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Install Guides by Loco Class General - Fitting Decoders General - Fitting Kit CT Elektronic Reference

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Heljan O Gauge Class 20 diesel with DCC Sound

Example 1: Direct-soldered MX696S

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

This kit is a bit of a push in terms of space - the MX696S is normally supplied with a screw terminal adapter (MX696KS version), but with the adapter plate there is no way to fit it in. Even just the decoder itself is very tight, and wires must be soldered directly to the feet of the pins of the decoder - it isn't ideal, but if you want to go with this approach then YouChoos can pre-solder the wires onto the decoder for you for a small extra charge.

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!

Example 2: OO Decoder with Booster board - easier fit

Kit: MX645PluX22; ADAPLU Booster Board; YLR302815 + IceCube25X18X10-1W speakers; SuperCap17000uF

This next approach is a much more comfortable fit, space-wise. We use a OO decoder, the MX645PluX22, but plugged into an ADAPLU booster board, which increases the motor power to 1.8A continuous - enough for the Class 20.

Before disconnecting any wiring from the factory PCB, we do a quick space-check to see what speakers we can slot underneath. YLR302815 (2W) goes in easily, and next to it an IceCube25X18X13-1W, which will help with the top-end frequencies.

Now with the wires disconnected, and factory PCB removed, we set about building a plastic platform for the ADAPLU booster board to sit on, secured in place with 4 blobs of BlackTack.

Slot in the SuperCap (covered in Kapton tape)...

...then start the process of soldering wires onto the approrpiate pads of the ADAPLU board. We find it easier to use Zimo decoder wire for these joints, then join those onto the wiring that comes with the model - some of the model's wires are very heavy gauge, and not easy to solder onto the relatively small pads of the ADAPLU.

Finally plug in the MX645PluX22 decoder (not shown here), and test all the lighting and the fan out.

Getting into the cab

We've been asked numerous times how we got into the cab of the Heljan Class 20, and it isn't straight-forward, so thought we'd share our approach for that too...

Basically, the rear glazing has to come out, but on many of these models it is impossible to prise it out from the outside, so instead, we make some discrete holes under the cab floor and use a long thin screwdriver to lever them out from the inside instead. This is actually quite easy, and we don't usually make the wipers pop off either!

You can see the 2 screws inside, which need to be removed in order to get the cab off. Bit of an odd design from Heljan!

We've added a cab light while we had it open too, which needed a small recess on the cab wall for the wire to safely route through.


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!