Fitting DCC sound to a Lima HST

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This article describes the process of fitting DCC sound to a Lima HST:

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Sound Decoder

My sound decoder of choice is the excellent ESU Loksound:

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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 or DCCSupplies bass-reflex speakers as the latter gives better sound reproduction.

Loco Preparation

Like most Lima locos, the HST has a very noisy motor and therefore, adding DCC sound is pointless unless the motor noise is rectified, especially as both power cars on my HST are powered.
I have used the ModelTorque motor on a few other Lima locos, however, while it provides significantly better performance, the noise level is largely unchanged.
Therefore, for this project, I have decided to replace the motor bogies completely.
When I started planning this project, I had just purchased one of the new Hornby class 121 DMU's and noticed the new motor bogie it had fitted. This motor bogie runs extremely well.
At the time, I wasn't aware that Hornby were going to be re-introducing the Lima HST into their 'RailRoad' range, complete with new motor bogie.
I discovered that the wheel base of the DMU motor bogie was exactly the same as that on the HST.
I contacted Simon Kohler of Hornby who kindly provided two DMU motor bogies:

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The power bogie shown here is fitted with Ultrascale wheels to match those I fitted on my power cars some years ago. The bogie has a can motor driving all wheels with all wheel pickup.

The following pictures show the Lima chassis, currently fitted with a TCS M1 decoder:

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Fitting the new Motor Bogie

The new Hornby motor bogie is a self-contained unit which requires a chassis mounting to be located above the bogie. Because the Lima motor bogie is suspended using the bogie side frames, it has no mountings above the bogie, therefore, to fit the new bogie requires a completely new mounting system. The main challenge is to ensure that any new mounting system maintains the chassis at the correct height above the rails and level with respect to the leading bogie.
I decided that a mounting plate suspended across the top of the motor bogie was the way to go. Trial and error with a few pieces of plastic strip found the correct height:

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The white plastic strips are Evergreen no. 199 6.3mm square section, cut to 14mm on the rear end of the chassis and 28mm above the tanks. The inset is 8mm wide and is cut in 2mm to clear the body retaining screw. The pictures show the allignments.

The mounting plate was laminated from three strips of 1mm thick plastic card, 16mm wide, 70mm long, with a 4.5mm hole drilled exactly 40mm from one end of the plate to the centre of the hole. The plate was then screw mounted, using the bogie to ensure that it hung centrally within the frame surround. Notice that the motor bogie will not fit in the frames centrally until the bearers upon which the Lima bogie frames rested are removed from the insides of the frames:

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Note that before fitting the motor bogie, there is a nodule of plastic which must be filed off otherwise the vehicle will tend to lean in one direction. This modification would also prevent wobble problems with other Hornby models which use the same motor bogie:

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Electrical Wiring

Now that the motor bogie is fitted, we can start the electrical wiring. We start by wiring all of the wheel pickups. The following pictures show the motor bogie and front bogie:

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Note that on the front bogie, the original Lima copper strip has been removed from inside the bogie and replaced with two separate strips touching each axle. One axle is reversed so that we have one wheel picking up from each side. In the future, I may expand this into all-wheel pickup.

Now we wire all of the pickups to some strip board attached above the motor bogie:

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The stripboard acts as a 'junction box' for all wiring and could have a DCC 8 pin socket attached.

Wiring the Sound Decoder

The sound decoder being fitted is an ESU LokSound, obtained from Howes Model Railways with their HST 'Valenta' sound package installed.
I have located the sound decoder on top of the original Lima steel weight, however, before doing so, the top of the weight was covered with insulation tape (even though the decoder is enclosed in its own insulated wrap) and the weight has been glued into the frames so that it does not move - this is also partly related to body modifications which will come later:

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The decoder has been 'hardwired' to the strip board.

Wiring the Lights

The Lima model only had white lights which were constantly illuminated and not directional. All of the Lima lighting has been removed and replaced with two LEDs for the white lights and a single red light:

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Wiring the Speaker (Version 1)

The standard speaker supplied with ESU decoders provides satisfactory performance for higher frequencies, however, it has no bass capability. Therefore, I fitted a DCC Supplies bass-reflex speaker above the leading bogie to significantly improve the sound rendition:

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Note that the 23mm speaker above the rear bogie has had the openings in its case sealed with blue-tac. The blue-tac will be painted black.
The two speakers are wired in parallel and this pair is then wired in series with a 50 ohm resistor to create a total 100 ohm load on the decoder. This is the same load as a standard 100 ohm speaker. In theory, this wiring arrangement reduces volume slightly, however, this wasn't actually noticeable, probably because the volume is already high as supplied.

Wiring the Speaker (Version 2)

Following various tests with the Howes and Olivias sound packages, the package produced by Ian Bishop was selected. In making this change, a number of other changes were made to the power cars:

An 8 pin decoder socket was fitted to replace the hard-wiring system
The Lima weight was removed
The speaker inside the DCCSupplies bass-reflex was removed and fitted facing down inside the fuel tanks with a suitable aperture cut on the underside of the fuel tanks
A whole new sealed sound chamber was constructed using lead flashing so as to replace the removed Lima weight. This sound chamber improves the bass significantly

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Bodywork Modifications

Having made significant modifications to the chassis, the time has now come to arrange for the body to be refitted to the chassis. The Lima body has a perspex frame inserted inside which fills the entire body. Its purpose is to provide for the window glazing on the rear end and to provide a retainer for the weight. The perspex was removed and cut near the rear window and just behind the cab. The section near the rear window was then glued in place. The original idea was that the section behind the cab would form the cab back, however, this was subsequently found to obstruct fitting the body when drivers were sitting on the seats! Note also how the leading vertical body attachment tube has been removed from inside the roof. It can no longer be used due to the presence of the speaker and decoder:

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Roof Modifications

Finally, we cut out the engine exhaust outlets on the roof:

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Motor Bogie Side Frames

Although Hornby have made a motor bogie which is compatible with the Lima chassis, the Lima bogie sideframes are not compatible with the Hornby motor bogie unless modified.
To modify the frames, the underplate was cut to clear the Hornby gears and some plastic strip spacers were then attached as in the following pictures:

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The bogie frames were then glued to the motor bogies.

Prototype Videos

Paxman Valenta MML HST 125 opening up leaving Derby
43105 and 102 take out 1S12 at Newcastle
HST Leaving Reading
SCREAMING VALENTAS at Cheltenham 1989

Videos of the final Product

ESU Loksound Decoder, sound and speakers supplied by:

Motor Bogies kindly provided by Simon Kohler of:

Bass Reflex Speaker supplied by:

Graham Plowman (1/2/2011)

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