Heljan O Gauge Class 50 Diesel with Zimo Sound

Kit: MX699KS; K50WP speaker in fuel tank; YLR453520 speaker in body

Heljan's late 2019 release of the Class 50 diesel is another great monster of a model. The usual twin motor with top fly-wheels and good body detail. Space inside is excellent - much improved with lowering of the weight, still giving excellent traction, but freeing up masses of room for an easier DCC Sound conversion.

Remove the 4 retaining screws under the sides of the bogies and the body pulls out easily, revealing all this space, and a central PCB with screw terminals for everything. Lighting resistors are on board this, which means we'll have to add our own resistors when the board is removed, to be replaced with the sound decoder.

The PCB is mounted on a couple of brackets, which are well designed in that you can reverse them, and reuse various supplied screws. Our Zimo MX699KS sound decoder screws nicely into the outer screws, though we have to be careful to rotate the brackets so that they fit properly.

Pickups and bogies are mirrored, as is normal for Heljan models, which means we need to take the red from one end, joined with the black from the other for one side of pickups, and vice versa for the other side of pickups. Same for the motor bogies - orange from one end plus grey from the other, and vice versa for the other motor connection. These are shown here screwed into the MX699KS. At least Heljan have sort of used standard DCC wire colours for this aspect!

Next we take out the wiring for the 2 switches on the underside - these are designed for DC to allow separate control of red tail lights and top overhead lights. This is not necessary under DCC as we will provide different function keys for separate control.

Here we wire in the fan PCB - chop off the plug from the main PCB end and connect the fan module's red to a common positive terminal on the MX699, and the fan module's black to FA5.

Next step is to begin wiring in the lighting. Here we start with the rear end lighting. Wiring colours are a bit odd, so some careful testing using a 12V DC supplied (with resistor) reveals which wire is which. Resistors are added - one for the overhead top lights, and another shared between the central headlight and red lights at that end.

Do something similar for the front end. We have F0Fwd as the front headlight, F0Rev as the rear headlight, FA1 for front red light, FA2 for rear red light, FA3 for front top light, FA4 for rear top light, and (as mentioned previously), FA5 for the fan. YouChoos can pre-configure a decoder with this mapping for you - just let us know at the time you order.

There is plenty of space inside, so we initially try out a pair of YLR453520 speakers in parallel. This works really well until the body is put on, at which point we get a massive amount of echo because the body space is so large. Therefore we must look for a different approach.

In the fuel tank we discover a decent space provided for a speaker - it even has a round recess for the magnet of a hifi style speaker to sink into. The K50WP is a 50mm speaker, but we cannot quite get it into the fuel tank without opening up the aperture slightly... a little filing allows this and so we thread the wires through, protect the terminals with some electrical tape, and secure the speaker to the tank with some Black Tack.

The K50WP and single YLR453520 are joined in parallel and screwed into the MX699's speaker terminals. Testing with the body on proves this combination to be a winner - the 50mm speaker in the tank provides good volume and clarity, and the YLR453520 provides just enough bass, and a little echo in the body to balance everything out.

That's it - pretty straight-forward install. Thanks to Heljan providing very good wire lengths, we can simply reuse all of the wiring provided by the factory, with the only real addition being the resistors. Sounds excellent and rolls very well.

Please note that these guides are provided as useful resources for you, as-is. YouChoos cannot be held responsible for errors in the information, or for any damage caused to your models or equipment if you choose to follow any of the steps detailed here.