Not only isn't the old Mainline Class 03 DCC-Ready, which means the decoder needs to be hard-wired in, but it's a split-frame chassis as well - something that strikes fear into the hearts of many modellers. Thankfully this type of chassis is pretty rare on D&E models and this particular example is dead easy to convert. You don't need to start cutting chunks of metal from the chassis and you don't need to take the whole thing apart. If anything, the Mainline Class 03 is an easier DCC conversion than many modern multiple units, so fear not!
The distinguishing feature of a split-frame chassis is that it's split into two, electrically live halves. Two large slabs of metal, just sitting there waiting to cause a short circuit. Care needs to be taken to avoid anything causing such shorts, not just decoder wiring but things like couplers and other metal detail parts, too. But from a purely DCC point of view, split-frame installations involve two unique steps - isolating the motor terminals from the two live halves of the chassis, and deciding which bit of the chassis is the easiest place to connect the red and the black decoder wires to.
Photo 1
All you need. The loco, a cross-head screwdriver, a decoder (in this case the Lenz LE0521) 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 has sat in it's box for as long as mine had then it's probably a good idea to test-run it before proceeding in order to check everything is OK.
Photo 2
The body is held on to the chassis by the couplers, so undo the screws circled in red in the photo and remove the couplers. Put the couplers and screws safely to one side as they'll be needed when you want to put everything back together again.
Photo 3
The body should then lift from the chassis very easily, revealing the split-frame in all it's glory.
Photo 4
This side view shows what you're up against - the only obvious free space is in the cab area. N gauge decoders are the natural choice as they'll fit far more easily, although some of the smaller, double-sided OO examples can be squeezed in, too. The latter might be visually obtrusive, however, as they'll probably need to be supported on a diagonal. As my loco had a stall current of only 0.4 amp, even less powerful decoders with only a 0.5 amp rating will be OK, so I went for the Lenz LE0521.
Photo 5
The motor is only connected to the chassis by a couple of contact strips, so the easiest solution is to take it out to work on. Remove screws A and B on one side...
Photo 6
...followed by C and D on the other. Loosen screw E just enough so you can coax the motor out through the top of the frame - you don't need to completely dismantle the chassis or worry about getting the wheels back in correctly.
Photo 7
The motor connects to the chassis via the two simple terminals with bent-over contact strips, as labelled in the photo. Carefully remove these, one at a time, by undoing the retaining screw. Make sure the little spring underneath doesn't fly across the room.
Photo 8
Snip off the contact strip from each terminal, which will effectively isolate the motor from the live chassis - it's as easy as that. It's a good idea to solder the orange and grey decoder wires to the terminals while you've got them loose, then replace each terminal when you've finished with it. Ensure the end sits firmly in the groove and holds the spring in place before securing with the original screw.
Photo 9
Pop the motor back into the chassis, taking care that the small gear on the motor meshes with the larger gear on the chassis. To connect the red and black decoder wires, simply trap them between the motor and the chassis. The bare wire end of the black lead goes behind screw A and the bare wire end of the red lead behind screw D. Replace and tighten all screws, not forgetting screw E which you loosened to get the motor out.
This is one of the easier split-frame installs, and if connected as per the photos everything should be safe. The motor has been isolated from the chassis and there are no exposed wires that need insulating. But study the photographs carefully until you're sure what goes where, as with a live, split-frame chassis any errors could easily blow your decoder.
Photo 10
Fit your decoder in the cab area, making sure everything is well insulated from the metal chassis to prevent short-circuits. With a typical N gauge decoder, the sticky-pad is sufficient insulation, but if in doubt cover the bare metal of the chassis with insulating tape first. I've routed the decoder wires between the two halves of the chassis (make sure they can't droop far enough to interfere with the gears) and insulated the spare lighting wires with a piece of tape. All of the wires should fit safely in the chassis gap and can be held in place with another small piece of tape across the top.
Photo 11
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 to 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 pop the body on and secure it by screwing the couplings back into place.
A few notes on decoder selection might be useful at this point. The Mainline Class 03 is a famously poor runner and a decoder can't be expected to magically cure mechanical or electrical problems. However, careful selection can get the most out of a model, so it pays to think carefully about the choices available. For this reason I tested a lot of decoders with this loco, going so far as fitting a temporary DCC socket so I could swap things around easily and try many of the larger decoders that wouldn't actually fit within the loco in reality. A few observations are: