When converting the Heljan Class 31 to DCC sound, in addition to sorting out where the decoder and speaker are going to fit, there are the lights and the fan to take care of. The body is removed from the chassis by undoing the two screws at each end. Take care when lifting it off the chassis as there are several wires connecting body and chassis via plugs and sockets. These are for the lights and fan which will in fact require more work than the basic DCC installation (photo 2).photo 2

The internal circuit board (photo 3) has resistors to control the voltage to the lights and diodes to control them directionally. Although it is possible to re-use this board I have found it brings more problems than it saves. Removing it also gives more space for the decoder. As an aside, these boards are not the same across the Heljan range which further complicates the issue and makes them even less attractive to retain.

Looking upwards inside the body, there is a second circuit board (photo 4) linked to the fan. This board regulates the voltage to the fan to about 1.25 volts. Putting 12 volts to the fan results in it screaming in agony at far too fast a speed. In some Heljan diesels the fan control circuitry is on the main board instead of a separate one.

Loudspeaker

Heljan have designed the battery box under the chassis to take a large circular speaker (photo 5). This will need securing with glue or mastic to stop it rattling and the wires will require feeding up to the top of the chassis.

On the Class 31 holes need to be drilled up through the chassis to take these wires but on other models the holes are ready drilled.

Alternatively a bass reflex speaker can be mounted in the roof (photo 6) next to the fan voltage regulator. This was the option I selected, as it was both easier and gave a better sound.

Decoder

The ESU XL decoder has a 3amp rating which will provide enough power for those twin motors – mine was loaded with a Howes sound project.

I mounted the decoder on two strips of square crosssection plastic (photo 7) to ensure the bare connections on the back can never touch the metal chassis. It was then epoxied onto the chassis. I used quick-set epoxy which is strong enough to hold it in place but not so strong that it cannot be removed if necessary.

The screw terminals are labelled for easy identification (photo 8).The motors can be wired in series or parallel. Series connection gives a slower loco, consuming less power – good for the smaller layout. However if you want a higher top speed then parallel connection is the way to go. In either case it is important to have the motors connected so they pull in the same direction - however if you get this wrong it is easily corrected.

Lights

I wired up the lights so that the headcode lights operate directionally on F0 and the red tail lights operate with each end on its own function. This gives complete flexibility as to what lights are on and when.

It is essential that the resistors on the discarded terminal board are replaced or the LEDs will be almost instantly destroyed if connected to the full voltage out of the decoder. For both front and rear lights I used a 2k ohm series resistor which gave a realistic brightness. The headcode lights were connected to the FL and RL decoder terminals, and the red tail lights to AUX 1 and 2. I retained the plug and socket arrangement for connecting the lights to the chassis wiring for convenience (the extra four pin connector was for the fan and cab lights). I was surprised to find that the loco was fitted with instrument panel lights but easily connected these to work with the headcode lights.

Another surprise was that the loco was fitted with cab lights. Disappointingly they were in effect part of the headcode lighting which meant they are on whenever the headcode lights are on – not very prototypical. I blocked off the cab light LED on the back of the headcode lighting board with a piece of insulating tape (photo 9) as it was impracticable to disconnect it without a risk of damaging the headcode lighting. To avoid losing the cab lighting, I simply added a small yellow LED in the roof (photo 10) and connected both cab lights via series resistors to AUX3.

Fan The fan was connected to the AUX4 terminal on the decoder so it could be turned on and off at will. Simply connecting up the decoder to the original wiring ensures the 12 volts from the AUX output is regulated down to 1.25 volts. In Heljan diesels where the fan voltage regulator is integral with the lighting board it is necessary to replace this circuitry with a purpose built regulator. With the editor’s permission this may be the subject of a separate article in the future.

Function Mapping I left the headcode lights on F0 but as various sounds and facilities used F1 to F14, I mapped the red tail lights to F15 and 16, and the cab lights and fan to F17 and F18 respectively. Function mapping is most easily done with a programmer such as the ESU Lokprogrammer, Sprog or similar but it can be done by manually changing CVs – however this does need a very clear head to interpret the decoder manual.

DCC Operation It is also worth noting that a conversion like this is not just for the DCC layout. With a few adjustments to the lighting circuit and decoder CVs it is entirely possible to operate a loco like this on a DC/analogue system. However only the engine sounds will be available and there will be no control of the various lights. With the editor’s permission I will write a further article on how this may be achieved.

Conclusion This conversion, though basically straightforward, did throw up a few surprises such as the cab and instrument lighting and the regulated fan. All that is needed now is to apply some number transfers to the cabsides and couple it up to a train.