Thankfully, they made it foolproof...

Though I had a good go at testing it...
At Warley I bought a Dapol Class 121 railcar in BR blue (and a Lenz Silver mini DCC decoder to go with it).

Due to pressure of time, and not having either a rolling road or continuous run available to check out the running of the unit, it has sat on a shelf since then.

At the Milton Keynes show, I bought a Dapol class 121 railcar in green with small yellow ends and an Uhlenbrock 73410 decoder.

Last weekend, I had some time so set up a continuous run and tested both units. Once I was convinced that they ran nicely on DC, it was time to take the plunge and convert the two units.

The Uhlenbrock decoder is slightly smaller and has shorter pins, rather useful as we shall see later.

I also had in stock one Dapol White lightbar, which I intended to use in the Blue unit, not convinced that they had fluorescent lighting, most references indicate that they had their tungsten bulb lighting replaced when they were refurbished.

The Lenz Silver had pin one marked with a spot of paint.

The roof comes off fairly easily. One thing to make sure of is that you keep your fingers away from the clips on the inside. On releasing the roof my finger tip curled under the roof edge and snapped off the clip.

As you can see, the dummy plug is easy to see and carefully labelled. Also note (and I just noticed this) that the pin numbers are printed on the dummy plug. Two other points. The first is that this is the green unit which has an illuminated headcode on DC (there are a pair of LED on the left end). The second is the small space available for the decoder.

So, it was on with the installation.

The first one to be worked on was the BR Blue one (blanked headcode, so no LEDs).
So, I installed the decoder (see above) based on the diagram in the PDF manual on the Uhlenbrock web site.

Having not noticed the pin numbers on the dummy plug, I managed to plug it in the wrong way round (Lenz mark pin one as the left hand pin with the pins pointing upward), Dapol have their socket arranged so that pin one is on the right. This means that the pin one marker is towards the PCB. Slightly frustrating, especially as I think I made the same mistake when I installed the decoders in the class 153 and class 156 from the same manufacturer.

So while I was working out what was wrong, I shuffled the two decoders round.

Above is the Lenz decoder in the blue unit, as you can see, it is a very tight squeeze.

As you can see, the Uhlenbrock decoder is physically smaller, plus it has shorter pins, which make installation in the slightly cramped Class 121 easier.

The Dapol White lightbar is too long to fit in the body, that will be used to light the St Ives Bay class 153.

Keeping your programming track clean is important, after all the fuss getting the decoder in the right way round, the first unit shuffled itself off the main part of the track onto a grubby bit which meant that it could not be programmed. A quick wipe of the track solved that.

Once programmed (imaginatively 1211 and 1212) I ran them on the continuous track. The head and tail lights worked fine, but the head code lights on the green unit failed to light.

Initially, I thought I might have burned them out when I had the decoder the wrong way round, but it appears to be a 'feature' of the models. A quick search of the Internet showed a number of people having had problems with the headcode (different decoders have different effects). I will have to as Dapol at Ally Pally later in the month.