DCC conversion and Lighting update of the Bachmann Class 70  Heavy Freight Diesel Locomotive



This page provides a summary of the process adopted to incorporate a DCC decoder and correctly operating external LED lighting in the Bachmann OO gauge model of the Class 70, heavy haul diesel locomotive.

Note that the smaller pictures can be enlarged by clicking on the image.

Class 70, 70006, viewed through the dirty window of a Northern Rail Class 156 at Carnforth in September 2010



Scope of the project:

DCC control: The unit has a 21 pin decoder socket built-in. The plan is to use a TCS 21 pin decoder, the EU621, which plugs directly on to the locomotive 21 pin DCC socket. The decoder has 6 function outputs which will enable full control of the locomotive lighting.

Lighting arrangements: On the real loco, daylight running uses the top and left hand lower forward marker lights plus the right hand headlight. Night running uses the top and right hand lower forward marker lights plus the left hand headlight. Both rear lights are used during day or night running if the loco is moving "light engine" i.e. without a train in tow. When towing a train, no rear lights are shown by the locomotive, but instead, a single rear facing, flashing red rear light is fitted to the rear wagon of the train. (To see a way to produce a realistic flashing rear warning light for DCC freight trains: click here).

The Bachmann model correctly incorporates the Class 70 light fittings. However, not all of the lights operate and the intensity of those that do, leaves a lot to be desired (e.g. the headlight is much dimmer than the marker lights). The main objective of this project is to model correct day and night operation of the lighting, with more representative relative levels of brightness.

The Bachmann model is only fitted with operational right hand (daylight) headlights. The right hand lower marker lights are in place, but non operational. So left hand (night running) headlights need to be added and the corresponding right hand lower marker lights need to be activated. The existing headlight needs to be much brighter and all the marker lights need to be reduced in intensity. The rear lights can be disabled (when the loco is at the head of a freight train) by means of a mechanical switch on the underside of the model. This operation will instead be controlled via the DCC controller. The forward cab light will also be activated during night running. All underside switches will be disconnected as their functions will now be automatic or switched via the DCC controller.

Note: The initial work was carried out on 70006. However, a more comprehensive set of photos has since been taken when subsequently modifying 70003, particularly on the main PCB. So revisions to the webpage attempt to make the work required clearer.



Day running lights

Night running lights

Rear lights


Separating the chassis and upper body shell:

Four well hidden posidrive screws are the first issue to be addressed. These should not be confused with the more visible screw heads nearer the centre of the loco. After removing the screws, the upper body shell can be eased away from the chassis by pulling the upper body sides apart and simultaneously extracting one end of the chassis. Two miniature 4 way connectors are used to provide power to the cab lights and top marker lights. These can be removed from the main pcb to completely separate the chassis and body shell assemblies.


The Lighting System Straight Out Of The Box:

The rear lights, headlight and lower marker lights are mounted on a small PCB mounted on the chassis and situated immediately behind the body shell mounted lower light housings.

The lighting PCB


One of the two PCB LED shrouds


Light-pipes in the front of the body shell


The missing light pipe for the left headlight


Circuit diagram for the Lighting PCB (minus the small RF suppression capacitors)


The cab light and top marker lights (also chip LEDs) fit on to a small PCB assembly screwed into the roof above each cab. Connections are via a pair of 4 way miniature flying lead connectors.


Circuit Diagram for the planned new lighting system: 


Sorting out the headlights:

The primary reason for the dim headlights is the large distance between the PCB mounted headlight LED and the input to the headlight light-pipe. The relationship between distance and light intensity approximates to an inverse square law. The distance between headlight lens and its light-pipe is approximately twice the distance between the marker lights and their light-pipe, so just as you would expect, the light level is about one quarter the intensity. (The LEDs run at the same current).  To produce brighter headlights, the distance between LED and light-pipe/lens needs to be minimised. The diagrams below show how I have done this. Also the successful solution to the new left hand headlight is illustrated.

Right Hand Lighting Mods

The original right hand headlight LED (LED 4) is removed from each lighting PCB. A new daylight white nano-light chip LED is mounted on a small rectangle of plasti-card, which is fixed into the right hand LED shroud, to position the new LED flush with the outside surface of the plastic shroud. This enables the new LED to beam directly into the adjacent light pipe/lens for the day headlight. The leads for the new LED are fed through a hole drilled into the side of the plastic shroud moulding, to be soldered as shown in the circuit diagram, when the shroud has been clipped back on to the lighting PCB. (Details shown below)

Left Hand Lighting Mods

The left hand light housing moulding is removed from the upper body shell by gently levering from side to side. The previously unused cavity behind the left hand headlight lens can be used as a very convenient location to mount another daylight white nano-light chip LED, after first painting the cavity interior white, and drilling a small hole in the outside corner to feed the LED leads through. When re-fitted, the housing can be secured in place with a small amount of Blu Tack squeezed between the housing and the upper body side. It may be necessary to open up the small hole in the lower tab of the housing to obtain a tight fit into the upper body shell.



