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Heljan O Gauge GWR Diesel Railcar

Heljan O Gauge GWR Diesel Railcar

Kit: MX645PluX22 in ADAPUS break-out board; YLR603010 speaker; SuperCap17000uF

Bit of a funny one this Railcar... on the face of it , a decent attempt by Heljan with simpler electrics and motor arrangement than their larger diesels. It comes with a 21pin DCC socket and 9 lighting features, all of which can be separately disabled using a 9-way bank of switches on the underside.

Access to the 21pin socket is easy, removing 2 screws to reveal the socket. Note that Heljan have designed the 21pin socket upside down compared to normal, so if you do end up using a 21pin decoder, it goes the wrong way up!! Just line up the missing 'index' pin to check you've got it right.

The PCB is fitted with a variety of sockets, the important ones being clearly marked. The only spare one is the speaker socket.

The body removes quite easily after taking out the 4 retaining screws, so fitting of drivers and passenger figures is very straight forward.

Lighting is split into 2 groups - those on the chassis (lower headlights and red lights, then via a strip PCB along the top, interior lighting, cab lights, and overhead headlights. The upper lighting is connected via a 6-way plug to the DCC PCB. All LEDs have their protective resistors next to them, so the main DCC PCB is quite simple.

The problem with 21pin...

Ok, so this is where the design starts to fall apart... Heljan have designed the 21pin board to expect the first 4 outputs to be full power (known as 'Open Collector') and the remaining 5 outputs to be 'logic level'. Unfortunately there are multiple 'standards' for 21pin, and this arrangement is the latest called RCN-121 (Railcommunity). The ESU LokSound V5 supports this format, but none of the current MX Zimo decoders do, so there is no way to access all of the lighting in the model with any 21pin Zimo.

Zimo's forthcoming MS range includes the MS440, which will support it, and this is what was pictured earlier in this guide, but at the time of writing, the firmware of the MS440 only gives control of the first 8 of these outputs, not the 9th, which means you get no control of the interior coach lighting. Not only that, but geared diesel mechanical sound projects are not yet supported by the MS440 either (that will come later in the year).

So that leaves us with 2 options if using Zimo... either live without access to all the lighting features (MX644C gives control of only 6 of these), or convert the model to use a different interface.

21pin DCC is a horrid format for exactly this reason - too many 'standards', and little compatibility above 6 lighting outputs. Our solution instead is to lose the 21pin PCB fitted by Heljan and put in a PluX22 break-out board instead. The great thing about PluX22 is that it is a solid standard, with no ambiguity - all outputs are full power, none of this logic-level nonsense!

The journey to PluX22...

For reference, here is the layout of Heljan's PCB, with wire colours shown, which we'll refer to as we rewire to our PluX22 board.

We have a couple of options to get to PluX22... we often suggest an ADAPLU booster board for O gauge, which provides fairly small solder pads for all the features, but increases the continuous motor output from 1.2A to 1.8A. If you're good at soldering, then this is a good option - it is also quite compact and fits in the aperture in the model very nicely.

The 2nd option (the farther PCB shown in these photos), is a simple break-out PCB which provides much larger solder pads around the edge. It is NOT a booster board, so the capabilities of the decoder are unchanged (1.2A continuous to motor and up to 2.5A peak for short periods), but in the Heljan Railcar, this should be absolutely fine and well within means. This simpler board is called an ADAPUS. It is slightly longer than the available recess, but the recess is cut-away internally, so does actually fit anyway. This is the the option we'll use for this guide.

The wider plug which goes to the bank of switches is not important for DCC control, as this will all be done through function key presses instead. This will just remain in the model, but unattached to anything. The 6way connector to the roof lighting board will need to be cut off (same as all of the 2pin plugs) and those wires soldered directly to the appropriate solder pads on the ADAPUS board. It might seem sacrilege to do this, but it is actually pretty straight-forward to do - just be methodical.

First the RED and BLACK pickup wires go on...

We work our way around the lighting wires. There is a solder tab for common positive at each end, so we use the most convenient one for the wire we are attaching. A special version of our Railcar project assumes the following mapping use of the physical outputs...

  • Front headlights: F0Fwd/LightF
  • Rear headlights: F0Rev/LightR
  • Front red: FA1
  • Rear red: FA2
  • Front top light: FA3
  • Rear top light: FA4
  • Front cab light: FA5
  • Rear cab light: FA6
  • Interior lights: FA7

Note that we have also added a pair of PURPLE wires for the speaker, and a GREY+BLUE wire pair for the stay-alive, which will put it at the end.

We test this as we went round (note the croc clips from our track). Our test decoder was just an MX637PluX22, but the MX645PluX22 would have been fine to test with too.

The sound decoder now goes in...

Some electrical tape, then BlackTack to mount the ADAPUS board, decoder downwards, and tidy all the wiring up with little blobs of BlackTack.

Tuck in the switch harness out of the way, which is no longer used. The speaker and stay-alive wires come through a convenient gap in the decoder housing.

At this point you can test all of the lighting again, now the seating assembly is back in position.

Our YLR603010 speaker is BlackTack'd to the underside, as is the SuperCap.

Body back on and on the test track, looking very nice. What seemed like it was going to be quite a long-winded process to convert to PluX22 wasn't actually too bad at all, thanks to some good wiring in the model, and the ADAPUS break-out board.

We have a special version of our Railcar project with FKeys for all of the lighting, assuming you go with the PluX22 approach - just select this variant on the Railcar project page.


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.

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