Fitting DCC Sound to a Hornby Class 121 DMU

This article describes the process of fitting DCC sound to Hornby class 121 DMU, together with some detailing:

Sound Decoder

My sound decoder of choice is the excellent ESU Loksound:

These decoders are a single package, combining traction control with 4 functions and sound. They are back-EMF decoders and provide excellent control. Because the decoders integrate traction and sound, the sound is properly synchronised with the motor operation.
The ESU decoders come supplied with a speaker and a baffle box into which the speaker fits. Wherever possible, I try to fit the larger ESU 40x20mm speaker and a bass-reflex speaker as this combination gives better sound reproduction. However, on this model, there isn't enough space for a bass-reflex and surprisingly, the sound recording actually sounds very good with a well sealed standard speaker.


The Hornby class 121 has its herritage in the former Lima model, however, Hornby have made significant improvements to the model in terms of its finish, flush glazing and most importantly, a completely new mechanism. The new mechanism comprises a superbly smooth and quite new motor bogie.
The model requires very little preparation for DCC sound because it is 'DCC Ready'. The only work required is to create space in the weight compartment for a speaker.
To prepare the space for the speaker, we drill holes in the bottom of the weight compartment. The lugs inside the compartment have yet to be removed. We still need to drill a hole for the speaker cable:

We then fit the speaker in place, after drilling a hole for its cable and then fit some of the weights back on top of it and clip the seating back into place. The DCCSupplies bass reflex speaker is a perfect fit:


This model is pre-wired for DCC and has an 8-pin DCC Decoder Socket. Installing the decoder is simply a matter of plugging it in and securing it in place with a double sided tape. However, there is very little space above the motor assembly and because the ESU decoder is fairly long, we must create space by unscrewing the 8 pin socket and moving it towards the front end. We plug the decoder in and sit it on top of the motor housing, suitable retained using insulation tape:

Detailing Modifications

The Hornby model is fairly basic in detail, so here we fit Craftsman exhaust pipes, steps, buffers and buffer beam detailing and a driver - in my opinion, DCC sound just doesn't sound right if there is no driver!

The Completed Model

The following picture shows the completed model, together with a green version of the same model which has been sound fitted and modified in the same way. Note that this model has had Ultrascale replacement wheels fitted.

CV Adjustments

As supplied by Ian Bishop, the decoder requires no CV changes other than the max volume CV (63) set to a volume of personal perefence. DMUs had little more than bus engines and consequently, were not very noisy.

Prototype Videos

Class 121 at Cardiff Bay
Class 121 'Bubble car' @ colne valley railway
Class 121 DMU 50th Anniversary 1960-2010 Chinnor

The Model in Action

Graham Plowman (8/8/2011)

Sound Package supplied by Ian Bishop (LegoManBiffo)
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ESU Loksound Decoder supplied by:
Howes Models Logo

Class 121 railcar supplied by:
Hornby Logo

Bass Reflex Speaker supplied by:
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