Modifications to the lighting PCB assemblies required to accommodate the modified Right Hand headlights and new circuit configuration.


Rear of PCB: one cut and one link


Remove original headlight LED4


Right hand LED shroud fitted with new day headlight


Connections for wires from new LED

(The C3 pad indicated, connects to pad B1)


Circuit Changes, 4k7 resistors are added between main PCB pads A1, B3 and the wires to the lighting PCBs

(These are to reduce the intensity of the lower marker lights)


Fitting the new Left Hand (Night) headlights LEDs:


New left hand headlight LED glued

into the light housing cavity


Left light housing refitted to body shell

showing wires routed into main body cavity



Completing the wiring of the upper body shell assembly:

1. The new headlight wiring:

The enamel wire from the new left hand headlights is routed along the underside of the locomotive roof and around the outside of the fixing lugs. Series 1k resistors are soldered to the negative end of the LED wires.

Normal decoder size multi-strand copper wire is used to link the other end of the resistors to a twin connector. These wires are purple and green in colour to correspond to the new socket wiring in the locomotive chassis.

The positive LED wire ends are linked together with normal decoder wire (red in this case) with a further red wire connecting the positives to another connector. 

The enamel wires are spot glued to the inner surfaces of the upper body shell. The resistors and normal decoder wire are fixed to the roof underside using insulating tape, which also protects any exposed connections from the danger of short circuits.


2. Modifications to the original wiring of the upper body shell assembly:

In order to reduce the intensity of the upper front marker lights, extra series resistors (56k) are introduced into the wiring.  The cable to pin 1 of each 4 way plug is cut, in the straight section nearest to the plug, to enable a series 56k resistor to be soldered in position. The resistor and solder joints are completely enclosed in heat shrink sleeving.


Inside the upper body shell




Modifications to the main PCB.

The new circuitry can be accommodated on the original printed circuit board after a few minor modifications to the PCB:

There is quite a lot to do. I've divided the work up into several stages in case anyone is brave enough to give it a try!



The original main PCB assembly with the DC operation plug fitted to the decoder socket



Stage 1

Remove the decoder socket plug (only required for DC operation). 

Remove the 4 surface mounted diodes and 4 surface mounted resistors.

Add a wire link between the pads of R15 (a large SMD 470 ohm resistor).




Stage 2

Remove the black covers from the central wiring pins (P1-P5 & MO+,MO-).

Disconnect the motor wires MO+ and MO- and mark these for future re-assembly.

Disconnect wires from tabs P1 to P5. Trim the wires so that they can be parked with no danger of touching live circuitry.

Safely stow wires P1-P5 (These are connected to the under body switches which will no longer be used).

Solder wire links to connect tabs P1, P2 & P4.



Stage 3

Un-bolt the PCB from the chassis and turn it through 180 degrees to show the board underside.



Make the two track cuts as indicated in the marked up image above and add the purple decoder wire feed as also shown above, to result in the image shown below.

Turn the PCB back to its original position and bolt it back in place on to the chassis.




Stage 4

Add 4 diodes as shown below. (Note: Diodes near ends of PCB: Remove wire A3. Solder diode anode to PCB pad A3. Solder wire A3 to cathode [ringed end] of diode.)

Add white and yellow decoder wires as shown below. (Solder yellow decoder wire to left diode cathode and solder white decoder wire to right diode cathode.)

Add purple decoder enamel coated wire between pad P5 to the 21 pin decoder connector as shown below.

Connect purple wire from PCB underside to pad P5.



Stage 5

Add purple wire between lower central diode cathode [ringed end] and pad P5.

Add green wire between upper central diode cathode and the unused pad between P1 and P2.

Add green wire between (the pad between P1 and P2) and pad B3 on the right hand end of the picture.



Stage 6

Add transistor rear lights switch + brown decoder wire:

Solder the transistor emitter lead to the left R16 pad.

Solder a 10k resistor between the right R16 pad and the transistor base lead.

Solder a 56k resistor to the transistor base lead. Connect enamel coated wire between the other end of the 56k resistor (above the pcb) and the brown wire terminal on the 21 pin decoder connector as shown below.

Solder a brown wire between the transistor collector lead (above the pcb) and pad P3 on the PCB. (This tracks to the rear light positive connections at B2 at both ends of the PCB.)




Stage 7

One wire at a time, disconnect the wires to PCB pads A1 and B3 at each end of the PCB and fit 4k7 resistors between the pad and its wire. Use heat shrink sleeving to eliminate any danger of shorts to the nearby diode connections for the A1 pad resistors. (The 4k7 resistors reduce the intensity of the lower front marker lights).

