Installing a DCC decoder in a Hornby Class 121

Click on each image for a more detailed view.

121 and Zimo 63R decoder

This installation should be one of the easiest as the loco is DCC ready, but it isn't!

Manufacturers really do need to make greater efforts to make the inside of their locos more accessible for those who want to convert them to DCC operation. Getting the body off is a real challenge and refitting it 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.

This install shows a Zimo MX631R decoder (which has now been superceded) but any decoder would also fit as there is plenty of space inside.

M7 Rear fixing screw

To remove the body there are two screws on the underside, as shown in the photo. This is the easiest part of the installation!

Try following the instructions provided by Hornby with the loco. They make separating the body from the chassis look easy.

It isn't!!

Non-motor end of locoMotor end of loco


Try to gently separate the body from the chassis by levering the body side away from the chassis - it is a very tight fit at the ends. There seems to be plenty of sideways movement but it took 20 minutes to eventually separate the two parts. The reason it is so difficult is that there are two lugs, one each end of the loco, securing the body to the chassis. The lugs fit into gaps in the glazing inside the body at each end. If you try levering the end away there is not much space and the moulding could crack if you apply too much pressure - so be warned!!!


M7 with decoder fitted


The loco is DCC ready, having a socket installed at the front of the loco and once the body is off installing the decoder is straightforward.

Pin 1 (for the pin with the orange wire attached) is clearly labelled, both on the dummy plug and on the socket itself. Firmly push the plug attached to the decoder into the socket, being careful not to bend the pins.



Decoder in situ

The decoder wires will need to be folded so that they do not obstruct any movement of the motor bogie and that they do not obstruct reassembly of the model. A 4-function decoder was used as the intention is to install a lighting kit. Disappointingly the model does not come with any lighting installed. In this day and age it would be reasonable to expect models to have such items installed during the manufacturing process.

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.

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

Refitting body of 121

When refitting the body, first slide the rectangular lug at the non-motor end of the chassis into the body.

You will then find that you have to remove the two exhaust pipes at the motor end as they will not go over the buffer beam. Push firmly to reseat the body. Be careful not to bend the pins on the pipes when pushing them back in! (Voice of experience!!)

The loco performed faultlessly using the Zimo MX631R without any programming of the decoder being required, except to change the loco number.

For an alternative plug and play decoder the TCS DP2X would suit.

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