Installing a DCC decoder in a Hornby M7 Steam Loco

Updated with notes about installing the TCS DP2X-UK plug and play decoder.

Click on each image for a more detailed view.

M1 and Zimo 620R decoder

This installation should be one of the easiest in a steam loco as the engine is DCC ready. However refitting the body on to the chassis is not as straightforward as it should be.

Before installing a decoder into any loco it is advisable to check the current being drawn by the loco.

To remove the body there are two screws, one at each end of the loco. You will need both chisel and phillips watchmakers type screwdrivers to carry out the installation.

This install shows a Zimo MX620R decoder (now replaced by the MX621R), but any very small decoder would also fit, such as the TCS M1-UK. The DP2X-UK decoder from TCS is a plug and play decoder with no wires fits easily and obviates the need for any modification of the body as outlined below.

M7 vac tankM7 Rear fixing screw

The screw at the front of the loco is hidden underneath the moulded vac tank. The tank needs to be VERY gently prised upwards from the front of the loco using a chisel type screwdriver being careful not to damage any of the delicate bodywork parts. You will, in any case, need to remove the vac tank if you intend using the coupling supplied with the model.

The rear screw can be located between the axle of the rear pony truck.

Gently separate the body from the chassis by rocking the body forwards and backwards - it is a very tight fit.

M7 with body removedM7 DCC dummy plug

The loco is DCC ready, having a socket installed at the front of the loco. The socket is hidden by a mass of wires and the capacitor. Lift the wires and gently lever the dummy plug out of the socket.

This reveals the DCC socket, with pin 1 (for the pin with the orange wire attached) clearly labelled. This installation used a Zimo 620R decoder, one of the smallest decoders on the market.

M7 with decoder fitted

Firmly push the plug attached to the decoder into the socket, being careful not to bend the pins.

The decoder wires will need to be folded so that the decoder sits on the lowest part of the motor assembly and will need to be secured in position. The Zimo decoder comes attached to a sticky pad, and this was used for this installation. However, you could use double sided carpet tape or a Sellotape Sticky Fixer as an alternative. Although this decoder is not vacuum sealed it was decided not to use the shielding supplied with the loco as the decoder had already been electronically isolated from the metallic parts of the model.

It is vital that the wires are carefully folded so that none are extending either side of the chassis, otherwise it will be impossible to reassemble the two parts.

If you wish to remove the capacitor this can be done by just cutting out the device, but this adaptation was not necessary with this decoder.


Inside the M7 bodyM7 with internal rib filed flush

With the Zimo decoder and all of the additional wires it was not possible to fit the body back onto the chassis so that the two parts sat comfortably together. Inside the body is a rib that extends around the inside of the boiler moulding.

It was necessary to file this rib down so that it was flush with the rest of the boiler moulding in order to get everything seated correctly.

When refitting the body slide the front part of the chassis into the body about half way along the body so that the decoder assembly and wiring slips into the boiler moulded shape.

These body modifications are not necessary with the TCS DP2X-UK as the decoder just fits.

M7 connection pipes


It is suggested that you do any programming of the decoder at this point, before replacing the body.

Replace the body carefully taking care with the delicate pipework at the rear. Make sure that the two connecting pipes, one each side of the loco, are relocated.

If you use the TCS DP2X-UK decoder, although this has no wires, make sure there are no wires from under the socket lying around the edge of the circuit boards as these will prevent the model being reassembled and could themselves get damaged in the reassembly process.

The loco performed faultlessly using the Zimo decoder without any programming being required, except to change the loco number. Similarly, an installation using the TCS DP2X-UK decoder also produced excellent performance characteristics.

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