Bachmann Class 108 DMU


Another loco aimed mainly at our extended "Little Histon" layout, to enable post 1967 period running up to closure of the station.

Hatton's photo of the DMU showing the 1967 BR blue color scheme.


After a few practical display issues with the earlier Derby Lightweight DMU, I need to try some changes on the DCC sound fit of the Class 108:

First, the original Class 108s photographed on our line in the late sixties and 1970 are exclusively two car units, so the centre car will not now be used.

The sound levels achieved in our earlier Derby Lightweight cars, using a low power audio decoder (MX648) and a sugar cube speaker driven at a level low enough to avoid distortion, are not high enough for diesel sounds in our display environment. I also had a few problems with our fairly short track, where the motor sounds incorporating automated gear changing, stayed at high revs despite the locomotive slowing to a stop......... So this time I am going to try Bif's Class 108 sound project on board an ESU LokSound V4, driving a conventional 4 ohm 40x20 speaker.  The speaker is mounted in its standard enclosure, facing downwards, semi-recessed under the floor of the trailer car. I propose to modify the decoder mounting space on top of the motor housing, removing the side & rear walls with a hack saw & file, to make sufficient room for the V4 decoder to fit sideways on, between roof and motor housing.

I am using an ESU LokPilot FX function decoder in the rear car to drive the lights, as this decoder can track the soft switched lighting, used in the front car LokSound decoder. The rear car decoder fits within the toilet enclosure. A link is used to disconnect the track feed to the rear car decoder, to enable separate programming of the LokSound while connected to the speaker via a 2 way connector between the cars. (The rear car can be programmed separately on dedicated track, when the 4 way connector plug is removed.)  

Another two way inter-car connector is connected to the track contacts, so that all 16 wheels contribute to the DCC feeds to both decoders, to minimise the chance of a brown-out. If I can find room, a 6800uF Stay alive capacitor may also be fitted in the front power car.

A cab light is installed within the power car.

A driver has taken up residence in the front cab and a few passengers are distributed in both cars.

The outer tension lock couplers have been removed.

The destination boards are now  "St Ives" and "Cambridge".

The sprung feeds to the motor are now hard wired and the contact tabs removed, to eliminate a likely source of trouble.  (All bogies will be hard wired in the event of poor conduction via the  contact tabs or excessive tab vibration noises.)


The circuit.


The Bachmann model as received, posed at Histon Station.


Running Lights:

Decades before the introduction of modern dual function LED combined rear and front marker light fittings..... The Class 108 operated on a similar basis..... except that the light source was a pair of fairly dim conventional bulbs mounted one above the other (top red, bottom white) within a common circular housing employing a flat glass front.  Bachmann's interpretation is a light pipe with one red and one warm white LED providing the illumination.


Internal Coach Lighting:

The roof mounted compartment lighting circuit includes all series resistors, so can be powered directly from the decoder function output. A pair of yellow LEDs, each with an individual 1k2 series resistor, are connected in parallel across the supply. A single white LED plus 3k9 series resistor, across the common supply, provides rear illumination for the destination panel. Changing the destination label is fairly straight forward. The original painted on destination can be removed from the rear of the clear plastic window by scraping with a wooden cocktail stick. The new destination label is printed from the PC (white lettering on black background) and glued to the rear of the window using transparent glu 'n glaze.

I initially powered the internal lights via diodes from the headlight and rear light negative lines...... however, it is apparent that independent software control of the LED brightness is needed to dim the running lights, while the internal lights need closer to max intensity. Also, when a direction change occurs, the running lights change using soft start and stop. This causes a brief fade of the supply to the coach lighting..... As there are enough function outputs, I'm changing to a dedicated internal light negative feed. 

In fact, the original Bachmann internal lighting is far too dim and the lightpipe does not evenly distribute the limited available light evenly. So plan B is to remove the light pipe sheet and install replacement chip LEDs glued to the roof PCB. (I'll try 4 in the trailer car and 3 in the power car (where the motor housing makes an end light redundant). The PCB will then be painted white.


Internal Lighting Plan B:

Starting in the trailer car, I've glued 4 DCC Concepts protowhite chip LEDs along the roof PCB centre line. I've located one in the first class compartment, 2 in the central compartment and one in the end compartment. I've positioned them slightly forward of the obvious central locations, in the hope of illuminating the passengers better. Initially, I've used the original 1k2 series resistors, each wired with two series connected LEDs. (Each LED is glued to a small thin plasticard insulator, which in turn is glued to the PCB.) I've also painted the PCB white. 


The new lighting is a definite improvement.


Fitting the speaker:

Unlike the power car, the trailer car has a plastic floor. So that has to be the primary target location. Near the centre of the car, the under-floor furniture can conceal a protruding 40x20 enclosure from virtually any direction (as long as the car stays on the rails.) The PCB between the floor and the seat mouldings carries the usual dual operation DC/DCC circuitry at exactly this location. However as I only use DCC, these parts are just not required, so the centre of the PCB can safely be removed while the essential interconnect tracks and a few essential components, can be hard wired around the speaker enclosure between the PCB and the car underside moulding. The speaker enclosure can be inserted through the gap cut in the PCB right up to the seat moulding underside. The standard ESU 50344 20x40 enclosure is nicely hidden in this location, by the surrounding engine, radiator and under floor tanks etc. (Which typically protrude well below the speaker.) 


Motor Control CVs:

The motor control is fine tuned via a number of relevant CVs. I found some jitter present at very low speed so I've made some changes to largely eliminate the issue. There are probably other combinations that would do the job, but these are the ones I am now running with:

CV Value CV Value CV Value
2 1 51 6 29 14
3 60 52 36 49 17
4 30 53 61 124 4
5 255 54 60    
6 90 55 30    
    56 150    





Re-Arranging the Function mapping:

Charlie's website Function List:

Bif confirms that since this was posted, The manual gear change is now on Key 7 and the on-board toilet flush is on Key 8.

(Also handbrake on/off is now on F16 and F17.)




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%

Revised map to put the most useful functions in the 0 to 9 range.


An expensive mistake:

As indicated above, initially I used an existing 4 way miniature connector to link circuitry in the two cars. The inner two conductors linked the trailer car speaker to the power car sound decoder. The outer two conductors linked the track contacts in both cars to give 16 wheel pick-up reliability to both decoders. If I accidentally connected the plug and socket the wrong way around, all that would happen is that the track contacts would short out and the DCC controller safety trip would activate.  However......  When connecting up in poor lighting to make final checks, I managed to insert the plug with a one pin offset.... this connected the sound decoder audio output to the DCC track and immediately killed the LokSound audio circuit. So I had to purchase another Bif 108 LokSound, via Charlie's website.

I've now added an additional two way connector on the other side of the cars, dedicated to the speaker connection, to prevent a similar disaster happening again.


Supplier website links:
 Web Link (click below)    Items concerned
Hattons    Class 108 Sourced here.
DC Kits D&E Videos    Source for Bif LokSound V4 and ESU speaker.
Digitrains    Source for LEDs


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