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Heljan O Gauge Class 20 diesel with DCC Sound

Kit: Zimo MX696S with YLR603010 and LS26X20X08-1W speakers

Here's the donor model, with MX696S shown in front.

Remove the 4 retaining screws. This reveals the factory PCB, and 2 large motors in typical Heljan style. The PCB and wiring is an improvement over previous models, as it at least uses standard DCC colours, and the PCB has useful markings on to help identify things.

Unscrew the wires from their terminals and remove the PCB, as we'll be hard-wiring the MX696S into place.

It will be a tight fit, and we'll have to be careful to insulate the decoder's pins from the metal chassis, but it will go just about spot on.

We don't have the space to use any of the standard adapters for the MX696S, so we solder standard DCC wire colours onto the pin feet directly. If you are not confident with soldering, YouChoos will do this for you before shipping the decoder - just ask!

Shown here is the fuel tank assembly removed from the model, a YLR-603010 speaker, and an LS40X20X09-1W Dumbo. Eventually we decide to go with the smaller LS26X20X08-1W dumbo (not pictured in the guide) instead of the LS40X20X09-1W, but the overall volume is similar. The YLR-603010 has its' lugs removed.

The pair of speakers are wired in parallel and Black Tack used to secure each in place. The YLR-603010 slots on its' side, reasonably deep as the decoder's components will drop down some distance into the aperture.

Methodically connect all the wires to the decoder - standard colours on the whole. Note that the Heljan motors are mirrored, and so are the wiring colours - you need the orange from one motor and the grey from the other to connect to one side of the decoder's motor output etc.. Also, you'll have to make a decision about lighting - we've decided to use 4 different AUX outputs from the decoder so that front and rear lights can be controlled independently from each other. In addition, the roof fan will need an AUX output to make it work, so we added a 5th AUX (FA3) from the decoder to control this. The project must be re-configured with new mapping to make this work of course.

It may look like a spider's nest, but there is plenty of room in the chassis block to tuck the wires into. Be neat and use the channels provided, and it will be much easier. Finally secure the decoder down with some electrical tape and test.

On track and all working!


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.

A bit of YouChoos poster art for your enjoyment!