Bachmann Derby Lightweight DMU

Updated to provide higher sound volume &

Faster engine/gear change response


Another loco aimed mainly at our extended "Little Histon" layout, to enable late 50s & early 60s period running through the station.

Hattons photo of the DMU pair

Revised configuration and reasons for change:

The initial configuration used a miniature Zimo sound decoder (MX648) which fitted within the Bachmann decoder bay on top of the motor housing. This delivers 1 Watt of audio power into an 8 ohm speaker. The speaker technology used was an 8 ohm Zimo Dumbo sugar cube style transducer housed within a resonator box. The decoder was equipped with a Paul Chetter sound project based on recordings from a preserved Derby Lightweight DMU. Although good results were obtained in the relative quiet of my (continuous) loft layout, performance on our Little Histon village layout proved problematic:

Even operating at maximum volume (limited by overload distortion from the speaker) the sound level was not sufficiently loud enough to be heard clearly against the "background chat" ambient noise, typically experienced at our village layout displays. Also, the engine sound system did not work well on our relatively short layout track length.  Specifically, the engine sound could not be dropped to idle quickly for a station stop, once the gear change sequence had started, often leaving the DMU engine racing away for several seconds after the train had been brought to a standstill at the platform.

So changes are needed:

To increase the available sound volume and speed up the response of the engine when stopping the train, the following changes are underway:

The side walls of the decoder bay in the top of the motor housing, have been removed, this enables an ESU LokSound V4 decoder to be mounted sideways within the bay, once its insulating cover has been trimmed at both ends and the wiring carefully bent back to minimise the occupied length. This can deliver nearly 3 Watts of audio power into a 4 ohm speaker. This time a conventional ESU 20x40mm speaker will be used, mounted in the standard ESU enclosure, which will be fitted semi-recessed in the floor of the trailer car, facing down towards the track. Legomanbiffo's sound project will be used. This seems a better match to our layout operation, with a faster engine sound transition to idle, when performing station stops on our short layout.     


Another Hattons photo.


Showing how to dismantle the trailer car:


This picture shows the process required to separate the chassis assembly from the upper body shell. The unit shown is the trailer car (with original speaker) but the same technique is applied to both cars. First step is to remove an external screw near the driver's end of each car. However, the front bogie has to be removed in order to access the screw. There are 3 clips on each side. The fingers are on the chassis assembly and the sockets (slots) are built into the lower edges of the window transparencies. The windows have a tendency to stay clipped and separate from the upper body shell when the car sides are pulled outwards. I found that it was usually necessary to carefully use a small craft knife blade to push each window slot back into position on the body side, while slipping a piece of card between the slot and the chassis finger to hold them apart. When all 6 clips have been treated in this way, the chassis and upper body shell separate easily. 



The lighting arrangements

The original cars each have twin forward marker lights and a single central rear light which provide directional running lights, controlled in DCC mode via the headlight and rearlight function outputs of the decoder in each car. Both cars are also fitted with passenger accommodation lighting, which in DCC mode is permanently on. 

The internal passenger lights and new cab light will now be independently switched by the decoders. (So when parked up on a DCC layout, the internal lights can be kept off.)

A spare TCS FL4 decoder is fitted in the rear car to provide the internal lighting and directional headlight plus rear light switching. This lacks the soft start available on the power car LokSound decoder and if this becomes annoying, I'll change it for an ESU LokPilot FX, which has this feature.

Inter-car connections

The speaker connections and track connections are made via a pair of two way connectors (one on each side of the connecting corridor).


TCS FL4 programming

Description CV Number Value
Address 1 17
Set green wire (rear car rearlight) to continuous dimmable forward only 51 12
Set green dim level (set to max as it flickers when dim) 64 30
Set purple wire (rear car head lights) to continuous dimmable reverse only 52 82
Set purple dim level (set to max as it flickers when dim) 153 30
Set pink wire (internal lights) to continuous 54 32
Green wire to F0 (forward) 35 1
Green wire to F0 (forward) 37 0
Purple wire to F0 (reverse) 36 2
Purple wire to F0 (reverse) 38 0
Pink wire to F4 40 32
Pink wire to F4 42 0


The FL4 soldered to the truncated 6 pin blanking plug.


LokSound V4 / TCS FL2 Circuit Diagram



Trailer car PCB board centre removed to accommodate the new speaker enclosure.

The original surface mount components are replaced with just 3 leaded LED series resistors.


"Inconspicuous" rather than invisible, when viewed from the side.


The Power Car:

The motor housing has a small bay on the top at the rear to accommodate a small DCC decoder. In order to mount the larger ESU LokSound V4, I've removed the side walls from the decoder bay, which will just enable the V4 to fit transversally across the top of the housing.


An additional problem with this power car has been a mechanical noise produced from the motor area. This was almost (but not entirely) eliminated by fitting blue tac blobs behind the motor bogie wheel contact tabs on the PCB.  This time I've removed the tabs completely and the electrical contacts have been hard wired.  There is very limited space available for wiring within the motor housing, so the bogie wires have been taken through holes drilled in the top of the motor housing, adjacent to the pivot point. These have been externally soldered to corresponding wires from the PCB & sleeved on the top of the motor housing, forward of the decoder bay.



Fitting the decoder etc in the power car:

The ESU LokSound V4 is a very tight fit when transversally inserted into the roof space. The insulating sleeving of the decoder has to be trimmed to squeeze it into place.  The 8 way DCC plug wiring also needs to be reduced in length, to keep the wiring tidy.


The inter-car connections:

Two way TCS miniature connectors are fitted, a pair on either side of the connecting corridor. One pair carries the speaker wires and the other connects the track pick-ups of each car in parallel, largely eliminating the need for stay alive capacitors, with all 16 wheels accessing the track signal.

The speaker connector can be safely connected either way around, whereas The wheel contacts must be connected white to white and black to black. However, accidental connection the wrong way around, simply cuts out the DCC controller due to the resulting track short circuit!


Function mapping:

Bif's original has been re-mapped to exactly match the more recent Class 105 and Class 108 DMU control arrangements....



0 Running lights
1 Sound on / engine start
2 Hi Horn
3 Lo Horn
4 Internal lights
5 Despatch whistle
6 Right away signals
7 Manual gear change
8 Door slamming
9 Wheel flange scrape
10 Cab light
12 Brake application
13 Buffering up
14 Toilet flush
15 Firebell or buzzer
16 Handbrake on
17 Handbrake off
18 Detonators
19 Reduce volume to 50%




1) The power car noisy running issue has been completely eliminated by removing the power bogie wheel contact spring fingers and instead hard wiring the connections. 

2) The LokSound V4 driving a traditional 40x20mm ESU speaker unit, provides a huge improvement in the available undistorted sound level compared to the lower power Zimo decoder and Jumbo sugar cube style speaker unit originally used.

3) The Bif sound project works much better on a small layout than the original Zimo system, with revs dropping back to idle in response to station stops in a more speedy manner, automatically.



The custom destination labels.


To see the original Zimo based You Tube video, please click this line


A new video showing the upgraded unit will appear here in due course.


Supplier website links:
Hattons    DMU Sourced from Hattons
Digitrains    Source for Zimo stay alive super caps, speakers and LEDs
DC Kits    Source for ESU decoder and speaker


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