The Hornby Class 90 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 numerous Class 90s so far using these (plus many other Hornby locos with similar drives) and can confidently recommend them.
Photo 1
All you need. The loco, a screwdriver, a small piece of circuit board, a decoder (in this case the Lenz LE1025A) 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, but it also has a large screw holding the two together which is relatively unusual. Remove the screw (circled in red in the photo) first and keep it somewhere safe.
Photo 3
There are four clips beneath the doors 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. The body on the Class 90 will normally part company with the chassis quite easily as long as you're removed the screw.
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.
However, some Hornby electric locomotives, such as this one, have a working pantograph facility and an associated switch in the roof. This MUST NOT be used with DCC so disconnect the two black wires shown in the photo, this should also enable you to completely seperate the body and chassis. At this point you might like to completely remove the various bent pieces of metal that make up the sliding switch in the roof. I prefer to, but it's not essential.
Photo 5
On the motor bogie start by unsoldering the orange capacitor and any other gubbins clinging to the brush retaining clips - the silver metal things labelled 3 and 4 in the photo. Throw the capacitor in the bin as it hinders DCC operation and should not be used. I've numbered the 4 places you'll be connecting wires from the decoder for later reference, but here they still have the Hornby wiring connected.
Photo 6
Once you've unsoldered the wires you should be left with what is visible in this photo. 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
The first soldering 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 photograph and the red one goes to point 2. The red wire can be soldered directly in place, 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 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 attach it using the original screw.
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.
Now add two more wires connecting the motor terminals to the stripboard. The orange wire goes to the left hand terminal, labelled 3 in the photos, whilst the grey wire goes to the right hand one, labelled 4.
Photo 8
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 to the side of the metal weight using a double-sided sticky pad, you might want to trim the wires a little to keep this neat.
Photo 9
Note that if you stick anything to the top of the weight as I've done you'll need to remove a piece of the cross-shaped plastic support 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 how I remove it using a pair of cutters.
Photo 10
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 the end with the brass nut inset into the roof aligns with the screw hole in the chassis. Finally replace the large screw, taking care not to trap any wires with it - you can see where it's going by peering in through the large hole where the bogie fits. Do not over-tighten this screw as it will pull the chassis too far up inside the body.