Installing a DCC decoder in a Bachmann Derby Lightweight DMU

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Derby Lightweight DMU

Before installing a decoder into any loco it is advisable to check the current being drawn by the loco. This DMU requires two decoders, one with an 8-pin plug for the motor car and a 6-pin decoder for the trailer car. The unit draws about 0.18 amps when running normally and 0.6 amps when stalled.

Why Bachmann insisted on using a 6-pin decoder socket for the trailer is a mystery and makes no sense when there are no 6-pin lighting only decoders currently available. The extra expense of having to purchase a motor decoder when there is no motor just defies all logic! The only justification seems to be the ease of locating the socket for the manufacturer.

Derby Lightweight DMU

Now it might be reasonable to expect one of the latest releases from a model railway manufacturer to have been designed for ease of access for decoder fitting. In this case, though, you would be wrong. This is one of the most difficult and frustrating models to install a decoder in.

Even the screw to release the body shell from the chassis isn't easy to access and the instructions with the model even suggest it might be necessary to remove the bogie. I'm always against unnecessary disassembly of a model but to design a model where the suggestion is that you might need to remove additional parts in order to access a small screw beggars belief. Surely the screw could have been located in a slightly more user-friendly location.

In fact it is possible to JUST access the tiny screw by moving the bogie to one side and using a very small screwdriver. However, the phillips headed screw was done up so tightly that I couldn't get a proper fit with a phillips screwdriver because of the angle and had to resort to a very small flat-headed screwdriver.

Derby Lightweight DMU

The next challenge is separating body from chassis - the two are a VERY tight fit. You will need 6 pieces of strong thin card to place under the body clips once you have managed to release them from the chassis otherwise the body springs back into place. Be careful here as the clips on the body are actually part of the glazing assembly and too much leverage might loosen the glue holding the glazing in place.

Once the clips have been released gently pull the front end of the body away from the chassis and gradually work your way towards the back of the motor car. There are two clips just under the corridor connection which need to be carefully released to completely remove the body. Be careful not to damage any of the fine detailing on the model, both on the undercarriage and on the roof, whilst separating body and chassis.

Derby Lightweight DMU Derby Lightweight DMU

The decoder socket is located at the rear of the motor car. It had been hoped that one of the TCS DP2X range of decoders would fit but there isn't enough space for this decoder as it fouls the motor bogie. However, any wired decoder with an 8-pin plug would suffice provided the harness isn't too long. To that end a TCS T1-UK decoder was used, the top of the motor housing containing a recess for the decoder.

Wires from the plug to the decoder need to be carefully arranged so that they enter the top recess through the cutout but also so that they don't push the two metal lighting connectors out of position.

Make sure you insert the NMRA plug the right way round or the lights won't work and the motor car will be running the wrong way!

Replace the body and test that everything is working. Only then replace the screw.

Derby Lightweight DMU Derby Lightweight DMU Derby Lightweight DMU

The body is separated from the chassis in the trailer car in the same way as the motor car, except that there aren't any clips under the corridor connector, rather a single lug at the back which secures the body to the chassis.

The 6-pin decoder socket is located in the toilet at the back of the trailer car. It is easy to access and the pin numbers 1 and 6 are clearly labelled. Remove the blanking plug and insert the new decoder, in this case a Digitrax DZ125IN, though a TCS EUN651 would also suit, making sure you insert it the right way round.

Reassamble and test. You can program the trailer car separately from the motor car if you wish - the two are set up to run in the right direction lighting-wise using F0. Internal lights are on permanently in both cars. Don't forget to replace the screw securing body to chassis!


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Copyright Bromsgrove Models 2006