The Hornby HST is typical of most Hornby diesel and electric models in that it's not DCC-Ready but it's a fairly easy job to hard-wire a decoder using a soldering iron. The sample shown here is a recent production model equipped with pick-ups on all of the (blackened) wheels. Older examples will need a slightly different technique, but nothing difficult.
Of course, there are many ways to convert locos and we all have our own preferences, these are merely mine, offered in the hope that they might help someone, somewhere make a start. For example, you can solder the decoder leads directly to the motor terminals - I just prefer a central mounting point as it makes later alterations to things such as the lighting circuits that little bit easier. Decoders, too, are an area where tastes will vary, but the one suggested here is a reliable performer and well matched to the loco. It's a good place to start if you haven't already developed your own opinions, I've converted five HSTs so far using them (plus many other Hornby locos with similar drives) and can confidently recommend them.
Photo 1
All you need. The loco, a small piece of circuit board, a decoder (in this case the Lenz LE1024A) and the double-sided sticky pad supplied with it. Also you'll need your soldering outfit and a craft knife or wire stripper - not pictured because they're not very photogenic! If your loco is brand new like this one then it's probably a good idea to test-run it before proceeding in order to check everything is OK.
Photo 2
Most Hornby locos have bodies that clip onto the chassis, and this one is no exception. There are five clips (circled in yellow in the photo) which need to be released, if you have difficulties then wedge a piece of card in the gap to stop them popping back into place.
Photo 3
You'll probably find that the body on the HST is a little reluctant to give up it's grip on the chassis. As the front tucks into the nose of the loco the back needs to come out first (as in the photo) but it's not easy. A fairly strong pull on the fuel tanks will be needed before the back will unclip - don't be afraid to bend the chassis at this stage.
Photo 4
With the body off and the innards laid bare, you can begin to see where things need to go. Current Hornby production has easily visible terminals for everything and no hidden pitfalls such as screws that need to be isolated. Everything is ideal for attaching a decoder to.
Photo 5
Start by unsoldering the orange capacitor (see previous photo) and any other gubbins clinging to the brush retaining clips - the silver metal things labelled 3 and 4 in this photo. Throw the capacitor in the bin as it hinders DCC operation, and if you've unsoldered the wires you should be left with what is visible in the picture. I've numbered the 4 places you'll be connecting wires from the decoder for later reference.
Photo 6
If you're unsure about wielding a hot soldering iron this close to the plastic chassis and want a little more room to work in, just unclip the bogie from the chassis and drop it out from the bottom. Again, the 4 connection points are numbered for later reference.
Photo 7
Now turn your attention to the other end of the chassis and discard the light bulb and related wiring. I prefer to fit replacement lighting, using either LEDs or the kits from Express Models but I'm not going to cover that here - this is just about fitting a decoder. Unclip the light grey plastic representation of the cab interior to expose the light bulb. Remove the metal pins holding the bulb in place to enable you to pull it out, then replace the grey moulding, making sure the wires aren't trapped underneath it.
Photo 8
Unclip the bogie and drop it out underneath the chassis as shown to enable you to remove the unwanted wiring. Cut off the two wires (the thicker red and black ones in this example) that used to connect to the bulb. Cut as close to the pick-ups as you can comfortably get, but make sure you don't cut or damage the longer wires that take the power to the other end of the loco. Clip the bogie back in place once the surplus wiring has been discarded.
Photo 9
Returning to the power bogie end of the loco, the first task is to connect the red and the black wires that will take the track-power from the pick-ups to the decoder. As I prefer to take all connections to a scrap of stripboard stuck to the metal weight I've cut the two pieces of wire long enough to reach that far.
The black wire needs to connect to point 1 in the earlier photographs and the red one goes to point 2. The red wire needs to be soldered in place of the shorter red wire that's already there, but the black one is slightly more difficult as it needs to be attached to a terminal that in turn is screwed to the bogie. As the Hornby one is crimped in place I discard it (that's it lying in the foreground) and solder the replacment black wire to a new M3 solder tag, product code RG86T from Maplin. I then bend this to match the discarded one and screw it in place using the original screw before clipping the bogie back into the chassis.
Photo 10
Next solder these red & black wire to a piece of stripboard (product code FL17T from Maplin) stuck to the top of the weight with a double-sided sticky pad. Also solder the matching black and red wires from the other bogie as shown, red to red and black to black. One piece of stripboard will do many installations if you cut it into 9-strip chunks as I've done here.
Note that if you stick anything to the top of the weight like this you'll need to remove a piece of the two ribs inside the bodyshell directly above it or the body won't seat properly on the chassis. If you look at the photo you can see where I've snapped a couple of chunks off using a pair of pliers.
Photo 11
Now add two more wires connecting the motor terminals to the stripboard. The orange wire goes to the left hand terminal, labelled 3 in earlier photos, whilst the grey wire goes to the right hand one, labelled 4. It doesn't really matter whether you solder these wires to the blobs of solder where Hornby's connections were, or further out on the 'ears' as I've done here.
Photo 12
The final wiring operation is to solder all nine wires from the decoder to the stripboard. The red, black, orange and grey wires need to match the corresponding colours from the motor bogie, but the other wires can be soldered to any other free strip. These wires can power lights if you wish, but aren't used here as we're just concerned with the motor. Hopefully the photograph will make this clearer.
The decoder can be stuck down into the fuel tank using a double-sided sticky pad, you might want to trim the wires a little to keep this neat.
Photo 13
Before putting the body back on the chassis it's as well to check that everything is working. The safest way to do this is to put the loco on the programming track and try and read information from the decoder - the instructions for your particular DCC system will tell you how to do this. If all is well (and I've never yet had a problem) then clip the body back on, making sure that you feed the chassis front into the nose first.