘U’ shaped part of the cast weight that sits in the lower half of the boiler space.
Fit the decoder by connecting the red and black wires to the two bundles of pick-up wires, using heatshrink material to insulate the joints. Then connect the orange and grey wires to the motor. Don’t worry if you end up getting the orange and black wires round the wrong way, you can always reverse the connections or use CV29 to make sure the locomotive runs forward!
Connect the speaker to the purple wires. Use a very small screwdriver to open up the small spring contacts on the speaker before inserting the wires – no soldering is needed. With its basic set-up the decoder fits in the bottom of the boiler space and the speaker sits above it under the cast weight.
Select a suitable capacitor that will fit under the weight; there is room between the speaker and the motor. A DCCconcepts capacitor pack will do the trick. If you want to fit the largest possible capacitor, use a file to make a shallow slot in the underside of the weight to accommodate the larger diameter.
Soldering the capacitor pack to the decoder needs a steady hand because the negative side of the capacitor needs to be soldered to the GROUND pad on the circuit board. This is the gold-plated pad immediately to the right of the grey motor wire. A fine tipped soldering iron and a steady hand are in order here. The other end of the pack is easy, just solder to the decoder blue wire and insulate the joint.
I have also added a red LED into a small hole drilled in the firebox door, with a suitable resistor (approximately 100ohms) connected to the white and blue decoder wires. A few CV tweaks allow the LED to operate when the ‘firebox shovelling’ sound is activated.
When re-assembling, remember to refit the front motor plate, routing the orange and grey wires so they are not trapped. Refit the weight ensuring that the new components underneath fit snugly. The clips that hold the front of the cab to the footplate are a very tight fit – the trick is to angle the cab away from the motor and push down to get the clips to click into place.
With a little work, just over £100, time and patience you too can have a sound-fitted J94 for your layout.

The Hornby Hunslet Austerity 0-6-0ST before works starts.


Remove the screw from the chimney and two screws from underneath to loosen the cab and tank moulding.


To expose the motor and wiring, undo the screws holding the die-cast ballast weight.


With the weight off, the existing decoder and ‘Stay Alive’ pack is on show.


New Zimo decoder is small enough to fit into the hollow under the tank casting.


Install the new  Zimo MX645 decoder and 'sugar cube' speaker with custom sound from Digitrains (see text above for details).



Remove the U-shaped part of the cast weight. A further modification is the firebox glow LED, which is synchronised with the ‘firebox door open/coal shovelling’ sound function.

Warning: Make sure you insulate all exposed wires before replacing the body. Job done!


Zimo MX648 decoder with Digitrains sound
T: 01522 527 731



Ian Mellors' Summat Colliery was Layout Of the Month in the November 2013 edition of BRM.

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