Fitting DCC Sound to a Heljan Class 35

This article describes the process of fitting DCC sound to a Heljan class 35 locomotive:

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.

Loco Preparation

The Heljan class 35 does not require any chassis block modifications to fit a standard speaker, provided an improvised baffle box is installed - there is insufficient space for a standard box.
In this installation, an improvised baffle 'box' was installed using blue-tac. The result is probably a better air-tight sealing than a regular box.
To fit larger 20x40mm speakers would either require chassis block modifications or a reduction in height of the speaker baffle box. There is enough space to mount a 20x44mm speaker on blue-tac, however, the PCB 'wires' poking up need to be snipped off so that they don't cause an electrical short when they touch the speaker frame. The wires can be seen snipped on the edge of the blue-tac in the right hand picture below.
Alternatively, a speaker could be located in the fuel tanks, facing down, although again, this would need to be a small speaker as the tanks are small.


This locomotive is pre-wired for DCC and has an 8-pin DCC Decoder Socket. The loco also has directional lighting. Installing the decoder is simply a matter of plugging it in and securing it in place.

Bodywork Modifications

Sound is not very effective unless it is allowed to 'escape' the loco, otherwise it is 'caught' inside the model, reverberates and can create strange noises in the bodywork which are not heard on the prototype. To this end, I normally drill out exhaust outlets and replace fan grills with proper meshes. Generally it is a good idea to place speakers under these meshes or in the fuel tanks facing downwards.

CV Adjustments

As supplied, the decoders have the max volume CV (63) set to 50. The class 35 is very noisy locomotive! At the default level, the loco sounds like a tractor. By reducing CV63 to a value of 40, the 'tractor' sound is reduced to an acceptable level, but unfortunately, the sound appears to have 'highs' and 'lows' and appears to 'fade', so finding an acceptable compromise somewhere between 35 and 63 is the order of the day.

Results - The Model in Action

Class 35 - stopping

Videos of real class 35's:

Click Here to view D7076 on the East Lancashire Railway
(HD)D7017 Hymek(&33048)Lydeard-Vexford-Washford-BlueAnchor-Vexford 12th June 2010
Wesmek (Hymek D7076) - Test Run - Sunday 28th June 2009
Hymek D7076 hellfire summer diesel gala 2011 ELR
Mega clagg D7076 Test run with new Maybach 870 engine - 29-6-11
Hymek D7076 is Back.! - East lancs Railway 10-7-11

Graham Plowman

ESU Loksound decoder, sound and speakers supplied by:
Howes Models Logo

Locomotive manufactured by:

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