Re-connect the motor wires, but reverse the polarity by soldering the MO+ wire to the MO- pad and the MO- wire to the MO+ pad. (If you don't reverse the polarity, you will find that the cab equipped with a driver is at the rear of the locomotive for the default direction of travel.)..... At least for the first release of Bachmann 70003 and 70006!



Stage 8
Fit the sockets to enable connection to the new left hand headlights in the upper body shell assembly.
Wire plug 1 with a green wire from the un-named pad between P1 and P2 and with a purple wire from pad P5.
Wire plug 2 with a red wire from the pad shown in the image below (common positive).
Stage 9
Finally ....... After a slight oversight, the missing green decoder enamelled wire is fitted between the un-named pad between P1 and P2, ..... and the 21 pin decoder connector as shown below:



TCS EU621 Decoder in place. The chassis is ready to be re-combined with the upper body shell.



Programming the EU621 Decoder CVs:


CV number


White (end 1 day forward)



Yellow (end 2 day reverse)



Green (end 1 night &cab forward)



Purple (end 2 night &cab reverse)



Brown (rear light enable F & R)


White via buttons  0 (F)



Yellow via buttons  0 (R)



Green via button  1



Purple via button  1



Brown via button 9


Brown via button 9


Pink (forward & reverse double strobe) 54 37
Pink via button 6 40 128


CV3=8   CV4=12  CV61= 1





Day running forward lights

Night running forward lights (with cab light)

Rear lights


Results of initial testing of the original 70006 loco:

The modified headlamps work very well with a dramatic increase in intensity. The top marker lights look about right with 56k of additional series resistance. One of the lower marker lights on 70006 was over-dimmed with 4k7 extra series resistance, so just for this light, the resistor was changed to 2k2.  (70003 looks fine with the same 56k added to the top markers and 4k7 resistors on all lower marker lights.)

As referred to above, the main surprise was that the default direction of the loco is in the direction of the un-manned cab. I had originally wired up 70006 assuming the fan end with the model driver in place was the default forward end. This issue is simply resolved by swopping the two motor connections to the main PCB. (As now described in the modification details above).

The final solution used for the lighting levels is shown below and the resistor values used appear in the (now updated) circuit diagram above.


Daylight running

Night running


Adding warning strobe lights to the lamp brackets, upper body shell mods:

A pair of orange strobe lights, modelled on the design of Dorman rear warning lights, were fabricated from 1mm plasti-card, using DCC Concepts amber nano-lights as the lighting source. The lights received an undercoat of gloss black paint, before receiving the white top coat, to prevent unwanted light glow from the lamp body.

Tiny holes less than 1mm in diameter were drilled for the lamp wires, in the edge of the central front body panel, immediately behind the lamp brackets. The nano light wires were threaded through these holes, to emerge below the cabs inside the upper body shell. The lamps were then super glued to the lamp brackets (beside the left hand headlights).  To avoid clashes with the loco lighting PCB assemblies, the wires were routed around the side of the upper body shell just below the cab floors, to emerge into the main body cavity alongside the new headlight wiring.

With thin flex wires now soldered to the ends of the enamelled wire & the joints insulated with heat shrink sleeving, the necessary connections are made to the original common positive wire and a new pink wire terminal, with a 1k series resistor incorporated.  


Modifications to the chassis assembly for the new warning strobe lights:

Access to the pink function wire of the decoder is provided by soldering thin enamelled copper wire directly to pin 4 of the 21 pin socket.  One of the unused speaker connection pads is isolated from the PCB track and is used to interface the thin enamel wire to the normal flex attached to the connector. The opportunity was also taken to tidy up the 70006 connector wiring in the space provided in the chassis block for a speaker. 

After re-assembly of the chassis and upper body shell plus a brief test, the holes in the cab fronts are filled and yellow paint is used to make the wires and filler less conspicuous.


The TCS decoder "double strobe" option is used via function button 6 (when in the yard) to provide twin short bright flashes that repeat approximately every second.

The effect works well, but the short bright flashes are difficult to capture accurately on the camera. Below is an animated GIF simulation.


Class 70 strobe lights simulation

(The real strobe pulse is shorter and much brighter)


     Class 70 Pair Gallery




Daylight running

Night running




Supplier website links:


Rails of Sheffield    The Freightliner Class 70 unit 70006 was purchased from this excellent web supplier.
Hattons    The Freightliner Class 70 unit 70003 was purchased from this excellent web supplier.
Bromsgrove Models    Excellent source of DCC decoders and specialist LED devices for this type of project.
Rapid Electronics    Excellent source of sensibly priced electronic components.
TCS (Train Control Systems)    A U.S. company. My preferred DCC decoder family with good programming data and advice on their website.
Maplin    Convenient local source for resistors and transistors, although the prices reflect that convenience.


Photos of the real class 70 were taken at Carnforth in September 2010.   The photos of the model were taken on the kitchen worktop at 200 ISO, either freehand with flash or using a tripod and lighting from an illuminated magnifier.    


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