DCC in a Hornby A1X Terrier

I’ve recently acquired a rather nice little Hornby Terrier loco. It’s one of the ex-Dapol tooling models, so while it’s not up to the standards of the more modern Hornby offering, it’s still a reasonable model that I’d like to get running. As it’s one of the old models, it doesn’t come with a DCC socket, and fitting a chip might be a challenge due to the severe shortage of space inside the body! I’ve therefore decided to forego the Zimo MX600 decoder I normally use (in my opinion the best decoder for the price on the market by far), and use it’s baby brother, the MX617F.

One Terrier and one DCC chip. The loco itself is no.3, Bodiam, in its as-preserved Kent and East Sussex Railway blue livery. Personally this is a nice loco to own, since I grew up close to the KESR and have spent a lot of time there over the years.

This model offers the same excellent motor control as the rest of the range, but with a reduced peak and sustained current output. This will not be a problem, as the motor in the model will not draw that much.

Even with such a small decoder (this one measures just 13x9x2mm), finding a place to fit it will still be a challenge. Let’s open it up!

To remove the body, unscrew the two screws at the front and back. The other three screws can be left alone.
With these two screws removed the body will pop off. The cab can then be detached from the body by releasing three plastic lugs underneath the footplate. In this photo I’ve already removed the nasty orange 0.1nF capacitor, since they have no place in a DCC-chipped loco, and can only hurt performance!

With the body off, we can see where we might be able to place the decoder. There is a small slot in the bunker at the back, above the grey metal weight, but I decided against this, since this would mean running the decoder harness through the cab, where it would be on display.

Having measured the space inside the motor cavity, I determined that there would just about be enough space for mount the decoder longitudinally on top of the motor, as shown below:

Test-fitting the decoder. There’s so little space that I have removed the factory-applied protective casing and replaced it with two layers of sellotape, which should insulate it just fine.

Before mounting the decoder, I glued 0.5mm plasticard shims to the dark grey magnets. This ensures that the motor armature will not hit the underside of the chip as it rotates. The decoder itself is hard-wired to the chassis, as there is simply no space for a socket of any kind!

Before I re-assembled the loco, I took the chance to fill up any spaces I could find with lead to improve electrical pickup and adhesion. I added one strip in the aforementioned slot in the bunker, and one strip in the motor cavity, right above the worm gear. They’re not huge weights, but every little helps.

Running in on the rolling road. The motor is quiet at low to medium speeds. I have not yet tuned the motor drive settings in the decoder, but it is already running very smoothly out of the box, which is a good sign.

This was a pretty satisfying decoder install, since this loco has been sitting on my shelf for months. Now I can finally run it! The next step is to fit Kadee couplings to it so it can actually pull something.